CSCI 465 D ata Communications and Networks Lecture 19 Martin van Bommel CSCI 465 Data Communications...

CSCI 465Data Communications and Networks

Lecture 19

Martin van Bommel

CSCI 465Data Communications & Networks

1

Local Area Network (LAN)

• Wide Area Network (WAN)– Cover large geographical area– Can be either public or private

• Local Area Network (LAN)– Cover single building or cluster of buildings– Privately owned– Network assets and network management

typically controlled by user organization


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LAN Key Elements

• Topology– Way in which end points (stations) interconnect

• Transmission medium• Wiring Layout• Protocol• Medium Access Control


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LAN Topologies


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LAN Topology: Bus and Tree

Bus:• Multipoint medium• Stations attach to linear

medium (bus) via tap• Full-duplex between

station and tap• Transmission from any

stations travels entire medium (both directions)

• Terminators required at each end of bus


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Tree:• Generalization of bus• Branching cable with no

closed loops• Cable(s) begin at headend,

travel to branches which may have branches of their own

• Each transmission propagates through network, heard by all stations

Bus and Tree Issues• One station could dominate medium– Frames - Transmit small block at a time

• Two stations transmitting at same time will lead to garbled messages– Have medium access control protocols– Stations monitor control information in frames

• All stations hear all transmissions– Frames include headers with address

• Frames removed by terminators at endsCSCI 465

Data Communications & Networks6

LAN Topology: Ring• Set of repeaters joined by point-to-point links in a

closed loop• Receive data on one link & retransmit on another– links unidirectional– stations attach to repeaters

• Data transmitted in frames– circulate past all stations– destination recognizes address and copies frame– frame circulates back to source where it is removed

• Medium access control– determines when a station can insert frame


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LAN Topology: Star

• Stations connect to common central node– Usually via two point-to-point links– One for transmission and one for reception

• Central Node (Hub)– Operates in broadcast fashion– Physical star – logically a bus– Only one station can transmit at a time– Hub could act as a frame switch • send only to recipient


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Choosing a LAN Topology

• Factors to consider include reliability, flexibility/expandability, and performance

• Bus/tree is most flexible• Tree topology easy to lay out• Ring provides high throughput, but reliability

problems – single failure cripples network• Star can be high speed for short distances, but

has limited expandability – natural layout


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LAN Transmission Media• Unshielded Twisted Pair (UTP)– Early LANs used voice-grade cable– Scaling up for higher data rates not practical

• Shielded Twisted Pair (STP)– Much higher performance

• Baseband coaxial cable– Digital signaling – used in original Ethernet

• Broadband coaxial cable– Used in cable TV systems– Expensive, difficult to install and maintain

• Optical fiber – used in high-speed backbonesCSCI 465


Selecting Transmission Media

• Capacity– Can it support expected traffic?

• Reliability– Can it meet requirements for availability?

• Types of data supported– Is it well-suited to the applications involved?

• Environmental scope– Can it provide service in required environments?


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LAN Protocol Architecture


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IEEE 802 Layers

• Physical Layer– Encoding / decoding of signals– Preamble generation / removal - synchronization– Bit transmission / reception– Transmission medium and topology• Considered below physical layer of OSI model• Critical in LAN design


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IEEE 802 LLC and MAC

• Logical Link Control Layer (LLC)– Provides interface to higher levels– Performs flow and error control

• Medium Access Control (MAC)– Assemble data into frame – address, error control– Disassemble frame

• address recognition and error detection

– Govern access to transmission medium

• For same LLC, several MAC options providedCSCI 465


LAN Protocols in Context


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Logical Link Control (LLC)

• Transmission of link level protocol data units (PDUs) between stations

• Must support multi-access, shared medium– Some details of link access performed by MAC layer

• Addressing – source and destination LLC users– Referred to as service access points (SAPs)


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LLC Services

• Unacknowledged connectionless service– Datagram style service– Delivery of data is not guaranteed• no flow or error control mechanisms

• Connection mode service– Logical connection set up between two users• flow and error control provided

• Acknowledged connectionless service– Datagrams are to be acknowledged


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Medium Access Control (MAC)

• Controls access to the transmission medium• Key parameters– Where• Centralized – greater control, single point of failure• Distributed – more complex, but more redundant

– How• Synchronous – capacity dedicated to connection

– Similar to circuit switching – not optimal in LANs

• Asynchronous – response to demand– Round robin, reservation, or contention


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Asynchronous MAC Systems

• Round robin - each station is given turn to transmit data– Decline or transmit up to pre-defined limit– Waste time polling idle stations

• Reservation - divide time on medium into slots– Stations reserve future slots– Good for stream traffic – lengthy, continuous data

• Contention - all stations contend for time– Good for bursty traffic – short, sporadic data– Simple to implement and efficient under light load– Tend to collapse under heavy load


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MAC Frame Handling

• MAC layer receives data from LLC layer• PDU is referred to as a MAC frame with fields:– MAC control– destination MAC address– source MAC address– LLC – data for next higher layer– CRC - frame check sequence (FCS)

• MAC layer detects errors and discards frames• LLC optionally retransmits unsuccessful frames


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CSCI 465 D ata Communications and Networks Lecture 19 Martin van Bommel CSCI 465 Data Communications...

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