Local Area Networks (LANs) SMU CSE 5344 / 7344 1 LAN/MAN - - PowerPoint PPT Presentation

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Local Area Networks (LANs)

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LAN/MAN Technology Factors

• Topology
• Transmission Medium
• Medium Access Control Techniques

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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

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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

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Star Topology in Practice

• Parallel cables feeding from the hub
• Look like this

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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

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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.

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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

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Choice of Topology

• Reliability
• Expandability
• Performance

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LAN Operations

• LAN properties
• Control layer – managing bits
• Communication layer – getting

attention

• Accommodating multiple access

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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 – ?

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LAN Operations

• LAN properties
• Control layer – managing bits
• Communication layer – getting

attention

• Accommodating multiple access

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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)

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Protocol Layers (cont’d)

• Physical Layer

– Encode/decode signals – Bit transmission/reception – Modulation

PLCP Sublayer PHY layer Management PMD Sublayer

DSSS FH IR OFDM

PHY

Physical Layer Convergence Procedure (PLCP) Physical Medium Dependent (PMD) sub-layers. Direct Sequence Spread Spectrum Frequency Hoping

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MAC Frame Format

• MAC control - information such as priority
• Destination MAC address
• Source MAC address
• LLC data
• CRC (Frame Check Sequence field)

LLC DSSS FH IR OFDM

PHY MAC

WEP

MAC Mgmt

PLCP Sublayer PHY layer Management PMD Sublayer MAC sublayer MAC Layer Management

PHY Service Interface

LLC

MAC Service Interface MAC Mgmt Service Interface

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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

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Medium Access Control

• Provides a means of

controlling access to a shared medium

• Two techniques in

wide use

– CSMA/CD – Token passing

• LLC frames data,

passes it to MAC which frames it again

– MAC control (e.g. priority level) – Destination physical address – Source physical address

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LAN Operations

• LAN properties
• Control layer – managing bits
• Communication layer – getting

attention

• Accommodating multiple access

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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

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(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%)

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Pure ALOHA

Success (S), Collision (C), Empty (E) slots In pure ALOHA, frames are transmitted at completely arbitrary times.

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Pure ALOHA

Vulnerable period for the shaded frame.

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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

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Efficiency of Aloha

S = throughput = “goodput” (success rate) G = offered load = Np

0.5 1.0 1.5 2.0 0.1 0.2 0.3 0.4

Pure Aloha Slotted Aloha

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Dynamics of Aloha: Effects of Fixed Probability as We Vary the Number of Active Users

Lesson: if we fix p, as N varies: 1) the efficiency is low; 2) may have an undesirable stable point

n: number of backlogged stations

m successful transmission rate new arrival rate desirable stable point undesirable stable point

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Summary: Problems of Aloha Protocols

• Low efficiency

– Pure Aloha – Slotted Aloha

• Undesirable steady state at a fixed

transmission rate, when the number

• f backlogged stations varies

Need a better access protocol

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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

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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

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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

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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!

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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

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CSMA/CD

How long to wait for collision detection?

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CSMA/CD Collision Detection

instead of wasting the whole packet transmission time, abort after detection.

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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

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CSMA/CD

Collision detection:

– easy in wired LANs: measure signal strengths, compare transmitted, received signals – difficult in wireless LANs: receiver shut off while transmitting

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Persistent and Nonpersistent CSMA

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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

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Collision-free Protocols

Bit-map protocol:

– 1 bit per station overhead – Contention (or reservations) slots (bits 0..N-1) followed by actual transmission of data frames (d bits each) – At low load, channel efficiency = d/(d+N), since d data bits are transmitted for every N bits of contention slots. – At high load, channel efficiency = d/(d+1)

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Collision-Free Protocols

Binary Countdown Protocol

• Scheme attempts to eliminate the contention slot overhead

– the 1 bit per station scalability problem by using binary station addressing

• Each station has a unique binary address.

– Stations who wish to contend for a slot transmit their address bit by bit prior to actual data frame transmission. – If a station with a 0-bit contends with a 1-bit, then the station that’ sent “0” stops contention. – This continues until the last address bit is transmitted.

• No collisions as higher-order bit positions are used to

arbitrate between stations wanting to transmit

– Higher numbered stations have a higher priority

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IEEE 802.x Standards

IEEE 802.2:

Logic link control (LLC) layer of data link layer

IEEE 802.3:

Ethernet

IEEE 802.4

Token bus, an old protocol

IEEE 802.5: Token ring IEEE 802.6:

Distributed queue dual bus (DQDB) protocol, similar to FDDI

IEEE 802.9:

Integrated voice and data networking, including ISDN

IEEE 802.11: Wireless LAN IEEE 802.12: 100Base-VG IEEE 802.13: 100Base-X IEEE 802.14: Cable modem

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IEEE Standard 802.3

1-persistent CSMA/CD:

– A station keep listening to the cable until it is idle; – it then starts transmitting its data frame

• the moment it detects a collision, it terminates its

transmission

• after a collision, a station waits for a random time and starts

repeating the above steps

Ethernet is only one product that follows the 802.3

• standard. There could be several variations of this

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IEEE Standard 802.4: Token Bus

Why 802.4? 802.3 does not support priorities, it is not deterministic Token Bus

– physically linear or tree-structured, but logically a ring – Each station has a unique address – Each station knows the address of its left and right neighbors in the logical ring. – The cable is a broadcast medium So logical neighbors are not necessarily physical neighbors. – Each station has several priority queues (0,2,4,6).

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IEEE Standard 802.5: Token Ring

Not a broadcast medium; a physical ring topology

– A token (a special bit pattern) circulates around the ring when all stations are idle

• The length of a bit on the cable

– If the speed of propagation is 200 meters/usec and if the speed

• f transmission is 1 Mbps or 1 bit/usec
• each bit occupies 200 meters on the ring.
• A 1 km ring can hold 5 bits
• Consists of ring interfaces to which stations are connected

– Each bit arriving at an interface is copied into a 1-bit buffer – May be modified and copied out into the ring again – Copying and inspection introduces a 1-bit delay

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DQDB - IEEE 802.6

• Shared medium
• Fixed length packets
• Dual bus
• Designed for metropolitan area networks

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Synchronization and Timing

• Transmission consists of steady stream of fixed

size slots

• Head (A/B) responsible for generating the slots

for bus A/B

• Operation controlled by a 125 µs clock
• Number of slots per cycle depends on the physical

bandwidth

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Protocol Architecture

• Physical Layer
• DQDB Layer
• Service Layer (LLC)

– Connection-less data service – Connection oriented service – Isochronous service (voice, video)

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MAC Protocol

• Distributed Queue Access Control
• Pre-arbitration – for isochronous (BW) set
• Distributed queue arbitration

– Reservation-based distributed scheme – Implemented through two counters (RQ – request - and CD -countdown) – At light load the delay is very small similar to CSMA/CD – At heavy loads highly efficient as in the case of token ring

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Bandwidth Balancing (BWB)

• Protocol slightly unfair to the nodes towards the

middle of the bus

– Two active nodes, separated by D timeslots – Downstream node sets RQ bit – Upstream node uses all available slots – Takes D timeslots for Request to arrive – Takes D timeslots for empty slot to arrive – In general, P(available slots) decreases the further downstream a node is connected

• Without BWB, a node may transmit a segment when

CD is 0 and the current QA slot is free

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Bandwidth Balancing (BWB)

• Without BWB, a node may transmit a segment when

CD is 0 and the current QA slot is free

• BWB is done by restricting transmission to a

fraction of unused slots

• Achieved by artificially incrementing RQ after

transmitting “n” segments

– BWB counter incremented every time a node transmits a segment – When counter reaches 0 the node must skip next free slot

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End of Class 2