4b ethernet_medium access control sublayer

8/3/2019 4b Ethernet_Medium Access Control Sublayer

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The Medium Access control(MAC) Sublayer

4b : ETHERNET


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IEEE standards

802.3: Ethernet

802.11: Wireless LAN

802.15: Bluetooth 802.16: Wireless MAN


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Ethernet Cabling

Four TypesType Cable Max Seg. Nodes/seg Remark

10Base5: Thick coax 500m 100 Original cable/ obsolate

10Base2: Thin Coax 185 m 30 No hub Needed 10BaseT: Twisted Pair 100 m 1024 cheapest system

10BaseF: Fiber Optics 2000m 1024 Best between bldgs


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Ethernet Cabling

10Base5 (Thick Ethernet) 10Mbps Base: Baseband transmission For Coax medium its length is given in round number

to multiples of 100m Its max segment length = 500 m

Signal-regenerating repeaters Thick Coax

Advantages: Low attenuation, excellent noiseimmunity, superior mechanical strength

Disadvantages: Bulky, difficult to pull, transceiverboxes too expensive

* Wiring represented a significant part of total installedcost.


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MAU device is physically hooked on main cable.

50 meter AUI cable from MAU to station.

10Base5 (Thick Ethernet)

AUI: Attachment unit interface cable

MAU Connectors: Multistation Access Unit


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Ethernet Cabling

10Base2 (Thin Ethernet) Cheapernet 10 Mbps

185 meter segment length

Signal-regenerating repeaters

Transceiver was integrated onto the adapter

Thin Coax (coax thinner and lighter)

Advantages: Easier to install, reduced hardwarecost, BNC connectors widely deployed lower

installation costs. Disadvantages: Attenuation not as good, could

not support as many stations due to signalreflection caused by BNC Tee Connector.


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BNC connector : Bayonet Navy Connector.


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(a)

(b)

transceivers

Figure 6.55

Thick Ethernet Cable

Thin Ethernet Cable


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10BASET (Twisted pair Ethernet)

10 Mbps

100 meter segment length

Signal-regenerating repeaters Transceiver integrated onto adapter

Two pairs of UTP

Hub-and-spoke topology {Hub in the closet}

Advantages: could be done without pulling newwires. Each hub amplifies and restores incomingsignal.


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Hub Concept

Separate transmit and receive pair ofwires.

The repeater in the hub retransmitsthe signal received on any input paironto ALL output pairs.

Essentially the hub emulates a

broadcast channel with collisionsdetected by receiving nodes.


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Hub Concept

10Base T


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Single collision domain

Twisted Pair Ethernet

hub

Hub Concept

10BaseT


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Switched Ethernet

High-Speed Backplane or

Interconnection fabricswitch


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Switched Ethernet

* Basic idea: improve on the Hub concept

The switch learns destination locations byremembering the ports of the associated

source address in a table.The switch may not have to broadcast to all

output ports. It may be able to send theframe onlyto the destination port.

a big performance advantage over a hub,if more than one frame transfer can go

through the switch concurrently.


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Switched Ethernet

The advantage comes when the switchedEthernetbackplane is able to repeat morethan one frame in parallel (a separatebackplane bus line for each node). The frame is relayed onto the required output

port via the ports own backplane bus line.

Under this scheme collisions are stillpossible when two concurrently arriving

frames are destined for the same station.Note each parallel transmission can

take place at 10Mbps!!


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Figure 4-20.A simple example of switchedEthernet.

Switched Ethernet

Note: Tanenbaums discussionconsiders a more powerful switch

that reduces collisions evenfurther!!


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Switched Ethernet Hub

Since servers are often shared bymultiple nodes, one can employ aswitching hubwith a port whichoperates at a higher rate than theother ports.

Extra buffering inside hub to handle

speed mismatches. Can be further enhancedby higher

rated port full-duplex.


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Ethernet

Switch

Ethernet

Switch

Server

100 Mbps links

10 Mbps links

Figure 6.57

Fast Ethernet

Switch

Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks


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Features of Ethernet

1-persistent, CSMA-CD

Binary Exponential Backoff.

Manchester encoding.


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Features of Ethernet

Operational Description Ethernet stations sense the channel. When the channel is free the station

transmits a frame. Stations monitor the ether during the

transmission. If a collision is detected by any station, the

transmission is terminated immediately and

a jam signal is sent. Upon collision, stations backoff using a local

counter and then retransmit.


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A begins to

transmit at

t=0A B

B begins to

transmit att= tprop-

B detects

collision at

t= tprop

A B

A B

A detects

collision at

t= 2 tprop-It takes 2tpropto find out if channel has been captured

Figure 6.22

Collision Detection [worst case]

Leon-Garcia & Widjaja: Communication NetworksCopyright 2000 The McGraw Hill Companies


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frame contention frame

Figure 6.23

Frame seizes the channel after 2 tprop

On 1 km Ethernet, tpropis approximately 5microseconds.

Contention interval = 2 tprop Interframe gap =9.6 microsecondsModeled as slotted schemewith slot = 2 tprop

Ethernet


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Binary Exponental Backoff

Upon a collision, the sending stationsincrement a localcounter K. The backoff interval is randomly selectedusing a uniform distribution over the L = 2K slots.

K is initially set to 0.

Thus upon collision, the value of L is doubled locally for

each sending station.


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Binary Exponential Backoff (BEB)

Slotted ALOHA shown to be unstable when

p > 1/n

Since Ethernet permits up to 1024 stations, backoff

continues until K = 10, L = 2

10

, and p = 1/2

10

Normally K is incremented up to 10, but BEB is set for 16retries. After 16 retries, MAC gives up trying to send

frame.


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Preamble SDDestination

Address

Source

AddressLength Information Pad chksum

7 1 2 or 6 2 or 6 2 4

Total size of Ethernet frame : 64 to 1518 bytesSync Start

frame

0 Single address

1 Group address

Destination address is either single address

or group address (broadcast = 111...111)

Group address : Multicast

Addresses are defined on local or universal

basis

246 possible global addresses

0 Local address

1 Global address

802.3 MAC Frame

Figure 6.52Leon-Garcia & Widjaja: Communication Networks


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Preamble SD

Destination

Address

Source

Address Type Information Pad chksum

7 1 2 or 6 2 or 6 2 4

64 to 1518 bytesSync Start

frame

Ethernet Frame

Figure 6.53Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks


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FastEthernetandGigabit Ethernet


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Fast Ethernet (100BASE-T)

How to achieve 100 Mbps capacity?

Media Independent Interface provides threechoices.

LLC

MACConvergence Sublayer

Media Dependent Sublayer

Media Independent Interface

Data Link

Layer

Physical

LayerMII


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Fast Ethernet [IEEE 802.3u]

Three Choices

Figure 4-21.The original fast Ethernet

cabling.*Concept facilitated by 10Mbps/100Mbps Adapter Cards


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100 BASE T


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FastEthernet Details

UTP Cable has a 30 MHz limit

Not feasible to use clock encoding (i.e.,NO Manchester encoding)

Instead use bit encoding schemes withsufficient transitions for receiver tomaintain clock synchronization.


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100 BASE T4

Can use four separate twisted pairs of Cat3 UTP

Utilize three pair in both directions (at 331/3 Mbps) with other pair for carriersense/collision detection.

Three-level ternary code is used 8B/6T.

Prior to transmission each set of 8 bits isconverted into 6 ternary symbols.


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100 BASE T4

The signaling rate becomes

100 x 6/8

------------ = 25 MHz

3

Three signal levels : +V, 0, -V

Codewords are selected such that line is

d.c.balancedAll codewords have a combined weight of 0

or 1.


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100 BASE T4

36 = 729 possible codewords.

Only 256 codewords are requires, hencethey are selected:

To achieve d.c. balance Assuming all codewords have at least two signal

transitions within them (for receiver clocksynchronization).

To solve d.c. wander, whenever a string of

codewords with +1 are sent, alternatecodewords (inverted before transmission)are used.

To reduce latency, ternary symbols are

sent staggered on the three lines.


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100 BASE T4

Ethernet Interframe gap of 9.6microseconds becomes 960

nanoseconds in Fast Ethernet.100 m. max distance to hub; 200

meters between stations.

Maximum of two Class II repeaters.


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100 Base TX

Uses two pair of twisted pair, onepair for transmission and one pair forreception.

Uses either STP or Cat 5 UTP.Uses MTL-3 signaling scheme that

involves three voltages.

Uses 4B/5B encoding.

There is a guaranteed signaltransition at least every two bits.


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100 BASE FX

Uses two optical fibers, one fortransmission and one for reception.

Uses FDDI technology of converting4B/5B to NRZI code group streamsinto optical signals.


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Fast Ethernet Repeaters andSwitches

Class I Repeater supports unlike physicalmedia segments (only one per collisiondomain)

Class II Repeater limited to single physicalmedia type (there may be two repeaters percollision domain)

Switches to improve performance can addfull-duplexand have autonegotiation forspeed mismatches.


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Collision Domains


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Gigabit Ethernet History

In February 1997 the Gigabit Ethernet Allianceannounced that IEEE802.3z Task Force met toreview the first draft of the Gigabit Ethernet

StandardAccording to IDC by the end of 1997 85% of allnetwork connections used Ethernet.

Higher capacity Ethernet was appealing becausenetwork managers can leverage theirinvestment in staff skills and training.

1000 BASE X (IEEE802.3z)was ratified in June1998.

G b h ( 000 S


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Gigabit Ethernet (1000 BASEX)

Provides speeds of 1000 Mbps (i.e., one billionbits per second capacity) for half-duplex andfull-duplex operation.

Uses Ethernet frame format and MAC technology

CSMA/CD access method with support for onerepeater per collision domain.

Backward compatible with 10 BASE-T and 100 BASE-T.

Uses 802.3 full-duplex Ethernet technology.Uses 802.3x flow control.

All Gigabit Ethernet configurations arepoint-to-point!

Gigabit Ethernet Architecture Standard


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Gigabit Media Independent Interface (GMII)

(optional)

Media Access Control (MAC)

full duplex and/or half duplex

1000 Base T

PMA

transceiver

1000 Base X PHY

8B/10B auto-negotiation 1000 Base TPCS

Unshielded twisted pair

IEEE 802.3ab

1000 Base-LXFiber optic

transceiver

1000 Base-SXFiber optic

transceiver

1000 Base-CXCopper

transceiver

MultimodeFiber

ShieledCopper Cable

Single Mode orMultimode Fiber

IEEE 802.3z

g

Source - IEEE


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Gigabit Ethernet Technology

Figure 4-23.Gigabit Ethernetcabling.1000 BASE SX fiber - short wavelength

1000 BASE LX fiber - long wavelength

1000 BASE CXcopper - shielded twisted pair

1000 BASE T copper - unshielded twisted pair

* Based on Fiber Channel physical signaling technology.


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Gigabit Ethernet (1000 BASE-T)

LLC

MAC

Medium

Physical Layer

Data Link

Layer

GMII Gigabit Media Independent Interface

Media Dependent Interface

Gigabit Media Independent


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Gigabit Media IndependentInterface

GMIIAllows any physical layer to be used

with a given MAC.

Namely, Fiber Channel physical layercan be used with CSMA/CD.

Permits both full-duplex and half-duplex.


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1000 BASE SXShort wavelength

Supports duplex links up to 275meters.

770-860 nm range; 850 nm laserwavelength

(FC) Fiber Channeltechnology

PCS (Physical Code Sublayer)includes8B/10B encoding with 1.25 Gbpsline.

Only multimode fiber


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8B/10B Encoder


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8B/10B Encoding Issues

When the encoder has a choice for codewords, it alwayschooses the codeword that moves in the direction of balancingthe number of 0s and1s. This keeps the DC component of thesignal as low as possible.


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1000 BASE LXLong wavelength

Supports duplex links up to 550meters.

1270-1355 nm range; 1300 nmwavelength using lasers.

Fiber Channel technology


Either single mode or multimode fiber.


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1000 BASE CXShort haul copper jumpers

Shielded twisted pair.

25 meters or less typically withinwiring closet.


Each link is composed of a separateshielded twisted pair running in eachdirection.


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1000 BASE TTwisted Pair

Four pairs of Category 5 UTP.

IEEE 802.3ab ratified in June 1999.Category 5, 6 and 7 copper up to 100

meters.

This requires extensive signalprocessing.


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Gigabit Ethernet compared toFiber Channel

Since Fiber Channel (FC) alreadyexisted, the idea was to immediatelyleverage physical layer ofFC into

Gigabit Ethernet.

The difference is that fiber channelwas viewed as specializedfor high-

speed I/O lines. Gigabit Ethernetisgeneral purpose and can be used as ahigh-capacity switch.


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Gigabit Ethernet

Viewed as LAN solution while ATM is WANsolution.

Gigabit Ethernet can be shared (hub) or

switched.Shared Hub

Half duplex: CSMA/CD with MAC changes:

Carrier Extension

Frame BurstingSwitch

Full duplex: Buffered repeater called{BufferedDistributor}


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Gigabit Ethernet

Figure 4-22. (a) A two-station Ethernet. (b) A multistationEthernet.

Carrier Extension


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Carrier Extension

Based on Raj Jains slide

RRRRRRRRRRRRRFrame

Carrier Extension

512 bytes

For10BaseT : 2.5 km max; slot time = 64 bytes

For1000BaseT: 200 m max;slot time = 512 bytes Carrier Extension :: continue transmitting control

characters [R] to fill collision interval.

This permits minimum 64-byte frame to be handled.

Control characters discarded at destination.

For small frames net throughput is only slightly betterthan Fast Ethernet.

F B i


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Frame Bursting

Based on Raj Jains slide

512 bytes

ExtensionFrame Frame Frame Frame

Frame burst

Source sends out burst of frames without relinquishingcontrol of the network.

Uses Ethernet Interframe gap filled with extension bits

(96 bits) Maximum frame burst is 8192 bytes

Three times more throughput for small frames.

Buffered Distributor


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Buffered Distributor

A buffered distributor is a new type of 802.3

hub where incoming frames are buffered inFIFOs.

CSMA/CD arbitration is inside the distributorto transfer frames from an incoming FIFO to

all outgoing FIFOs.802.3x frame-based flow control is used to

handle congestion.

All links are full-duplex.

Hub

4b ethernet_medium access control sublayer

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