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