CHAPTER 7

ELEMENTS OF NETWORK CONNECTIVITY

Lesson 1: Connectivity Devices . . . …… 268

Lesson 2: Connection Services . ………. 297

Introduction

This chapter discusses the uses of modems, repeaters, bridges, routers, brouters and gateways

and concludes with a look at remote access computing. It will discuss how you can expand

a network across the street, or around the world.

Lesson 1: Connectivity Devices

These devices will begin with the most basic of communication device the modem. Modems

are in most computers you purchase today, plus if you connect to the Internet or use a FAX

machine, you are using a modem. Devices that expand a LANs include repeaters, bridges,

routers, brouters, and gateways.

Modem Technology

A modem is a device that makes it possible for computers to communicate over a telephone line,

plain and simple as that. When computers are too far apart to be joined by a standard computer

cable, a modem can enable communication between them. Modems are cheap and plentiful, but

too slow and unreliable for long-term solutions, a faster dedicated line will work better.

Basic Modem Functions

Computers cannot simply be connected to each other over a telephone line, because computer

communicate by sending digital electronic pulses, and a telephone line can send only analog

waves.

A digital signal has a binary form. The signal can have a value of either 0 or 1. An Analog signal

can be pictured as a smooth rounded curvy line that can represent an infinite range of values. A

modem at the sending end converts the computer’s digital signals into analog waves and transmits

the analog waves onto the telephone line. A modem at the receiving end converts the incoming

analog signals back into digital signals for the receiving computer.

In other words, a sending modem Modulates digital signals into analog signals, and a receiving

modem Demodulates the analog signals back into digital signals.

NOTE: To use digital lines, you must install a special digital card in the computer.

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Modem Hardware

Modems are known as DCE, Data Communications Equipment and share the following characteristics:

A serial (RS-232) communication interface

An RJ-11 telephone-line (a four-wire telephone plug)

Modems are can be external or internal. An internal modem is more common, and a better way to go.

An internal modem, is installed in a computer’s expansion slot like any other circuit board. An external

modem, is a small box that is connected to the computer by a serial (RS-232) cable running from the

computer’s serial port RJ11C connector to connect to the wall.

DTE stands for Data Terminal Equipment.

**** C:\DEMOS, 1-7 EXCELLENT ***

**** REVIEW A FEW TIMES ****

Modem Standards

Modems must be standardized so they can be compatible. Below are some of the common industry

standards:

1. Hayes-Compatible – In the early 1980’s this company developed the Hayes

Smartmodem. All modems were measured against

standards for the Hayes. Early Hayes modems sent

data at 300 bits per second. Hayes modems were

compatible, very good but very expensive. Hayes

Standards are still used today, but Hayes are now out of

Business. Other manufacturers currently offer speeds

of up to 56,600 bps or more.

2. International Standards -- The ITU International Telecommunications Union,

developed the standards for modems. They called them

the V series.

Modem Performance

Bps and Baud rate are not the same. Baud rate is the speed at which the sound waves that carries

a bit of data over the telephone lines oscillates. Baud rate does not apply as much any more

because modems have increased their speed, so much.

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CCITT changed to ITU, meets every 4 years to discuss standards for Modem Compressions. Each large

company usually have a representative on the panel. For example, IBM, Hewlett Packard, Microsoft etc.

Types of Modems

Just for information basis, the high pitch sounds you hear on a modem before it connects is the negotiating,

the sending and receiving are communication to verify that they are sending the information in the same manner.

Modems send data at 100 bits/second, quite slow, whereas a computer in the classroom can send data at

100 bytes/second.

The two types of communications methods for modems are Asynchronous and Synchronous.

(Async) is the most commonly used, because it uses regular telephone lines. Modems that send the data using

Async uses about 20% as overhead.

Below is a sample of an Asynchronous serial data stream:

BYTE N

BYTE 3

BYTE 2

BYTE 1

Both the sending and receiving devices must agree on the start and stop bit sequence. The receiving

computer uses the start and stop bits markers to schedule its timing functions so it is ready to receive

the next byte of data.

Communication is not synchronized. It is simple, the sending computer just send the data, and the

receiving computer just receives the data. The receiving computer then checks the data to ensure it

matches what was sent. Asnyc is cheaper and more common than sync.

Async transmission over telephone lines can happen at up to 28,800 bps., however if you boost the

signal it can reach 115,200 bps.

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Error Control

Because of the potential for error, async can include a special bit called a parity bit, which is used in

an error-checking and correction scheme called parity checking. The number of bits sent must match

the number of bits received.

Improving Transmission Performance

1) Signaling or channel speed describes how fast the bits are encoded onto the

Communication channel.

2) Throughput measures the amount of useful information going across the channel.

NOTE: Although compressing data can improve performance, it is not an exact science. A text

file for example can be compressed more effectively than a complex graphic file. It is even possible

to have a compressed file that is actually larger than the original.

Coordinating the Standards

Async, or serial modems are less expensive than synchronous modems because the async modem

does not need the circuitry and the components to handle the timing involved in synchronous

transmission that synchronous modems require.

Synchronous Communication (usually only on Mainframe)

Relies on a timing scheme coordinated between two devices to separate groups of bits and transmit

them in blocks known as frames. Because the transmission of the frames is timed, start/stop bits are

not required. The transmission stops at the end of one frame and starts again with a new one. This

is more efficient than async. especially if the packets are large. When small packets are sent, this

increase in efficiency is less noticeable.

BYTE N

BYTE 3

BYTE 2

BYTE 1

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Synchronous

Synchronous protocols perform a number of jobs that asynchronous protocols do not:

1) format data into blocks

2) add control Information

3) Check the information to provide error control

The Primary protocols in synchronous communication are:

1) Synchronous Data Link Control (SDLC)

2) High-level Data Link Control (HDLC)

3) Binary Synchronous Communications Protocol (bisync)

Note: Synchronous communication is used in almost all digital and network communications. For

example if you were using digital lines to connect remote computers, you would use synchronous

modems rather than asynchronous modems to connect the computer to the digital line. Generally,

their high cost and complexity have kept synchronous modems out of the home market. This form

is very uncommon, unless you are working on a Mainframe. Also, if one of the clocks it out it will

not work. To Perfectionalist.!!!

Asymmetric Digital Subscriber Line (ADSL)

This technology converts the existing twisted-pair telephone lines into access paths for multimedia

and high-speed data communications. These new connections can transmit more than 8Mbps to

the subscriber and up to 1 Mbps from the subscriber.

This technology needs special hardware, including an ADSL modem on each end of the connection.

It requires broadband cabling, which is only use in a few locations, and there is a limit into the

connection length.

NOTE: ADSL is known as a physical layer transmission protocol for unshielded twisted-pair media.

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Expanding a Network Using Components

As companies grow, so do the needs of the current LANS. You know your LAN is too small when:

1) The cable begins to get crowded with network traffic.

2) Print jobs include longer wait times

3) Traffic-generating applications, such as databases, experience increased response times.

Each topology has its limits, but there are some components you can add to increase

the size of the network within its existing environment.

1) Segment existing LANs so that each segment becomes its own LAN

2) Join two separate LANs.

3) Connect to other LAN’s and computing environments to join them into a

Larger comprehensive WAN.

The components that enable engineers to accomplish these goals are:

Hubs Routers

Repeaters Brouters

Bridges Gateways

HUBS

A hub can be used to expand a LAN, but it cannot be used to expand a LAN to a WAN.

Connecting a Hub to a LAN can effectively increase the number of workstations. Hubs cannot

switch between cable types or access methods. A hug cannot convert from a LAN to A WAN.

NOTE: It is important to be careful when connecting hubs. Crossover cables are wired differently

than standard patch cables, and one will not work correctly in place of the other. Crossover cables

are used to connect 2 computers and not 2 hubs.

REPEATERS

As signals travel along a cable, they degrade and become distorted in the process called attenuation.

If a repeater is installed it can in a way boost the signal as it travels farther. Repeater boosts the

packet, works at the physical layer of the OSI model. It does not filter the data, sends everything

good and bad. A repeater must be using the same access methods. For example, it cannot go

from Ethernet to Token Ring. But, it can adapt to different cable types, it can go from coaxial to

10 Base T Twisted-Pair.

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NOTE: The following does not use up the signal, Patch Cables/Extender/Barrel Connection. This will

extend a LAN, but it is not a good long-term solution.

How they work:

A repeater works at the physical layer of the OSI reference model, and it regenerates the network’s

signal and resend them out on other segments.

For a repeater to work, both segments that the repeater joins must use the same

Access method. The two most common access methods are CSMA/CD and token

Passing. A repeater cannot connect to a segment using CSMA/CD to a segment

using the token-passing access method. In other words the repeater cannot

translate an Ethernet packet into a Token Ring packet.

Some considerations, repeaters are the cheapest way to expand a Network. When

the need arises to extend the physical network beyond its distance or node limit-

ations, consider using a repeater to link segments if neither segment is generating

much traffic or limiting costs is a major consideration.

Repeaters send every bit of data from one cable segment to another, even if the data

Consists of malformed packets or packets not destined for use on the network.

That means that problems can spread through the network from the repeater, much

as a virus spreads.

Repeaters will also pass a broadcast storm along from one segment to the next,

back and forth along the network. Broadcast storms overload the bandwidth limit.

This will really slow down the network.

When to use a Repeater:

1) Connect two segments of similar or dissimilar media.

2) Regenerate the signal to increase the distance transmitted

3) Pass all traffic in both directions.

4) Connect two segments in the most cost-effective manner

NOTE: SUMMARY – Repeaters improve the networks performance by dividing

the network into segments, thereby reducing the number of computers

per segment. Don’t forget the 5-4-3 rule.

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DO NOT USE A REPEATER IF:

There is heavy network traffic

Segments are using different access methods

Data filtering is needed.

BRIDGES

Like a repeater, a bridge can join segments or workgroup LAN’s. If you have a problem area on

the LAN, you can separate those computers by using a Bridge. This is what Bridges can be used for:

1) Expand the length or a segment

2) Allow for an increase number of computers on the network

3) Reduce traffic bottlenecks, where computers are excessively attached.

4) Divide an overloaded network into two networks.

5) Link unlike physical media such as twisted-pair and coaxial Ethernet.

How Bridges Work

Because bridges work at the data-link layer of the OSI model, all info at the higher levels is not

available to them.

A MAC-layer Bridge:

1) Listens to all traffic

2) Checks the source and destination address of each packet

3) Builds a routing table, as information becomes available

4) Forwards packets in the following way:

a) If the destination is not on the table, the bridge forwards the packet to all

Segments

b) If the destination is on the table, the bridge forwards the packet to that

Segment.

A Bridge works on the principle that each network node has its own address. A bridge forwards

packets based on the address of the destination node.

Bridges actually have some degree of intelligence, in that they learn where to send data. The bridge

uses the computers RAM to store information. The bridge uses the RAM to build a routing table

based on source addresses. At the start the Bridges routing table will be empty, but as nodes

transmit packets it will start to fill up the table.

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Creating a Routing Table

Bridges build their routing tables based on the addresses of computer that have transmitted data on the

network. Bridges use source addresses, the address of the device that initiates the transmission to

create a routing table.

When the bridge receives a packet, the source address is compared to the routing table. If the

address is notthere it is added. The bridge then compares the destination address with the

routing-table database.

1) If the destination address is in the routing table and is on the same segment as the

source address, the packet is discarded. This filtering helps to reduce network

traffic and isolate segments of the network.

2) If the destination address is in the routing table and not in the same segment

as the source address, the bridge forwards the packet out of the appropriate

port to reach the destination address.

3) If the destination address is not in the routing table, the bridge forwards the

packet to all its ports except the one on which it originated.

SUMMARY If a bridge knows the location of the destination node, it

forwards the packet to it. If it does not know the destination, it

forwards the packet to all segments.

Segmenting Network Traffic

A Bridge can segment traffic because of its routing table. Therefore, bridges can use

routing tables to reduce the traffic on the network by controlling which packets get forwarded to

other segments. This control theory is known as “segmenting network traffic”.

If the Bridge knows the MAC address, it will send the packet if it is in the LAN.

The Bridge sends through a broadcast, and it resides in the Data-Link Layer of the OSI.

A Bridge can extend a Network by 185M, it builds up MAC addresses, and puts them in the table.

The spanning-tree algorithm reads the Bridge Manual upon installation, just a little tid-bit of

information for on the job!!!

Bridges can be used on a dedicated phone line. A bridge rebuilds the packet and repackages it.

There are fewer collisions. The bridge also has the capability of transforming multiple NICs into a

bridge. A bridge is inexpensive, it is more than a hub, but less than a router.

You can have several Bridges in a network.

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Remote Bridges

Bridges can connect networks through the telephone lines. Only one bridge is necessary to link

two cable segments. If you implement two remote bridges connected with synchronous modems

to a dedicated, data-grade telephone line will do this.

Differentiating between Bridges and Repeaters

Bridges work at the higher layer than repeaters. Bridges are more intelligent. This means that

bridges can send data packets over long distances using a variety of long-distance media.

Bridge Considerations

Bridges have the features of repeaters, but can have more nodes. They provide better network

performance than repeaters. Because bridged networks have been divided, fewer computers

compete for available resources on each segment. In otherwords, it you had an Ethernet network

divided by bridges, each segment would have less data collision, there would be less overall traffic

on the network, and the operating system would work more efficiently. Although each network

would be separate, the bridge would pass appropriate traffic between them.

Implementing a Bridge

A bridge can be a stand alone piece of equipment or it can be installed in the server.

Network administrator like to use bridges because they are:

Simple to install and transparent to users

Flexible and adaptable

Relatively inexpensive

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ROUTERS

Routers are used in more complex systems then say a bridge. A router knows the address

of each segment, but can also determine the best path. It is similar to a type if air traffic controller.

A router filters and will not send out bad packets, it will kill them. A router is a better traffic

controller than a bridge, but a little slower. The router resides on the Network layer, and can

bypass routes if there is a faster path, so it is in an essence smart!!!

The router works at the level of the Network layer of the OSI model. This enable them to switch

packets and route them across multiple networks. Routers have access to more information

than bridges, that is why they can make in a sense more decision making than bridges. Router

can provide the following:

1) Filtering and isolating traffic

2) Connecting Network segments

If using RIP:

A router can be a cost saver, because there are less hops, and it finds a cheaper route. Satellite

routes can only be used if regular routes are not available, but don’t forget you usually get charged

for accessing a satellite. NT only supports RIP.

OSPF – Open Shortest Path First, you can advise the router, it is built into the logistics, that it is not

to use for example the satellite route, too expensive. Don’t forget if it is a static RIP run for the hills,

may have to manually enter the router. Also, routers do not use broadcasts.

NONROUTABLE PROTOCOLS

NetBEUI and LAT

How Routers Work

Routers maintain their own routing tables, usually consisting of network addresses, host addresses can

also be kept if the network architecture calls for it. To find out the destination address of an incoming

packet the router does the following:

1) All known network addresses

2) Instructions for connection to other networks

3) The possible path between routers

4) The costs of sending data over those paths

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NOTE: The routing table can be maintained in two different ways, by bridge or by via router. When

the routing table is maintained by a bridge it contains MAC-sublayer addresses for each node, whereas

when maintained by a router contains network numbers. Although Manufacturers still call both a routing

table, it has a different meaning for bridges than it does for router, since there is different information

stored in each type of table.

Routers require specific addresses. They understand only the network numbers that allow them to

communicate with other routers and local NIC addresses. Routers talk to other Routers, but not to

remote computers.

When routers receive packets destined for a remote network, they send them to them to the router

that manages the destination network. In some ways this is an advantage because it means routers can:

1) Segment large networks into smaller ones

2) Acts a safety barriers between segments

3) Prohibits broadcast storms, because broadcasts are not forwarded

Routers can be slower than bridges because they have more things to perform. As packets are passed

from router to router, data-link layer source and destination addresses are stripped off and then

re-created. This allows a router to route a packet form different protocols. For example, it can route

from a TCP/IP Ethernet network to a server on a TCP/IP Token ring Network.

Because routers read only addressed network packets, they do not allow corrupted data to get passed

onto the networks. This prevents the event of passing broadcast storms over the network, so they are a

type of security benefit also.

Routers do not look at the destination node address; they look only at the network address. Routers will

pass information only if the network address is known. The router therefore can control the flow of data,

making it more efficient and quicker than a bridge.

Using the router-addressing scheme, administrators can break one large network into many separate

networks. This reduces traffic on the lines, and will reduce wait time for the users.

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Routable Protocols

Not all protocols are routable, here is a list of routable protocols:

DECnet

Internet Protocol (IP)

Internetwork Packet exchange (IPX)

OSI

Xerox Network System (XNS)

DDP (AppleTalk)

Not routable protocols are:

Local Area Transport Protocol (LAT), a protocol from Digital Equipment Corp.

NetBEUI (Net BIOS Extended User Interface)

Choosing Paths

Unlike bridges, routers can accommodate multiple active paths between LAN segments and choose

among redundant paths. Because routers can link segments that use completely different data

packaging and media-access schemes, there are often several paths available for the router to use.

This means that if one router does not function, the data can still be passed over alternate routes.

A router can also listen to a network to see where it is the busiest. It will use this information, and

send the data on a less congested path. A router decides the path the data packet will follow by

determining the number of hops between internetwork segments. Like bridges, routers build routing

tables and use these in routing algorithms such as the following:

1) OSPF (“open shortest path first”) is a link-state routing algorithm. They control

the routing process and allow routers to respond quickly to changes in the network.

2) RIP (Routing Information Protocol) uses distance-vector algorithms to

Determine routes. TCP/IP and IPX support RIP

3) NetWare Link Services Protocol (NLSP) is a link-state algorithm to be used with

IPX.

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TTL Time to Live, example can be TTL + 1, and TTL + 1, and TTL + 1, therefore now 3 for TTL.

This is used between Routers, and every time a router hops it adds 1 to the TTL to keep track of things.

For example to get to Australia on the Internet you would need only 4 to 5 hops on routers to get there.

Type of Routers

There are two types of routers, static and dynamic. Static routers need an administrator to set up and

configure each routing table, and set out the routes taken. If you accept a job with a static router run

for the hills, lots of manual entering. If you loose the router, you have to manually enter it all. Static

routers can be considered more secure because the administrator specifies each route.

Dynamic routers are designed to discover routes automatically, therefore take little time to setup or

configure. They can filter out incoming addresses. Routers filter better than bridges.

Difference between Bridges and Routers

Bridges and routers can be confusing even for engineers with LAN and WAN experience because

they appear to do the same things: both forward packets between networks and send data across

WAN links.

A question is often asked how do I decide when to use a bridge and when to use a router?

The bridge, which works at the MAC sublayer of the OSI data-link layer sees only a node address.

To be more specific, a bridge looks for a node’s MAC-sublayer address in each packet. If the

bridge recognizes the address, it keeps the packet local or forwards it to the appropriate segment.

If the bridge does not recognize the address it forwards the packet to all segments except the one

through which the packet arrived.

Broadcasting

Bridges send all broadcasts through the network, therefore creating a lot of chatter. Whereas the

router which works at the network layer of OSI takes more information into account than the bridge

does, determining not only what to forward but where to forward it. The router determines which

protocol to use as well. The router is more intelligent, drawing on all of its knowledge to send the

packet the quickest and more efficient route.

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BROUTER

A brouter is a combination of a bridge and a router. A brouter can act as a router for one protocol and

as a bridge for all the others. The brouter uses the MAC IP address. A brouter can router selected

routable protocols, and bridge nonroutable protocols.

GATEWAYS

A gateway is a translator, plain and simple. Gateways can be very slow, they are usually set-up as a

separate computer. Gateways requires RAM, CPU and bandwidth. Gateways must has the same

access methods on each end of the line.

A gateway links two systems that do not use the same:

Communication protocols

Data-formatting structures

Languages

Architecture

Gateways interconnect heterogeneous networks, for example, they can connect Microsoft Windows

NT Server to IBM’s Systems Network Architecture (SNA). Gateways change the format of the

data to make it conform to the application program at the receiving end.

Some gateways use all seven layers of the OSI model, but gateways typically perform protocol

conversion at the application layer.

Lesson 2: Connection Services

Carriers

A modem is useless unless it can communicate with another component. All modem communication

takes place over some kind of communication line or cable.

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The issue is simple: it is difficult and expensive to move data quickly over long distances. The three

factors an administrator must take into account when considering how to implement modem communications are:

Throughput

Distance

Cost

There are two types of modem connections:

1) Dial-up Lines

Are common telephone lines. They are slow, and require users to make a connection for

each communication. Some digital lines can support data transmissions speeds of up to 56 Kbps

using error correction.

2) Leased (Dedicated) Lines

They provide full-time, dedicated connections 24/7. The quality of the line is often higher.

They range in speed from 56Kbps to 45 Mbps or more.

VPNs are Virtual private networks.

RAS (Remote Access Service)

A RAS and a DUN (Dial-up Network) can convert a LAN to a WAN. Because many Internet service

providers use telephone-line access, a RAS server often serves as an Internet interface for its network.

A Windows NT Server allows 256 inbound connections, whereas Windows NT Workstation client

allows only one.

RAS Connections

The physical connection to a RAS server can be made using several different media. These include the following:

PSTN This service is otherwise known as the public telephone system

X.25 This packet-switched network service can be used to make dial-up or

direct connections.

ISDN This service provides high-speed remote access, but at greater cost than dial-up

Connection. An ISDN connection requires an ISDN card in place of a modem.

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RAS Protocols

RAS supports three connection protocols:

SLIP (Serial Line Interface Protocol) does not support dynamic IP addressing or the

NetBEUI or IPX protocols, it cannot encrypt logon information, and it is

Supported only by RAS clients.

PPP (Point-to-Point) is better than SLIP, it supports IPX, NetBEUI, AppleTalk,

and DECnet protocols. It also supports encrypted passwords.

PPTP (Point-to-Point Tunneling) is an essential to VPN technology. PPTP very

secure transmission over TCP/IP networks, it is encrypted. This enables

highly private network links over the public Internet.

WANs OVERVIEW

Most WANs are combinations of LANs and other types of communication components connected

by communication links called WAN links. WAN links include:

Packet-switching networks

Fiber-optic cable

Microwave transmitters

Satellite links

Cable television coaxial systems (broadband uses analog)

Communications between LANs will involve one of the following transmission technologies:

Analog Uses (PSTN) Public Switched Telephone Network

Digital Used for voice-grade communications, will be using ADSL,

technology is coming in the future.

Packet Switching

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Digital Connectivity

When organizations require fast, secure transmissions they turn to DDS or Digital Data Service Lines.

DDS provides point-to-point sync communications at 2.4, 4.8, 9.7 or 56 Kbps. Point-to-Point digital

circuits are dedicated circuits that are provided by several telecommunications carriers. The carrier

guarantees full-duplex bandwidth by setting up a permanent link from each endpoint to the LAN.

The main advantage of digital lines are they provide 99.99999% error free transmissions.

Digital lines are available in DDS, T1, T3, T4 and switched 56.

Because DDS uses digital communication, it does not require modems. Instead, DDS sends data from a

bridge or router through a device called a Channel Service Unit/Data Service Unit (CSU/DSU). This

device converts the standard digital signals that the computer generates into the type of digital signals

(bipolar) that are put on the sync communication environment. It also contains electronics to protect

the DDS service provider’s network.

For higher digital data speeds, T1 service is commonly used. It is Point-to-Point transmission, that uses

two-wire pairs to transmit a full-duplex signal at a rate of 1.544 Mbps. T1 transmits digital voice, data

and video signals.

Multiplexing (Muxing), for a diskless workstation you send to a MUX.

T1 uses technology called multiplexing. Several signals from different sources are collected into a

component called a multiplexer and fed into one cable for transmission.

A T1 cable can carry 1.544 megabits of data per second. T1 has 23 B Channels =1.54Mbps.

Provide voice and data-grade service from 6 Mbps to 45 Mbps. These offer the highest-capacity

leased-line service available today.

Switch 56

Works if you have a LAN to LAN connection up to 56 Kbps. You can set your usage time, for

example 9-5 M-F. Each computer using this service must be equipped with a CSU/DSU

(Channel Service Unit/Data Service Unit) that can dial up another Switched 56 site.

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Virtual Circuits

Many packet-switching networks use Virtual Circuits. SVC or Switched Virtual

Circuits, the end computers use a specific route. PVC Permanent virtual Circuits,

Are similar to leased line, except the customer only pays for the pays for time used.

Sending Data across a WAN

As an administrator you can use these technologies to send data:

X.25 (guaranteed delivery, using a PAD, but very slow, checks at each NODE)

Frame relay (Uses PTP)

ATM (Asynchronous Transfer Mode)

ISDN (Integrated Services Digital Network)

FDDI (Fiber Distributed Data Interface)

SONET (Synchronous Optical Network)

SMDS (Switched Multimegabit Data Service)

FDDI

Fiber Distributed Data Interface, is high speed token-ring network that uses fiber-optic.

There are two rings one is redundant, or a back-up. The main or primary ring is used most of the

time. The token takes as many packets as it can hold, and then it goes an transmits them

separately. You need a FDDI router to run a FDDI system.

FDDI (Hub) can be set-up as a star (star-wired ring)

FDDI hardware and software, Windows NT2000 does not come with software. The contractor

would have to supply the H/W and software, and you would have to get a

reputable subcontractor for the fiber-optic cable installation.

FDDI uses beaconing, if a cable is removed it sends out beacons to see where the break is in the

cable. Always have at least 3 metres of extra cable at each end of the network.

SONET

Sonet is at the physical layer of the OSI.

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Page 322 Case Study 7.

1) Check the users, ensure that all users are down, not just one.

2) Check all local H/W cables, before calling in service people. Check your

Hubs.

3) Call your other LAN, see it they are up and running.

4) Call support if all looks okay on your end, see if a part of the line

is down.

NetBEUI

CPU