Local Area Networks (LANs) SMU CSE 5344 / 7344 1
LAN/MAN Technology Factors • Topology • Transmission Medium • Medium Access Control Techniques SMU CSE 5344 / 7344 2
Topologies Topology: the shape of a communication system • Most popular topologies for LAN: – Star – Ring – Tree – Bus • Logical topology vs. Physical topology – Logical topology: The way data passes through the network – Physical topology: physical structure of the network SMU CSE 5344 / 7344 3
Star Topology • Central component of star network is called a hub • Separate connections to the hub • More expensive than linear topology because • of cost of concentrators • more cables SMU CSE 5344 / 7344 4
Star Topology in Practice • Parallel cables feeding from the hub • Look like this SMU CSE 5344 / 7344 5
Ring Topology • Computers are connected in a closed loop • Connections go directly from one computer to another • First passes data to second, second passes data to third, etc • Ring ease synchronization; may be disabled if any cable is cut • IBM token ring implementation. A token is passed around • May be disabled if any cable is cut SMU CSE 5344 / 7344 6
Tree Topology • Point-to-point wiring for individual segments • Common backbone • Overall length of each segment is limited by the type of cabling used • If the backbone line breaks, the entire segment goes down • More difficult to configure and wire than other topologies. SMU CSE 5344 / 7344 7
Bus Topology Shared cable • Each computer has its own connection to the shared cable • Shared medium forms the backbone of the network • Synchronization – only one computer transmits at a time • Bus requires fewer cables; may be disable if main cable is cut SMU CSE 5344 / 7344 8
Choice of Topology • Reliability • Expandability • Performance SMU CSE 5344 / 7344 9
LAN Operations • LAN properties • Control layer – managing bits • Communication layer – getting attention • Accommodating multiple access SMU CSE 5344 / 7344 10
LAN Architecture Properties • Data transmitted as addressed frames • No routing required Necessary OSI Layers – Layer 1 - Physical layer – Layer 2 - Data link layer – Layer 3 – ? SMU CSE 5344 / 7344 11
LAN Operations • LAN properties • Control layer – managing bits • Communication layer – getting attention • Accommodating multiple access SMU CSE 5344 / 7344 12
Functions of LAN Protocol Layers • Highest Level – Provide one or more SAPs – Assemble data into frames, with address and CRC fields – On reception, disassemble frame, perform address recognition and CRC validation – Govern link access (e.g., CAC) SMU CSE 5344 / 7344 13
Protocol Layers (cont’d) • Physical Layer – Encode/decode signals – Bit transmission/reception – Modulation PLCP Sublayer PHY layer PHY Management PMD Sublayer DSSS FH IR OFDM Physical Layer Convergence Procedure (PLCP) Direct Sequence Spread Spectrum Frequency Hoping Physical Medium Dependent (PMD) sub-layers. SMU CSE 5344 / 7344 14
MAC Frame Format • MAC control - information such as priority • Destination MAC address • Source MAC address LLC MAC Mgmt MAC • LLC data Service Service • CRC (Frame Check Sequence field) Interface Interface MAC MAC Layer LLC sublayer Management WEP MAC PHY Service MAC Mgmt Interface PLCP Sublayer PHY layer PHY Management PMD Sublayer DSSS FH IR OFDM SMU CSE 5344 / 7344 15
Logical Link Control • Specifies addressing method and controls data exchange • Independent of topology, medium, and medium access control • Unacknowledged connectionless service – higher layers handle error/flow control, or simple apps • Acknowledged connectionless service – no prior connection necessary • Connection-mode service – devices without higher-level software SMU CSE 5344 / 7344 16
Medium Access Control • LLC frames data, • Provides a means of passes it to MAC controlling access which frames it to a shared medium again • Two techniques in – MAC control wide use (e.g. priority level) – CSMA/CD – Destination physical address – Token passing – Source physical address SMU CSE 5344 / 7344 17
LAN Operations • LAN properties • Control layer – managing bits • Communication layer – getting attention • Accommodating multiple access SMU CSE 5344 / 7344 18
Overview of MAC Protocols How do you access a shared media? – Channel Partitioning, by time, frequency or code • Time Division, Code Division, Frequency Division – Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD – “Taking-turns” • polling • token passing SMU CSE 5344 / 7344 19
(Pure) Aloha • Station sends a frame whenever it has one • Waits for a time equal to the round-trip (RTT) for the signal • If the station does not receive an acknowledgment by then, resend the frame • Channel utilization very poor (18%) SMU CSE 5344 / 7344 20
Pure ALOHA Success (S), Collision (C), Empty (E) slots In pure ALOHA, frames are transmitted at completely arbitrary times. SMU CSE 5344 / 7344 21
Pure ALOHA Vulnerable period for the shaded frame. SMU CSE 5344 / 7344 22
Slotted Aloha • The stations synchronize using a central clock transmission time divided into equal slots • Stations are allowed to transmit only at the beginning of the slot • Improved channel utilization (37%) due to reduced conflict time SMU CSE 5344 / 7344 23
Efficiency of Aloha S = throughput = “goodput” 0.4 (success rate) 0.3 Slotted Aloha 0.2 0.1 Pure Aloha 1.5 0.5 1.0 2.0 G = offered load = Np SMU CSE 5344 / 7344 24
Dynamics of Aloha: Effects of Fixed Probability as We Vary the Number of Active Users desirable stable point successful transmission rate new arrival rate undesirable stable point m 0 n : number of Lesson: if we fix p, as N varies: 1) the efficiency is low; backlogged SMU CSE 5344 / 7344 25 2) may have an undesirable stable point stations
Summary: Problems of Aloha Protocols • Low efficiency – Pure Aloha – Slotted Aloha • Undesirable steady state at a fixed transmission rate, when the number of backlogged stations varies Need a better access protocol SMU CSE 5344 / 7344 26
Carrier Sense Multiple Access (CSMA) Based on the observation that signal propagation delay is much smaller than the transmission time • Tp : signal propagation delay = distance/signal velocity • signal velocity • ≈ 3 * 108 m/s: free space, optical fiber (300m/us) • ≈ 2 * 10 8 m/s: copper medium (200m/us) • TTx : transmission delay = N/R • N=number of bits per frame • R = bit rate • time to generate bit stream • determined by data rate & frame length SMU CSE 5344 / 7344 27
Carrier Sense Multiple Access (CSMA) • Listen before you talk – don’t interrupt • If the channel is free send the frame and wait for the acknowledgment • If the channel is busy – Non-persistent retry – 1-persistent retry - most popular – p-persistent retry SMU CSE 5344 / 7344 28
Carrier Sense Multiple Access (CSMA) • non-persistent CSMA – On finding channel busy, station backs-off for a random amount of time and tries later • 1-persistent CSMA – On finding channel busy, station continues listening and transmits when channel becomes idle • p-persistent CSMA – On finding channel idle, station transmits with a probability of p, backs-off and tries again when channel is busy SMU CSE 5344 / 7344 29
CSMA Collisions spatial layout of nodes along Ethernet Collisions can occur: propagation delay means two nodes may not hear each other’s transmission Collision: • entire packet transmission • time wasted • still not very efficient! SMU CSE 5344 / 7344 30
CSMA/CD (Ethernet) • Extension of CSMA – polite conversation – collisions detected within short time • Listen even after transmission has started • If a collision is detected during transmission, – cease transmission – reduces channel wastage – wait a random amount of time SMU CSE 5344 / 7344 31
CSMA/CD How long to wait for collision detection? SMU CSE 5344 / 7344 32
CSMA/CD Collision Detection instead of wasting the whole packet transmission time, abort after detection. SMU CSE 5344 / 7344 33
Binary Exponential Backoff • If colliding for the first time, wait 0 or 1 time slots (random) • Second time wait 0, 1, 2, or 3 slots • Third time wait anywhere from 0-7 slots • After n collisions wait anywhere from 0- 2^n –1 give up after 16 SMU CSE 5344 / 7344 34
CSMA/CD Collision detection: – easy in wired LANs: measure signal strengths, compare transmitted, received signals – difficult in wireless LANs: receiver shut off while transmitting SMU CSE 5344 / 7344 35
Persistent and Nonpersistent CSMA SMU CSE 5344 / 7344 36
Collision-free Protocols Collision-free protocols: – Assume a fixed number of stations (N) each with a unique address 0..N-1 wired into hardware. – Uses contention slots where stations can broadcast their intent to transmit SMU CSE 5344 / 7344 37
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