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Dated: Aug. 11, 2004
Related CategoriesNetwork+ Certification
- What is a Computer Network?
- Types of Networks
- Network Categorization w.r.t Distance
- Specialized Types of Networks
- Network Topologies
- Network Cabling
- Wireless Networks
- Signal Transmission
- The OSI Model
- Ethernet Network Architecture
- Token Ring Network Architecture
- Apple Talk
- Putting Data on the Cable
- Network Devices
- IP Addressing
- Name Resolution for TCP/IP
- Domain Name System
- Understanding Windows Internet Naming Services
- Examining your Network with Commands
- Network Operating Systems
- Maintaining Your Network
The CompTIA Network+ Exam certifies that the successful candidates knows the layers of the OSI model, can describe the features and functions of network components and has the skills needed to install, configure, and troubleshoot basic networking hardware peripherals and protocols. A typical candidate should have A+ certification or equivalent knowledge, but A+ certification is not required. In addition to A+ certification level knowledge, candidates are encouraged to have at least 9 months of experience in network support or administration.
It also includes discussion on new technologies such as wireless networking and gigabit Ethernet. The scope of networking systems are broadened somewhat placing an increased emphasis on Linux/Unix, Windows 9x, Windows NT, Windows 2000 and including AppleTalk as a network protocol. There is also more of an emphasis on hands-on experience knowledge needed in the areas of network implementation and network support including troubleshooting scenarios. This study guide gives you an overview what you needs to pass Network+ Exam.
What is a Computer Network?
A network is any collection of independent computers that communicate with one another over a shared network medium.A computer network is a collection of two or more connected computers. When these computers are joined in a network, people can share files and peripherals such as modems, printers, tape backup drives, or CD-ROM drives. When networks at multiple locations are connected using services available from phone companies, people can send e-mail, share links to the global Internet, or conduct videoconferences in real time with other remote users. As companies rely on applications like electronic mail and database management for core business operations, computer networking becomes increasingly more important.
Types of Networks
Peer to Peer
A peer to peer network is one in which lacks a dedicated server and every computer acts as both a client and a server. This is a good networking solution when there are 10 or less users that are in close proximity to each other. A peer to peer network can be a security nightmare, because the people setting permissions for shared resources will be computer idiots and the right people will never have access to the right resources. Thus is only recommended in situations where security is not an issue.
This type of network is designed to support a large Number of users and uses dedicated server/s to accomplish this. Clients log on to the server/s in order to run applications or obtain files. Security and permissions can be managed by 1 or more administrators which cuts down on the aforementioned computer illiterates from medling with things that they shouldn't be. This type of network also allows for convenient backup services, reduces network traffic and provides a host of other services that come with the network operating system (NOS).
This is also a client/server based model that is most often seen in UNIX environments, but the clients are "dumb terminals". This means that the client may not have a floppy drive, hard disk or CDROM and all applications and processing occur on the server/s. As you can imagine, this requires fast and damn expensive server/s. Security is very high on this type of network, although a similar level of security can be achieved using an NT server and setting appropriate permissions.
Network Categorization w.r.t Distance
LANs (Local Area Networks)
A network is any collection of independent computers that communicate with one another over a shared network medium. LANs are networks usually confined to a geographic area, such as a single building or a college campus. LANs can be small, linking as few as three computers, but often link hundreds of computers used by thousands of people. The development of standard networking protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organizations.
MANs (Metropolitan area Networks)
WANs (Wide Area Networks)
Wide area networking combines multiple LANs that are geographically separate. This is accomplished by connecting the different LANs using services such as dedicated leased phone lines, dial-up phone lines (both synchronous and asynchronous), satellite links, and data packet carrier services. Wide area networking can be as simple as a modem and remote access server for employees to dial into, or it can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the expense of sending data sent over vast distances.
The Internet is a system of linked networks that are worldwide in scope and facilitate data communication services such as remote login, file transfer, electronic mail, the World Wide Web and newsgroups.
With the meteoric rise in demand for connectivity, the Internet has become a communications highway for millions of users. The Internet was initially restricted to military and academic institutions, but now it is a full-fledged conduit for any and all forms of information and commerce. Internet websites now provide personal, educational, political and economic resources to every corner of the planet.
Specialized Types of Networks
With the advancements made in browser-based software for the Internet, many private organizations are implementing intranets. An intranet is a private network utilizing Internet-type tools, but available only within that organization. For large organizations, an intranet provides an easy access mode to corporate information for employees.
VPN (Virtual Private Network)
VPN uses a technique known as tunneling to transfer data securely on the Internet to a remote access server on your workplace network. Using a VPN helps you save money by using the public Internet instead of making long-distance phone calls to connect securely with your private network. There are two ways to create a VPN connection, by dialing an Internet service provider (ISP), or connecting directly to Internet.
What is a Network topology?
- Bus consists of a single linear cable called a trunk.
- Data is sent to all computers on the trunk. Each computer examines EVERY packet on the wire to determine who the packet is for and accepts only messages addressed to them.
- Bus is a passive topology.
- Performance degrades as more computers are added to the bus.
- Signal bounce is eliminated by a terminator at each end of the bus.
- Barrel connectors can be used to lengthen cable.
- Repeaters can be used to regenerate signals.
- Usually uses Thinnet or Thicknet both of these require 50 ohm terminator
- Good for a temporary, small (fewer than 10 people) network
- Its difficult to isolate malfunctions and if the backbone goes down, the entire network goes down.
- Computers are connected by cable segments to a centralized hub.
- Signal travels through the hub to all other computers.
- Requires more cable.
- If hub goes down, entire network goes down.
- If a computer goes down, the network functions normally.
- Most scalable and reconfigurable of all topologies
- Computers are connected on a single circle of cable.
- Usually seen in a Token Ring or FDDI (fiber optic) network
- Each computer acts as a repeater and keeps the signal strong => no need for repeaters on a ring topology
- No termination required => because its a ring
- Token passing is used in Token Ring networks. The token is passed from one computer to the next, only the computer with the token can transmit. The receiving computer strips the data from the token and sends the token back to the sending computer with an acknowledgment. After verification, the token is regenerated. relatively easy to install, requiring ;minimal hardware.
- The mesh topology connects each computer on the network to the others
- Meshes use a significantly larger amount of network cabling than do the other network topologies, which makes it more expensive.
- The mesh topology is highly fault tolerant.
- Every computer has multiple possible connection paths to the other com-puters on the network, so a single cable break will not stop network communications between any two computers.
Primary Cable Types
- Consists of a solid or stranded copper core surrounded by insulation, a braided shield and an insulating jacket.
- Braided shield prevents noise and crosstalk.
- More resistant to interference and attenuation than twisted pair cabling.
- Both thin and thick cables can use BNC cable connectors, BNC barrel connectors BNC T connectors BNC terminators.
- Can transmit data, voice and video.
- Offers moderate security ----> better than UTP/STP
Thinnet - RG-58 cable
- 0.25" thick.
- Uses BNC twist connector, BNC barrel connectors BNC T connectors 50 ohm terminators
- Can carry signals 185 meters or 607 feet.
- Each cable must have a terminator whose impedance matches the cable type
Thicknet - RG-8 and RG-11 coaxial cable
- 0.5" thick
- used for 10Base5 networks, linear bus topology
- Transmits at 10 Mbps
- Uses DIX or AUI (Attachment Unit Interface) connector - also known as DB-15 connector to connect to external transceivers.
- Can carry signals 500 meters or 1640 feet.
- much less flexible and far more bulky and harder to install than thinnet
- better security than thinnet
- better resistance to electrical interference than thinnet.
- MORE expensive than thinnet.
- Consists of two insulated copper wires twisted around each other.
- Twisting cancels out electrical noise from adjacent pairs (crosstalk) and external sources.
- Uses RJ-45 telephone-type connectors (larger than telephone and consists of eight wires vs. Telephone's 4 wires).
- Generally inexpensive.
- Easy to install
Unshielded Twisted Pair (UTP)
- Maximum cable length is 100 meters or 328 feet (10BaseT).
1.Cat 1 Voice grade telephone cable.
2.Cat 2 Data grade up to 4 Mbps, four twisted pairs.
Category 3 and above is needed for Ethernet networks. Cat 3, 4, and 5 use RJ-45 connectors
1.Cat 3 Data grade up to 10 Mbps, four pairs w/3 twists/ft.
2.Cat 4 Data grade up to 16 Mbps, four twisted pairs.
3.Cat 5 Data grade up to 100 Mbps, four twisted pairs
- UTP is particularly susceptible to crosstalk, which is when signals from one line get mixed up with signals from another.
- easily tapped (because there is no shielding)
- 100 meters is shortest distance => attenuation is the biggest problem here.
Shielded Twisted Pair (STP)
- Uses a woven copper braid jacket and a higher quality protective jacket. Also uses foil wrap between and around the wire pairs.
- Much less susceptible to interference and supports higher transmission rates than UTP.
- Shielding makes it somewhat harder to install.
- same 100 meter limit as UTP.
- harder to tap
- used in AppleTalk and Token Ring networks
Fiber Optic Cable
- Consists of a small core of glass or plastic surrounded by a cladding layer and jacket.
- Fibers are unidirectional (light only travels in one direction) so two fibers are used, one for sending and one for receiving. Kelvar fibres are placed between the two fibres for strength.
- Good for very high speed, long distance data transmission.
- NOT subject to electrical interference.
- Cable can't be tapped and data stolen => high security
- Most expensive and difficult to work with.
- Immune to tapping.
- can transmit at 100 Mbps and way up to 2 Gbps up to 2000 meters without a repeater.
- Supports data, voice and video.
- needs specialized knowledge to install => expensive all round.
- Used where cable isn't possible - remote sites; also when mobility is important.
- Use transceivers or access points to send and receive signals between the wired and wireless network.
Techniques for transmitting data
- Infrared transmission consists of four types;
1. Line of sight
2. Scatter: good within 100 ft.
4. Broadband optical telepoint: used for multimedia requirements; as good as cable.
- Laser requires direct line-of-sight.
- Narrow-band (single frequency) radio
- Cannot go through steel or load-bearing walls.
- Requires a service handler.
- Limited to 4.8 Mbps
- Spread-Spectrum Radio
- Signals over a range of frequencies.
- Uses hop timing for a predetermined length of time.
- Coded for data protection.
- Quite slow; Limited to 250 Kbps.
Point to Point Transmission
- Transfers data directly from PC to PC (NOT through cable or other peripherals)
- Uses a point to point link for fast error-free transmission.
- Penetrates objects.
- Supports data rates from 1.2 to 38.4 Kbps up to 200 feet indoors or 1/3 of a mile with line of site transmission. Also communicates with printers, bar code readers, etc
Uses wireless public carriers to transmit and receive using;
- Packet-radio communication. Uplinked to satellite, broadcast only to device which has correct address.
- Cellular networks. CDPD same as phone, subsecond delays only, real time transmission, can tie into cabled network.
- Satellite stations. Microwave, most common in USA, 2 X directional antennas, building to building, building to satellite
Baseband Transmission -- Digital
- Baseband transmission uses digital signaling over a single frequency.
- Entire communication channel is used to transmit a single signal.
- Flow is bi-directional. Some can transmit and receive at the same time.
- Baseband systems use repeaters to strengthen attenuated signals.
Broadband Transmission -- Analog
- Broadband uses analog signaling over a range of frequencies.
- Signals are continuous and non-discrete.
- Flow is uni-directional and so two frequency channels or two separate cables must be used. If enough bandwidth is available, multiple analog transmission systems such as cable TV AND network transmissions can be on the same cable at the same time. if this is the case, ALL devices must be tuned to use only certain frequencies Uses amplifiers for signal regeneration.
The OSI Model
- International Standards Organization (ISO) specifications for network architecture.
- Called the Open Systems Interconnect or OSI model.
- Seven layered model, higher layers have more complex tasks.
- Each layer provides services for the next higher layer.
- Each layer communicates logically with its associated layer on the other computer.
- Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.
- Serves as a window for applications to access network services.
- Handles general network access, flow control and error recovery.
- Example Protocols are NCP, SMB, SMTP, FTP, SNMP, Telnet, AppleTalk
- Determines the format used to exchange data among the networked computers.
- Translates data from a format from the Application layer into an intermediate format.
- Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
- Redirector operates at this level.
- Example protocols are NCP, AFP, TDI
- Allows two applications running on different computers to establish use and end a connection called a Session.
- Performs name recognition and security.
- Provides synchronization by placing checkpoints in the data stream.
- Implements dialog control between communicating processes.
- Example protocol is NetBIOS
- Responsible for packet creation.
- Provides an additional connection level beneath the Session layer.
- Ensures that packets are delivered error free, in sequence with no losses or duplications.
- Unpacks reassembles and sends receipt of messages at the receiving end.
- Provides flow control, error handling, and solves transmission problems.
- Example Protocols are NetBEUI, TCP, SPX, and NWLink
- Responsible for addressing messages and translating logical addresses and names into physical addresses.
- Determines the route from the source to the destination computer.
- Manages traffic such as packet switching, routing and controlling the congestion of data.
- Example Protocols are IP, IPX, NWLink, NetBEUI
Data Link Layer
- Sends data frames from the Network layer to the Physical layer.
- Packages raw bits into frames for the Network layer at the receiving end.
- Responsible for providing error free transmission of frames through the Physical layer.
- Transmits the unstructured raw bit stream over a physical medium.
- Relates the electrical, optical mechanical and functional interfaces to the cable.
- Defines how the cable is attached to the network adapter card.
- Defines data encoding and bit synchronization.
Ethernet Network Architecture
- Baseband signaling.
- Linear or star-bus topology.
- Usually transmits at 10 Mbps with 100 Mbps possible.
- Uses CSMA/CD for traffic regulation.
- IEEE specification 802.3.
- Uses thicknet, thinnet or UTP cabling
- Media is passive => it draws power from the computer
10 Mbps Topologies
- (10 = 10 Mbps; Base= Baseband; T = Twisted Pair)
- 10 Mbps, baseband over UTP.
- Usually wired in a physical star with a hub or multiport repeater. Internally it uses a bus signaling system like other Ethernet configurations
- Maximum segment length 100 meters (328 feet).
- Minimum between computers 2.5 meters (8 feet).
- 1024 nodes maximum on the LAN
- Category 3, 4 or 5 UTP.
- RJ-45 connectors, 4 twisted pair.
- Coaxial or Fiber backbone for larger LAN's
- (10 = 10 Mbps; Base= Baseband; 2 = 2x 100 meters)
- 10 Mbps, baseband over thinnet.
- Uses bus topology.
- Maximum segment length 185 meters (607 feet).
- Minimum between computers 0.5 meters (20 inches).
- Maximum of 30 computers per segment.
- (10 = 10 Mbps; Base= Baseband; 5 = 5 x 100 meters)
- 10 Mbps, baseband over thicknet.
- Also called Standard Ethernet
- Designed to support a backbone for a large department or building. Transceivers attach to the thicknet cable and the cable AUI connector plugs into a repeater. The branching segments of thinnet plug into the repeater and connect to the computers on the network.
- Uses bus topology.
- Maximum segment length 500 meters.
- Minimum between transceivers 2.5 meters (8 feet)
- 100 computers per segment, 300 per network.
- Transceiver is attached to main segment with a vampire tap.
- DIX or AUI connector is used to attach the transceiver to the network card. Maximum computer to transceiver distance is 50 meters. This distance is not included in the 5-4-3 calculation.
- (10 = 10 Mbps; Base= Baseband; FL =fibre optic)
- Allows long cable runs between repeaters, like between buildings
- Maximum segment length 2000 meters.
- 10BaseFL - Used for linking computers in a LAN environment.
- 10BaseFP - Used for linking computers with passive hubs from maximum cable distance up to 500m
- 10BaseFB - Used as a backbone between hubs.
- Baseband signal over a fiber-optic cable.
- Need concentrator (fiber-optic hub) ® Star wired (star topology). Either active or passive
- Long distance.
- Very expensive. Difficult to install.
100 Mbps Topologies
100VG-AnyLAN (IEEE 802.12)
- 100 Mbps data rate.
- Star topology over Category 3, 4 and 5 UTP.
- Uses demand priority access.
- Combines element of traditional Ethernet and Token Ring and supports Ethernet and token ring packets.
- Faster than Ethernet
- Demand priority access method => two priority levels, low and high
- Intelligent hubs can filter individually addressed frames for enhanced privacy.
- Uses RJ-45.
- Uses star topology and defines how child hub can be connected to a parent hub to extend the network.
100BaseT? (Fast Ethernet)
- Uses CSMA/CD on a star-wired bus.
- There are 3 specifications:
- 100BaseT4: Uses pair category 3, 4 or 5 UTP.
- 100BaseTX: Uses 2-pair category 5 UTP or STP.
- 100BaseFX: Uses 2-strand fiber-optic
Token Ring Network Architecture
- IEEE 802.5 specification.
- Star wired ring topology (logical ring)
- Uses token passing access method.
- Can have higher transmission speeds than Ethernet
- It has larger frames than Ethernet => more can get transferred over the wire in any given time.
- Uses IBM STP Types 1, 2 and 3 cabling. (Can be UTP)
- Transmits at 4 and 16 Mbps. (16 Mbps cards will slow down to 4 Mbps if put on that kind of network, but the 4 Mbps cards can't speed up.
- Baseband transmission
- Data travels in one direction only
- Each computer acts as a unidirectional repeater
- Deterministic method of cable access. Computers cannot use the cable unless they have the token. Therefore, computers can't force their way onto the network like CSMA/CD (Ethernet)
- First computer online is assigned to monitor network activity.
Token Ring Components
- Multistation Access Units (MSAU's)
- Smart Multistation Access Units (SMAU's)
- Computers attach directly to the MSAU in a physical star to form a logical ring.
- Each MSAU has 10 connection ports ==> can support 8 clients with 2 ports for ring in and ring out.
- Each ring can have as many as 33 MSAU's
- Up to 12 MSAU's can connect to each other
- The MSAU can sense if a computer is down and then disconnect it from the ring => built-in fault tolerance
- Most token ring systems use IBM type 3 cabling.
- Token ring networks are well suited to fiber optic cable: data travels in only one direction in it.
- local talk
- CSMA/CA access method
- 3 things happen when devices attached
1. device assigns itself an address randomly
2. device broadcasts the address to see if it's used
3. if not, the device will use it the next time it's online again
- bus or tree
- max. 32 devices
- Apple share
- file server on an AppleTalk network
- divided into zones
- allows protocols to run on ethernet coaxial cable
- TokenTalk -802.5 which allows Macintosh to connect to token ring network
Protocol Stacks (or Suites)
- IBM SNA (Systems Network Architecture)
- Digital DECnet
- Novell NetWare
- Apple AppleTalk
- APPC-IBM's peer to peer SNA protocol used on AS400's
- FTAM: an OSI file access protocol.
- X.400: international e-mail transmissions.
- X.500: file and directory services across systems.
- SMTP: Internet e-mail.
- FTP: Internet file transfer
- SNMP: Internet network management protocol.
- Telnet: Internet protocol for logging on to remote hosts.
- Microsoft SMB: client shells and redirectors.
- NCP: Novell client shells or redirectors.
- AppleTalk and AppleShare: Apple's protocol suite.
- AFP: Apple's protocol for remote file access.
- DAP (data access protocol): DECnet file access protocol.
- TCP (transmission control protocol): internet protocol for guaranteed delivery of sequenced data.
- SPX (sequenced packet exchange): Novell protocol suite.
- NWLink: Microsoft implementation of IPX/SPX.
- NetBEUI: establishes communications sessions between computers and provides the underlying data transport services.
- ATP, NBP: Apple's communication session and transport protocols.
- IP (Internet Protocol): packet forwarding and routing.
- IPX: (Internetwork Packet Exchange): Novell's protocol for packet forwarding and routing.
- NWLink: Microsoft implementation of IPX/SPX.
- NetBEUI: Transport for NetBIOS sessions and applications.
- DDP (datagram delivery protocol): An AppleTalk data transport protocol.
The IEEE protocols at the Physical Layer
802.3 (CSMA /CD - Ethernet)
- Logical bus network
- Can transmit at 10 Mbps
- Data is transmitted on the wire to every computer but only those meant to receive respond
- CSMA /CD protocol listens and allows transmission when the wire is clear
802.4 (Token Passing)
- Bus layout that used token passing
- Every computer receives all of the data but only the addressed computers respond
- Token determines which computer can send
802.5 (Token Ring)
- Logical ring network; physical set up as star network
- Transmits at 4 Mbps or 16 Mbps
- Token determines which computer can send
- Provides communications in a heterogeneous environment.
- Routable, defacto standard for internetworking.
- SMTP, FTP, SNMP are protocols written for TCP/IP
- Disadvantages are size and speed.
- NetBIOS extended user interface.
- Originally, NetBIOS and NetBEUI were tightly tied together but, NetBIOS has been separated out to be used with other routable protocols. NetBIOS acts as a tool to allow applications to interface with the network; by establishing a session with another program over the network
- NetBIOS operates at the Session layer.
- Small, fast and efficient.
- Compatible with most Microsoft networks.
- Not routable and compatible only with Microsoft networks.
- Protocols incorporated in a packet switching network of switching services.
- Originally established to connect remote terminals to mainframe hosts.
- Xerox Network System.
- Developed for Ethernet LANs but has been replaced by TCP/IP.
- Large, slow and produces a lot of broadcasts.
IPX/SPX and NWLink
- Used for Novell networks.
- Small and fast.
- Advanced Program to Program Communication
- Developed by IBM to support SNA.
- Designed to enable application programs running on different computers to communicate and exchange data directly.
OSI Protocol Suite
- Digital Equipment's proprietary protocol stack
- Defines communications over Ethernet, FDDI MAN's and WAN's.
- DECnet can also use TCP/IP and OSI protocols as well as its own protocols
Putting data on the Cable
This stands for "carrier-sense multiple access with collision detection" and is the method used on ethernet networks whereby all computers on the network check the cable for traffic before attempting to transmit a packet. If more than 1 transmits at the same time then there will be a collision and both computers will wait a random amount of time and retransmit.
Stands for "carrier-sense multiple access with collision avoidance". This access method prevents collisions by having computers broadcast an intent to send a packet. This is the access method used by Localtalk and is sometimes described as "chatty". This broadcasting of intent to send can cause excess network traffic and slow things down.
Token passing is the access method used by token ring networks. With this method, a packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This is sort of like the "conch" in Lord of the Flies. Piggy had all of this crap that he wanted to whine about all of the time, but could only do so if he possessed the conch.
This access method is used with 100VG-AnyLAN networks. The repeaters, bridges, routers or hubs search the network for requests that are waiting to be sent. If 2 or more requests are received by the network hardware at once, the data with the highest priority is sent. Priority for different data types can be controlled by the administrator. A real advantage is that computers can receive and transmit at the same time with this access method.
Network Adapter Cards
The role of the network Adapter card it to:
- Prepare data from the computer for the network cable
- Send the data to another computer
- Control the flow of data between the computer and the cabling system
- EXTEND the network segment by REGENERATING the signal from one segment to the next
- Repeaters regenerate BASEBAND, digital signals
- Don't translate or filter anything
- Is the least expensive alternative
- work at the Physical layer of OSI
- Both segments being connected must use the same access method e.g. an 802.3 CSMA/CD (Ethernet) LAN segment can't be joined to a 802.5 (Token Ring) LAN segment. Another way of saying this is the Logical Link Protocols must be the same in order to send a signal.
- BUT repeaters CAN move packets from one physical medium to another: for example can take an Ethernet packet from a thinnet coax and pass it on to a fiber-optic segment. Same access method is being used on both segments, just a different medium to deliver the signal
- They send every bit of data on => NO FILTERING, so they can pass a broadcast storm along from on segment to the next and back. So you want to use a repeater when there isn't much traffic on either segment you are connecting.
- There are limits on the number of repeaters which can be used. The repeater counts as a single node in the maximum node count associated with the Ethernet standard [30 for thin coax].
- Repeaters also allow isolation of segments in the event of failures or fault conditions. Disconnecting one side of a repeater effectively isolates the associated segments from the network.
- Using repeaters simply allows you to extend your network distance limitations. It does not give you any more bandwidth or allow you to transmit data faster.
- Why only so many repeaters are allowed on a single network: "propagation delay". In cases where there are multiple repeaters on the same network, the brief time each repeater takes to clean up and amplify the signal, multiplied by the number of repeaters can cause a noticeable delay in network transmissions.
- It should be noted that in the above diagram, the network number assigned to the main network segment and the network number assigned to the other side of the repeater are the same.
- In addition, the traffic generated on one segment is propagated onto the other segment. This causes a rise in the total amount of traffic, so if the network segments are already heavily loaded, it's not a good idea to use a repeater.
- A repeater works at the Physical Layer by simply repeating all data from one segment to another.
- Have all the abilities of a repeater
- Take an overloaded network and split it into two networks, therefore they can divide the network to isolate traffic or problems and reduce the traffic on both segments
- Expand the distance of a segment
- Link UNLIKE PHYSICAL MEDIA such as twisted-pair (10Base T) and coaxial Ethernet (10Base2)
- They can link UNLIKE ACCESS CONTROL METHODS, on different segments such as Ethernet and Token Ring and forward packets between them. Exam Cram says this is a Translation Bridge that can do this - not all bridges - but my observation is questions don't necessarily mention the distinction.
- Bridges work at the Data Link Layer of the OSI model => they don't distinguish one protocol from the next and simply pass protocols along the network. (use a bridge to pass NetBEUI, a non-routable protocol, along the network)
- Bridges actually work at the MEDIA ACCESS CONTROL (MAC) sublayer. In fact they are sometimes called Media Access Control layer bridges. Here's how they deal with traffic:
- They listen to all traffic. Each time the bridge is presented with a frame, the source address is stored. The bridge builds up a table which identifies the segment to which the device is located on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important, especially if the network has a large number of workstations/servers.
- They check the source and destination address of each PACKET
- They build a routing table based on the SOURCE ADDRESSES. Soon they know which computers are on which segment
- Bridges are intelligent enough to do some routing:
- If the destination address is on the routing table and is on the SAME SEGMENT, the packet isn't forwarded. Therefore, the bridge can SEGMENT network traffic
- If the destination address is the routing table, and on a remote segment, the bridge forwards the packet to the correct segment
- If the destination address ISN'T on the routing table, the bridge forwards the packet to ALL segments.
- BRIDGES SIMPLY PASS ON BROADCAST MESSAGES, SO they too contribute to broadcast storms and don't help to reduce broadcast traffic
- Two segments are joined by a bridge on each side, each connected to a synchronous modem and a telephone line
- There is a possibility that data might get into a continuous loop between LANs
- The SPANNING TREE ALGORITHM (STA)
- Senses the existence of more than one route
- Determines which is the most efficient and
- Configures the bridge to use that route
- This route can be altered if it becomes unusable.
- Transparent bridges (also known as spanning tree, IEEE 802.1 D) make all routing decisions. The bridge is said to be transparent (invisible) to the workstations. The bridge will automatically initialize itself and configure its own routing information after it has been enabled.
- Determine the best path for sending data and filtering broadcast traffic to the local segment. They DON'T pass on broadcast traffic
- Work at the Network layer of OSI => they can switch and route packets across network segments
- They provide these functions of a bridge: filtering and isolating traffic and connecting network segments
- Routing table contains
1. all known network addresses
2. how to connect to other networks
3. possible paths between those routers
4. costs of sending data over those paths
5. not only network addresses but also media access control sublayer addresses for each node
- Routers require specific addresses: they only understand network numbers which allow them to talk to other routers and local adapter card addresses
- Only pass Packets to the network segment they are destined for.
- Routers don't talk to remote computers, only to other routers
- They can segment large networks into smaller ones
- They act as a safety barrier (firewall) between segments
- They prohibit broadcast storms, because broadcasts and bad data aren't forwarded
- Can join dissimilar access methods: a router can route a packet from a TCP/IP Ethernet network to a TCP/IP Token Ring network
- Routers don't look at the destination computer address. They only look at the NETWORK address and they only pass on the data if the network address is known => less traffic
- Routable protocols have Network layer addressing embedded For Example:DECnet, IP, IPX, OSI, XNS, DDP (Apple)
- Non-routable protocols don't have network layer addressing. For Example LAT, NetBEUI, DLC
- Routers can choose the best path for the data to follow
- Routers can accommodate multiple active paths between LAN segments. To determine the best path, it takes these things into account:
- If one path is down, the data can be forwarded over on alternative route
- Routers can listen and determine which parts of the network are busiest.
- It decides the path the data packet will follow by determining the number of hops between internetwork segments
- OSPF (Open Shortest Path First)
- It is a link-state routing algorithm
- Routes are calculated based on
- # of hops
- line speed
- TCP/IP supports OSPF
- RIP (Routing Information Protocol)
- RIP is the protocol used to determine the # of hops to a distant segment.
- Uses distance-vector algorithm to determine routes
- TCP/IP & IPX support RIP
- NLSP (NetWare Link Services Protocol) is a link-state algorithm for use with IPX
- There are 2 types of routers
- Static - manually setup and config the routing table and to specify each route
- Dynamic automatic discovery of routers and use information from other routers
There are many types of hubs:
- Passive hubs are don't require power and are simple splitters or combiners that group workstations into a single segment
- Active hubs require power and include a repeater function and are thus capable of supporting many more connections.
- Intelligent hubs provide packet switching and traffic routing
- The TRANSLATOR -- allows communications between dissimilar systems or environments
- A gateway is usually a computer running gateway software connecting two different segments. For example an Intel-based PC on one segment can both communicate and share resources with a Macintosh computer or an SNA mainframe. Use gateways when different environments need to communicate. One common use for gateways is to translate between personal computers and mainframes
- GSNW is a gateway to allow Microsoft clients using SMB to connect to a NetWare server using NCP.
- Gateways work at the Application --> Transport layer
- They make communication possible between different architectures and environments
- They perform protocol AND data conversion / translation.
- They takes the data from one environment, strip it, and re-package it in the protocol stack from the destination system
- They repackage and convert data going from one environment to another so that each environment can understand the other environment's data
- Gateway links two systems don't use the same protocols ,data formatting structure,languages and architecture
- They are task specific in that they are dedicated to a specific type of conversion: e.g. "Windows NT Server -> SNA Server Gateway"
- Usually one computer is designated as the gateway computer. This adds a lot of traffic to that segment
An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.
It is sometimes useful to view the values in their binary form.
Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address.
There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address.
Class A addresses begin with 0xxx, or 1 to 126 decimal.
Class B addresses begin with 10xx, or 128 to 191 decimal.
Class C addresses begin with 110x, or 192 to 223 decimal.
Class D addresses begin with 1110, or 224 to 239 decimal.
Class E addresses begin with 1111, or 240 to 254 decimal.
Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They should not be used for host addresses.
Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part belongs to the node (n).
Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn
Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn
Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn
In the example, 18.104.22.168 is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).
In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 22.214.171.124 specifies the network address for 126.96.36.199. When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. 188.8.131.52 specifies the example broadcast address. Note that this is true regardless of the length of the node section.
There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. They can be used by anyone setting up internal IP networks, such as a lab or home LAN behind a NAT or proxy server or a router. It is always safe to use these because routers on the Internet will never forward packets coming from these addresses Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP networks to minimize the amount of traffic each segment must receive.
Applying a subnet mask to an IP address allows you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number. For example, using our test IP address and the default Class B subnet mask, we get:
10001100.10110011.11110000.11001000 184.108.40.206 Class B IP Address
11111111.11111111.00000000.00000000 255.255.000.000 Default Class B Subnet Mask
10001100.10110011.00000000.00000000 140.179.000.000 Network Address
Default subnet masks
Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000
Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000
Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000
CIDR -- Classless InterDomain Routing.
CIDR was invented several years ago to keep the internet from running out of IP addresses. The "classful" system of allocating IP addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B address block of 65533 host addresses. Even more wasteful were companies and organizations that were allocated Class A address blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has ever been actually assigned to a host computer on the Internet. People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992 as a scheme called Supernetting. The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24) It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 5 years ago, there were less than 5000 network routes in the entire Internet. Today, there are over 90,000. Using CIDR, the biggest ISPs are allocated large chunks of address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network route on the Internet. It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth.
Name resolution for TCP/IP
Name resolution is a process that provides users with easy-to-remember server names, instead of requiring them to use the numerical IP addresses by which servers identify themselves on the TCP/IP network. The name-resolution services are the DNS and WINS.
Domain Name System(DNS)
DNS is a hierarchical naming system used for locating computers on the Internet and private TCP/IP networks. It is used to map Internet domain and computer names into IP addresses and vice versa. DNS works at the application layer and uses TCP and UDP for transport. TCP is only used if returned data is truncated. DNS was originally based on HOSTS files that were maintained by a centralized Network Information Center. Today it is based on a hierarchy of servers with a distributed hierarchical database throughout the network or Internet. One or more DNS servers are needed in most installations. DNS is required for Internet e-mail; Web browsing, and Active Directory. DNS is also required in domains with clients running Windows 2000. DNS is installed automatically when you create a domain controller (or promote a server to become a domain controller), unless the Windows 2000 software detects that a DNS server already exists for that domain. (Alternatively, you can explicitly select DNS as a component to install during or after Setup.)
DNS is a hierarchical naming structure with the following levels:
- Root designated by a dot (.).
- First level - This indicates country or type of organization such as "org", "com", and "net".
- Second level - Indicates the organization name and can be purchased for a yearly fee.
On the client side, a DNS resolver is used to send queries to DNS servers. The resolver is normally part of a library routine or it is built into the application. DNS uses zone files to keep name and IP address database information for the internet domain or hierarchial set of domains. Zones are a storage of information in a file for a DNS domain or DNS subdomains (DNS domains are not the same as Windows domains). DNS does not yet support dynamic configuration but has been modified for Windows systems to do so. Different aliases may be created by the administrator for the same host.
Three types of name servers as defined by how it relates to the zone information:
- Primary - Locally stored files exist on the name server database. The master zone file copy is stored here.
- Secondary - Gets data called a zone transfer from another server that is the zone authority.
- Caching Only - Caches name server information and does not contain its own files.
A primary and secondary name server should be used on a network. When a zone is defined, some server must be configured to be a master name server for the zone. There can be different master name servers for different zones. The master server provides copies of the zone information to the secondary DNS server. Name servers can be configured to get information from other name servers when the information is not found in the local database. These types are forwarders and slaves. Name servers as categorized by function:
- Master - The zone authority that contains the master zone files.
- Forwarders - A name server that passes name resolution requests to other name servers. This configuration is done on a per server basis.
- Slaves - Slave name servers are configured to use forwarders.
Query types are:
- Inverse - Getting the name from the IP address. These are used by servers as a security check.
- Iterative - Server gives its best answer. This type of inquiry is sent from one server to another.
- Recursive - Cannot refer the query to another name server.
The DNS zone file serial number is used to track DNS changes. The notify function is used to initiate zone transfers. Zone transfer types are:
- Full - AXFR Query - Secondary server refresh interval expires and it sends an AXFR query.
- Incremental - IXFR query - Only new or updated entries are copied.
Possible zones include:
- Forward lookup zone - Name to IP address map.
- Reverse lookup zone - IP address to name map.
- Standard primary zone (primary zone) - A master copy of a forward or reverse lookup zone.
- Standard secondary zone (secondary zone)
DNS Record types
- A - Address record allowing a computer name to be translated into an IP address. Each computer must have this record for its IP address to be located. These names are not assigned for clients that have dynamically assigned IP addresses, but are a must for locating servers with static IP addresses.
- CNAME - Canonical name allowing additional names or aliases to be used to locate a computer.
- MX - Mail Exchange server record. There may be several.
- NS - Name server record. There may be several.
- PTR - Pointer resource record.
- SOA - Start of Authority record defines the authoritative server and parameters for the DNS zone. These include timeout values, name of responsible person.
- SRV - Service locator resource record to map a service to servers providing the service. Windows 2000 clients will use this record to find a domain controller
- Scope - A range of IP addresses that the DHCP server can assign to clients that are on one subnet.
- Superscope - A range of IP addresses that span several subnets. The DHCP server can assign these addresses to clients that are on several subnets.
- Multicast scope - A range of class D addresses from 220.127.116.11 to 18.104.22.168 that can be assigned to computers when they ask for them. A multicast group is assigned to one IP address. Multicasting can be used to send messages to a group of computers at the same time with only one copy of the message. The Multicast Address Dynamic Client Allocation Protocol (MADCAP) is used to request a multicast address from a DHCP server.
Understanding Windows Internet Naming System(WINS)
Provides name resolution for clients running Windows NT and earlier versions of Microsoft operating systems. With name resolution, users can access servers by name, instead of having to use IP addresses that are difficult to recognize and remember. The purpose of WINS is to allow a NetBIOS name to be mapped to an IP address. Therefore computers using WINS must be using NBT (NetBIOS over TCP/IP). WINS was originally put in place to compensate for a shortcoming of NetBEUI which is the fact that it is not routable. Therefore on large Networks IP is used to transport NetBIOS and rather than using broadcasts, information is sent to the WINS server. WINS maps Windows computer names to IP addresses but does not do name lookups based on IP addresses. The use of Windows Explorer or NET commands invokes the NetBIOS interface. NetBIOS names, if repeated on another domain that is on the network, may cause a problem since there is no way to distinguish NetBIOS names between two domains. Each computer, when booted, sends a name registration broadcast. If there is no response, the computer will use the name it registered. A NetBIOS broadcast releases the computer name when the computer is shutdown gracefully. WINS reduces this broadcast traffic when using NBT. The registration and release is sent to the WINS server rather than being broadcast. The clients have the IP address of the WINS server and they are configured to use WINS before using NetBIOS broadcasts. A backup WINS server may be available on the network for fault tolerance.
Five NBT Name Resolution Methods
- B-node - broadcast - Uses UDP broadcast data grams. Default node type.
- P-node - Peer to peer - Uses a Net BIOS name server such as WINS. If a WINS server is not available, broadcasts are not used as a backup. The WINS IP address must be specified at each client.
- M-node - Mixed - Tries B-node, then P-node resolution.
- H-node - Hybrid - Tries P-node, then B-node resolution. After this attempt for Windows 2000, LMHOSTS and HOSTS files are used, and then the DNS server is used.
- Microsoft enhanced B-node - Checks address cache, which is loaded from the LMHOSTS file when the system boots. After checking address cache, a broadcast is sent, then the LMHOSTS file is checked if broadcasting did not resolve the query.
On the WINS server, there is a NetBIOS name for each service a NetBIOS computer offers. This uses the 16th hidden character of the NetBIOS names. Up to 25 records of groups, domain browsers, and multihomed computers may be registered.
WINS Proxy Agent
A WINS proxy agent can be configured to act as a relay for non-WINS clients. The WINS proxy agent can intercept client broadcast requests, forward them to a WINS server and return the response. It may also reply with the response without contacting the WINS server if the required information is in its cache. One WINS proxy is used on each subnet that has non-WINS clients. This means that machines that are not using WINS (Even Windows machines such as those without TCP/IP) can use a proxy agent to let them find resources on other subnets. There should be a maximum of two proxy agents per subnet. The agent must be a Windows based client, not a server. When NetBIOSs names are registered, both the proxy agent and the WINS server checks the name.
When two WINS servers are configured to communicate with each other replication occurs any time the data base on one of them changes. Servers are configured as a push or pull partner. A server can be both a push and pull partner. Push partners send update notices when a database change is made. A pull partner asks push partners for database entries more recent than their current listings. Only changes are replicated. Pull servers are used across slow links since pull requests can be set for specific times.
- A pull server will pull updates when it is started, then at chosen times thereafter.
- A push partner will send updates when a change threshold is reached. A threshold and update interval may be set.
Examining your network with commands
PING is used to check for a response from another computer on the network. It can tell you a great deal of information about the status of the network and the computers you are communicating with. Ping returns different responses depending on the computer in question. The responses are similar depending on the options used. Ping uses IP to request a response from the host. It does not use TCP. It takes its name from a submarine sonar search - you send a short sound burst and listen for an echo - a ping - coming back. In an IP network, `ping' sends a short data burst - a single packet - and listens for a single packet in reply. Since this tests the most basic function of an IP network (delivery of single packet), it's easy to see how you can learn a lot from some `pings'. To stop ping, type control-c. This terminates the program and prints out a nice summary of the number of packets transmitted, the number received, and the percentage of packets lost, plus the minimum, average, and maximum round-trip times of the packets.
NSLOOKUP is an application that facilitates looking up hostnames on the network. It can reveal the IP address of a host or, using the IP address, return the host name. It is very important when troubleshooting problems on a network that you can verify the components of the networking process. Nslookup allows this by revealing details within the infrastructure.
NETSTAT is used to look up the various active connections within a computer. It is helpful to understand what computers or networks you are connected to. This allows you to further investigate problems. One host may be responding well but another may be less responsive.
This is a Microsoft windows NT, 2000 command. It is very useful in determining what could be wrong with a network. This command when used with the /all switch, reveal enormous amounts of troubleshooting information within the system.
Network Operating Systems
A networking operating system designed using a Directory to manage certain resources. NT's primary file system is NTFS. Provides an inherently GUI console at the server. Clients - Windows NT Workstation best serves Windows NT Server because of the common NTFS file system and they are optimized to work best with each other. However, Windows95/98, Windows for Workgroups, DOS, UNIX, Macintosh, and even NetWare clients can be connected to a Windows NT environment.
A product in Microsoft's Windows line of operating systems. There are four versions of Windows 2000: Professional -- an operating system for business desktop and laptop systems. It is used to run software applications, connect to Internet and intranet sites, and access files, printers, and network resources. Server - both a Web server and an office server. Windows 2000 Server lets users build Web applications and connect to the Internet. Advanced Server - an operating for line-of-business applications and e-commerce. It contains all the functionality of the standard version of Windows 2000 Server, plus additional features for applications that require higher levels of scalability and availability. Data Center Server - developed to work in high-traffic computer networks, it is designed for enterprises that need reliable high-end drivers and software. It supports up to 32-way SMP and up to 64 GB of physical memory.
Windows XP is the newest operating system from Microsoft. The release of XP means that all the desktop versions are now built on the Windows NT/2000 code base (rather than the shakier foundation of Windows 95/98/ME). This has vastly simplified the range, as well as bringing the stability of this code base to home users for the first time. For anyone who runs Windows 3.1, 95, 98 or ME, it is strongly recommended as the benefits of XP will be huge. XP also has "remote" technology, taken from Microsoft's Terminal Server technology, with variations of it being included in both Home and Professional editions. The user can allow a remote helper to view their desktop, or optionally gain control of the keyboard and mouse, in order to troubleshoot a problem. Windows XP comes in two version, Home and Professional. The company has focused on mobility for both editions, including plug and play features for connecting to wireless networks. The operating system also utilizes the 802.11x wireless security standard.
A networking operating system designed using a bindery or Directory Service to manage most resources. NetWare’s primary file system is a combination of FAT (File Allocation Table) and DET (Directory Entry Table). Provides an inherently text based and command prompt console at the server.Novell NetWare works well with most popular clients such as DOS, Windows 3.11, Windows 9x, Windows NT Workstation.
UNIX is a command line operating system written in the C programming language. GUI interface can be achieved by installing special software such as X-Windows. Used mainly in a multi-user environment on minicomputers. Several different version available and allows a great deal of flexibility when performing network operations. Many UNIX protocols are the standard for today’s Internet.
A freely-distributable open source implementation of UNIX that runs on a number of hardware platforms, including Intel and Motorola microprocessors. It was developed mainly by Linus Torvalds. Because it's free, and because it runs on many platforms, including PCs, Macintoshes and Amigas, Linux has become extremely popular over the last couple years. Linux is an extremely powerful Unix operating system that is completely free. It has all the features of commercial operating systems including true multitasking, virtual memory, shared libraries, proper memory management and TCP/IP networking. It runs on many different processors including Intel x86, Motorola 68k series (Amiga and Atari), DEC Alpha, Sun Sparc, Mips and Motorola PowerPC.
Solaris is a multiuser, multitasking operating systems developed and sold by Sun Microsystems and it is one implementation of the UNIX operating system that draws on both the SystemV (AT&T) and Berkeley (BSD) systems. Its an extremely powerful enterprise wide Network operating system having inherent support of mobile computing, clustering technology and security innovations such as Kerberos V5, IP Sec as well as inclusion of IPv6.
Maintaining Your Network
In order to secure a system against loss of valuable data, establish some sort of fault tolerance program. This program will allow recovery of data should there be a disk failure. RAID (Redundant Array of Inexpensive Disks) is a method of disk configuration that will assist in this goal.
Level 0-Disk Striping
Level 1- Disk Mirroring
Level 1- Disk Duplexing
Level 2 Disk Striping w/ ECC
Level 3 Disk Striping w/ ECC stored as parity
Level 4 Disk Striping with large blocks
Level 5 Disk Striping with parity
Distributes data and parity information across all disks in the array. The data and the parity information are arranged so they are always on separate disks. A parity stripe block exists for each row across the disk. The parity stripe is used for disk reconstruction in case of a failed disk. Supports a minimum of three disks and a maximum of thirty-two disks.
Types of Backups
- Normal - Saves files and folders and shows they were backed up by clearing the archive bit.
- Copy - Saves files and folders without clearing the archive bit.
- Incremental - Saves files and folders that have been modified since the last backup. The archive bit is cleared.
- Differential - Saves files and folders that have been modified since the last backup. The archive bit is not cleared.
- Daily - Saves files and folders that have been changed that day. The archive bit is not cleared.
- Room conditions - It's important to setup the room with normal humidity to prevent electrostatic discharge (ESD). Air conditioning should be used to prevent the CPU from overheating. Be sure to put the equipment in a secured room to prevent someone from tampering with unsupervised equipment during off hours.
- Building contents and personal effects - Consider the effects of heat on electrical signals, electromagnetic interference (EMI) from power lines or unshielded power cables as well as TV and radio interference. A common source of EMI is fluorescent lights, elevator motors, large generators, and refrigerator magnets.
- Computer equipment- Computer equipment can affect the unshielded data cables with electromagnetic interference (EMI), such as monitor radiation or CPU power supplies. If the computer equipment is faulty then the network components may appear to have problems.
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