Chapter 5

LAN, MAN, WAN

BY Dr.Sukchatri Prasomsuk

Contents

• LAN• MAN• WAN

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Major local area networks (LAN)

Major local area networks (LAN)

• Ethernet.

• Token Ring.

• Token Bus.

• A key consideration affecting the LAN user is

that its ownership, operation and management

are private.

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EthernetEthernet

• Based on the bus topology.• Access a common medium without requiring a

centralized controller.• Cooperative participation.• Relatively high data transmission. (Meg)• Sharing of expensive peripherals but the major

use is sharing of data.

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EthernetEthernet

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EthernetEthernet

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Example :

EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

• Traditionally coaxial cable, but UTP and even

optic fiber is becoming popular.

• CSMA/CD

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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CollisionsCollisions

• When a collision is detected, a 32-bit jamming code is

transmitted.

• The station “backs off“ or waits a random time before

trying to re-transmit.

• Exponential Back Off.– Slot times.– 16 collisions Max.

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EthernetEthernet

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EthernetEthernet

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Importance of frame lengthImportance of frame length

• IEEE 802.3 standard.• Maximum distance between nodes: 2500m• 12.5 ms transmission time.• 125 bits @ 10 Mbits/s.• Minimum frame size : 512 bits.• Use differential Manchester encoding.

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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Transmission in LANsTransmission in LANs

• Key terms:– Transmitted signal.– Frame length.– Transmission speed.

• Pre-transmission actions.• Actions during transmission.• What happen at a collision.

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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EthernetEthernet

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Token RingToken Ring

• A special frame - the Token - circulates among contenders for access to the medium.

• No contention - there are no collisions.• Deterministic - one can predict when a station will

get access to the medium.• Standard defined by IEEE 802.5 at 4 or 16Mbit/s.

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Token RingToken Ring

• Topology

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Token RingToken Ring

• Send or receive message from its nearest upstream neighbor.

• Each station acts as a repeater.• The last node downstream is the first node

upstream.• There are two types of frames :

– The Token.– The Information Frame.

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Token Ring FramesToken Ring Frames

• The token :– Possible implementation of priorities.– The token is only the activator.

• The Information Frame :– Contains the data.– Upon receiving the token, the sender

generates the information frame and send it.– After receiving the frame, the receiver

generates a token and send it.

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Priority in the ringPriority in the ring

• Token itself carries two fields within it to manage priority.

• Receipt of the token is a permit to transmit provided the priority is high enough.

• If the priority is too low, the “requested” field of the token is amended if its own priority is higher than that of the “requested” field.

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Physical Token Ring AddressesPhysical Token Ring Addresses

• One, 6-byte address per node present in the adapter.

• Three bytes, the first half, are allocated by IEEE and the second half by the adapter’s manufacturer.

• Examples :– IBM 005Ah.– DEC 004Dh.

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Token Passing BusToken Passing Bus

• Very low cost.• Governed by the IEEE 802.4 standard.• 2.5 Mbit/s ARCNET.• The information and token frames are combined

into one.• All stations receive the token at the same time -

only the identified station acts on it, however.• Stations are numbered and the token is passed

in numerical order.

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Why MANs?

• Metropolitan Area Networks evolved primarily to

permit LANs to interconnect across a metropolis

or a country.

• Two major technologies :

– Distributed Queue Dual Bus (DQDB).

– Fiber Distributed Data Interface (FDDI).

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DQDB

• Adopted in 1990 by IEEE 802.6.

• It is a multiplexing method.

• Its medium is not limited to any one type.

• For both isochronous and delay insensitive data.

• The transfer of data messages between stations

on two LANs is handled to appear as if they

were on the same LAN.

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DQDB

• Two busses accessible to every node.• Failure of one node can not incapacitate the

network.• Empty frames are generated by the two sources

annihilated at the sinks.• Different dual bus implementations:

– Open dual bus.– Point-to-point bus.– Looped dual bus.

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How DQDB Works

• Empty frames travel from sources to sinks.• Each node may peek non-destructively at the

frames as they go by.• Empty, non-reserved frames may be used.• To write, a node logically “ORs” its message

with, and replaces the content of the empty frame.

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How DQDB Works

• To reserve a frame on Bus A, a node must capture an empty frame on Bus B and set the reserved flag.

• FIFO priorities are used.• Special frames for isochronous communication

(pre-arbitrated frames every 125 microseconds).• A flag in the header indicates a special frame.

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FDDI

• Standardized by ANSI X3T9.5.• Similar to IEEE 802.5.• Motivation for implementation was not only the

interconnection of LANs but the need for high speed networking.

• Support ring with maximum circumference of 100 km.

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FDDI

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FDDI

• Data transmission speed : 100 Mbits/s.• NRZI coding is used.• Dual counter-rotating rings.• Inner ring is mainly for back-up.• The bit error rate is low : 1 in 1012

• Each station is a repeater.• Self-healing nature.

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Wide Area Networks (WAN)

• To implement a WAN, data communications takes advantage of the existing voice-grade facilities and infra-structure.

• Analog signals are send over copper wire circuits.

• The frequencies of signals in the electrical circuits are bandpass limited. (300-Hz to 3300-Hz)

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Wide Area Networks (WAN)

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Frequency Division Multiplexing.

• Limit the voice channels to 4-kHz bandwidths. (Guard Bands)

• Trunk circuits between central offices is not loaded with bandpass limiting equipment.

• Frequency division multiplexing is used on these lines.

• Many logical connections uses one physical connection.

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Theoretical limit to bits per second

• 1949, Claude Shannon published “The Mathematical Theory of Communications.”

• The information carrying capacity of a communications channel is a product of two factors:– Bandwidth of the channel.– Signal to noise ratio.

• This measure is “Entropy”.

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Entropy

• With a typical signal/noise of 1000, we find the entropy to be about 10.

• Wit a maximum usable bandwidth of about 3000 Hz the info. capacity is :

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log2(1+S/N)

10 * 3000 = 30 000 bits per second

Modems

• Named for its ability to modulate and demodulate a signal.

• A Modem is Data Circuit Terminating Equipment, DCE.

• A PC or terminal is user Data Terminal Equipment, DTE.

• The sending modem sends information as variants of a signal whose constant, predictable properties are known to the receiver, carrier signal. (FSK, PSK)

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CODECs

• Sender Code analog signals into digital signals

and decode the signal at receiver.

• Companding is used to increase quality.

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Extended WAN technologies.

• ISDN : Integrated Service Digital Network.

• SONET : Synchronous Optical Network.

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Frame Relay

• Fast packet switching service.

• Connection oriented.

• Many logical connections over one physical

connection.

• Suitable for interconnecting LANs.

• Variable packet length.

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SummarySummary

• After completing this chapter, you should be able to perform the following tasks:– Describe Network Topology and operations

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Review QuestionsReview Questions

1. What is the smallest size of an Ethernet frame?

2. What is the largest size of an Ethernet frame?

3. Describe the difference between Ethernet and Token Ring topology?

4. What is the CSMA/CD?

5. What is the Token Bus?

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