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LAN / WAN / Extranet

and Network Topology

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Motivation

Local Area Networks (LAN) were motivated by:– Decreasing computer size– Decreasing computer cost– Realizing computers could help with

many tasks

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Interchangeable Media

The first data transfers:– Used:

• Magnetic tapes• Disks

Data transferred between computers in a method similar to using floppy disks.

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LAN Generations First

– CSMA/CD and token ring– Terminal to host and client server– Moderate data rates

Second– FDDI– Backbone– High performance workstations

Third– ATM– Aggregate throughput and real time support

for multimedia applications

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Third Generation LANs

Support for multiple guaranteed classes of service– Live video may need 2Mbps– File transfer can use background class

Scalable throughput– Both aggregate and per host

Facilitate LAN/WAN internetworking

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LAN technologies MAC protocols used in LANs, to control access to the

channel Token Rings: IEEE 802.5 (IBM token ring), for computer

room, or Department connectivity, up to 16Mbps; FDDI (Fiber Distributed Data Interface), for Campus and Metro connectivity, up to 200 stations, at 100Mbps.

Ethernets: employ the CSMA/CD protocol; 10Mbps (IEEE 802.3), Fast E-net (100Mbps), Giga E-net (1,000 Mbps); by far the most popular LAN technology

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A Computer Consists Of Circuit Boards Inside a computer are electronic

components on circuit boards.– Containing electronic components – Containing wires

Computers having different circuit boards for external devices.

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Circuit Boards Plug Into A Computer

Computers are built so it contains a set of sockets.– Using wires to connect sockets

together– Using wires to carry power and data– Plugging circuit boards into sockets to

control external devices

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Illustrations of the components visible in a computer when the cover has been removed. A circuit board can plug into each socket; wires connect the sockets to other components.

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Connecting Computers In Early Systems Transferring data between two

computers consisted of two circuit boards connected by a cable.

Figure 7.2 Illustration of an early computer communication system formed using two circuit boards plugged into sockets in two computers.

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Early Systems The computers use cables to

transfer data electronically.– Operating like an I/O device– Writing data to circuit board

Figure 7.3 Two pairs of interface boards connecting three computers. Each new computer added to the set requires a new pair of interface boards and an additional cable.

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Early Systems

Advantage of early LANs were speed. Disadvantages of early LANs were

inconvenience and cost. Requiring effort to:– Add a new computer– Connect incompatible hardware

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Connecting A Computer to A LAN

A computer needs additional hardware to connect it to a LAN.

The speed of the LAN does not depend on the speed of the computer attached to it.– Communication by heterogeneous

computers

14Computers connected to a LAN. Each computer attaches to the hub with a cable; the computers can then communicate directly.

• In many LAN systems, a cable connects each computer to a hub.

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NIC A computer needs network interface

hardware and a cable that connects to the LAN.

A computer uses the network interface to send and receive data.

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The Importance Of LAN Technology

LANs changed the way people used computer networks.– Sharing resources– Connecting machines within a

building

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Relationship To The Internet Xerox gave universities a

prototype of a new LAN technology.– Beginning of Ethernet– Developing the idea of inexpensive

and widely available LANs

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Many Independent Networks By late 1970s, many organizations

began installing Local Area Networks because they:– Were inexpensive.– Were easy to install– Could operate them independently of a

central administration.

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The Proliferation of LANs

Advantages– An organization

can:• budget funds • decide who has

access• devise policies for

use

Disadvantages– Independent groups

can:• Encourage

proliferation of different LAN technologies

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Engineers have devised many LAN technologies

LAN performance determines cost. LAN technology may only work

with specific computers.

Facts About LANs

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LANs Are Incompatible

Various LAN technologies are completely incompatible.– Connecting multiple LANs is not possible

• Engineered to operate over limited distance• May be electrically incompatible• Encoding information may not make sense to

another LAN

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IEEE802.3 Medium Access Control Random Access

– Stations access medium randomly Contention

– Stations content for time on medium

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IEEE 802.3 Frame Format

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10Mbps Specification (Ethernet) <data rate><Signaling method><Max segment

length>

10Base5 10Base2 10Base-T 10Base-FP

Medium Coaxial Coaxial UTP 850nm fiber

Signaling Baseband Baseband BasebandManchester

Manchester Manchester Manchester On/Off Topology Bus Bus Star Star Nodes 100 30 - 33

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100Mbps (Fast Ethernet)

100Base-TX 100Base-FX 100Base-

T4

2 pair, STP 2 pair, Cat 5UTP 2 optical fiber 4 pair, cat 3,4,5

MLT-3 MLT-3 4B5B,NRZI 8B6T,NRZ

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Gigabit Ethernet Configuration

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Gigabit Ethernet - Differences

Carrier extension At least 4096 bit-times long (512

for 10/100) Frame bursting

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Gigabit Ethernet - Physical 1000Base-SX

– Short wavelength, multimode fiber 1000Base-LX

– Long wavelength, Multi or single mode fiber 1000Base-CX

– Copper jumpers <25m, shielded twisted pair

1000Base-T– 4 pairs, cat 5 UTP

Signaling - 8B/10B

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Wide Area Technologies Exist WAN technology includes an

additional special-purpose computer at each site that:– Connects to the transmission lines– Keeps communication independent of

the computer

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Few WANs, Many WANs

WANs cost much more than LANs.– Require more planning– Require more hardware

Only a few companies build their own WAN.

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WANs And LANs Are Incompatible

Many Wide Area Networks and Local Area Networks exist.– Cannot connect a WAN to a LAN– Cannot interconnect the wires from

two different networks

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WANs for Voice

Requires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switching

As a result, the preferred method for voice transmission is circuit-switching

Most businesses use public telephone networks, but a few organizations have implemented private voice networks

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WANs for Data Public packet-switched networks (X.25) Private packet-switched networks Leased lines between sites (non-switched) Public circuit-switched networks Private circuit-switched networks

(interconnected digital PBXs) ISDN (integrated X.25 and traditional circuit-

switching)

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WAN Considerations Nature of traffic

– stream generally works best with dedicated circuits

– bursty better suited to packet-switching Strategic and growth control--limited

with public networks Reliability--greater with packet-

switching Security--greater with private networks

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Wireless LANs IEEE 802.11 Basic service set (cell)

– Set of stations using same MAC protocol– Competing to access shared medium– May be isolated– May connect to backbone via access point

(bridge) Extended service set

– Two or more BSS connected by distributed system

– Appears as single logic LAN to LLC level

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Wireless LAN—links clients within the vicinity of each other.

A network adapter card that is connected to a transmitter, called an access point, via a cable.

The transmitter located on a wall gives the signal an uninterrupted path to a wall-mounted receiver on the far side of the room.

Data packets are transmitted over the airwaves to the receiver, which is also connected to network clients by a cable.

Wireless Extended LAN—connections to clients a couple of miles away.

Similar connectivity is an Extended LAN. Transmitter and receiver are typically located outside

the buildings. Forms an electronic data communication bridge called a

wireless bridge. Data packets up to 25 miles away from the transmitter

use spread spectrum radio technology.

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Wireless LAN - Physical Infrared

– 1Mbps and 2Mbps– Wavelength 850-950nm

Direct sequence spread spectrum– 2.4GHz ISM band– Up to 7 channels– Each 1Mbps or 2Mbps

Frequency hopping spread spectrum– 2.4GHz ISM band– 1Mbps or 2Mbps

Others under development

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Wireless LANs Mobility Flexibility Hard to wire areas Reduced cost of wireless systems Improved performance of wireless

systems

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LAN Extension Buildings with large open areas

– Manufacturing plants– Warehouses

Historical buildings Small offices May be mixed with fixed wiring

system

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Single Cell Wireless LAN

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Multi Cell Wireless LAN

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Client/Server Architecture

Combines advantages of distributed and centralized computing

Cost-effective, achieves economies of scale

Flexible, scalable approach

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Intranets

Uses Internet-based standards & TCP/IP

Content is accessible only to internal

users

A specialized form of client/server

architecture

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Extranets

Similar to intranet, but provides access to controlled number of outside users– Vendors/suppliers– Customers

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There are six topologies used in the design of a computer network: • Bus • Star• Ring• Token Passing• Hubs• Hybrid

Every computer network has the same basic components:

• Cables or wireless connection• Network adapter cards that transmit and receive

information • Client software that makes all these components

work togetherTopology: The way in which components are assembled.

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Topologies Tree Bus

– Special case of tree• One trunk, no branches

Ring Star

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LAN Topologies

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A bus topology requires: • Clients are connected to the same cable known as a trunk, segment, or backbone. • All data packets are received by every client regardless of whether the data packet

is addressed. • Data packets not addressed to the client are ignored.• Data packets addressed to the client are accepted and processed by the client.• Data packets travel the complete length of the cable, then bounce back.• This is called signal bounce and continues until the signal loses energy and

dissipates. • The network operating system has the responsibility to keep the transmission

moving along the cable. • A client can malfunction and the network continues to operate, which is called a

passive topology. • A passive topology is easy to construct, highly reliable, and susceptible to slow

performance during heavy network traffic. • A break in the cable is commonly caused by an improper network connection.• Breaks are hard to track down because every client and device on the network

could be suspect. • A network outage does not shut down a client's operation. Clients work as a stand-

alone.

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Frame Transmission - Bus LAN

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Bus and Tree Multipoint medium Transmission propagates throughout medium Heard by all stations

– Need to identify target station• Each station has unique address

Full duplex connection between station and tap– Allows for transmission and reception

Need to regulate transmission– To avoid collisions– To avoid hogging

• Data in small blocks - frames Terminator absorbs frames at end of medium

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• Clients connect to a central device called a concentrator or hub.• Clients transmit and receive data packets to and from the concentrator.• It is the job of the concentrator to redirect data packets to the appropriate client.• Clients only receive data packets addressed to them. • Star topology reduces the network traffic clients must handle.• The concentrator detects if a client is not connected to the network and returns data

packets to the sender. • Failure of one client does not disable the entire network.• Network services become unavailable if the concentrator malfunctions. • Clients use 10BaseT network adapter cards that automatically detect trouble with

the concentrator. • The client then stops any transmission of data packets and operates as a stand-

alone computer until the concentrator becomes operational.

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Ring Topology Repeaters joined by point to point links in

closed loop– Receive data on one link and retransmit on another– Links unidirectional– Stations attach to repeaters

Data in frames– Circulate past all stations– Destination recognizes address and copies frame– Frame circulates back to source where it is removed

Media access control determines when station can insert frame

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Frame TransmissionRing LAN

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• Each client is connected consecutively to the single cable. • There are no ends to the ring network that must be sealed with a

terminator.• Data packets pass clockwise from one client to the next. • If the data packet isn't addressed to the client, the client resends the

data packet to the next client. • The client strengthens the signal, allowing the data packet to travel a

further distance.• Clients connect to a hub. Within the hub is the ring. • If one client is not working properly, there is a good chance the entire

network will fail. • This depends on the network operating system. • IBM token ring automatically ignores inactive clients.

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• A token ring is a ring-like topology. • Clients are connected together to form a ring.• Each data packet is transmitted to each client on the network. • A special packet, called a token, is used to control transmissions on the

network.• The token packet is delivered to a client clockwise along the network. • The client can accept the token and transmit a data packet. • Addresses, data, and other necessary information are added to the token

packet. • The modified token packet becomes the new data packet and is sent to

the hub for delivery to the destination.• The destination client acknowledges the data packet.• The client that sent the data packet creates a new token packet and

passes it to the next client.• The client can ignore the token packet, which is then passed along to the

next client on the ring. • This is called token passing. • Token packets can travel more than 10,000 times around the network per

second.

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Star Topology Each station connected directly to

central node– Usually via two point to point links

Central node can broadcast– Physical star, logical bus– Only one station can transmit at a

time Central node can act as frame

switch

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Hubs• A hub is a central processing device on a network.• A hub is used with the star and ring topologies as a concentrator for network

traffic. • Hub topology is used to divide large network requirements into smaller,

serviceable LANs called segments. • A hub enables a network administrator to monitor and manage network traffic.• A hub is used to link segments together; cabling used by various clients can be

mixed and matched.

Types of Hubs• Networks use one of two kinds of hubs:

• Passive hubs • Active hubs

Passive Hubs• A passive hub acts as a connector box known as a wiring panel. • A passive hub connects cables from all clients. • A passive hub only provides connectivity among network clients. It does not

interrupt transmissions on the network.

Active Hubs• An active hub ID known as a multi-port repeater.• An active hub joins together clients. • An active hub boosts the signal along the network like a repeater.• An active hub can, in some cases, redirect data packets to the appropriate client.

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Hybrid Topology• The disadvantage of each topology is reduced by combining topologies.• The combined topology is called a hybrid network. • There are two common hybrid networks: star bus and star ring topology.

Star Bus Topology• The star bus topology combines the star topology and the bus topology. • Clients of the smaller networks use the star topology to connect to its network

concentrator. • Network concentrators are linked together in a bus topology.• The star bus topology controls traffic flow on the network.• Traffic between concentrators is less demanding than traffic among clients. • The bus topology is a more direct and efficient design for connecting

concentrators.• The star topology avoids transmission conflicts among clients.• The star bus topology enables network expansion. • If network response time is slow because of an increase in traffic, clients can be

transferred to a different segment with its own concentrator.

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• The star ring topology combines the best of the star and ring topologies. • The star ring topology is referred to as the star wired ring topology. • The star ring topology divides the network into smaller networks each

having a concentrator or hub used to connect clients. • Concentrators are joined together using a master concentrator

(sometimes called the main hub) in the form of a ring. • The advantage is that network traffic is handled by a topology that best

suits the volume of traffic. • The network is scalable.• A hybrid network reduces the chance that a complete network failure

will occur. • Only clients directly linked to a concentrator will lose access to network

services if the concentrator malfunctions.

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Question ???