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Chapter 5
LAN, MAN, WAN
BY
Dr.Sukchatri Prasomsuk
Contents
• LAN
• MAN
• WAN
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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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Ethernet
• 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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Ethernet
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Ethernet
Example :
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
• Traditionally coaxial cable, but UTP and even
optic fiber is becoming popular.
• CSMA/CD
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Collisions
• 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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Ethernet
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Ethernet
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Importance 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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Ethernet
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Ethernet
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Ethernet
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Transmission 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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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Ethernet
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Token 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 Ring
• Topology
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Token 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 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 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 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 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 3300Hz)
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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
log2(1+S/N)
• 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 :
10 * 3000 = 30 000 bits per second
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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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Summary
• After completing this chapter, you should be able to
perform the following tasks:
– Describe Network Topology and operations
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Review 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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