EECS 122, Lecture 7robotics.eecs.berkeley.edu/~wlr/12202/Lecture Slides/lec07.pdf · EECS 122,...

EECS 122, Lecture 7Kevin Fall

[email protected] Walrand

[email protected]

EECS 122 - Fall & Walrand 198

Ethernet: OutlineTypical SetupNamesPhysical LayerFrameFast Ethernet; Gigabit Ethernet10Base5 Efficiency of CSMA/CDPerspective


Ethernet Typical Setup


Ethernet Names

Names of form[rate][modulation][media or distance]

Examples:n 10Base2 (10Mb/s, baseband, small coax, 200m)n 10Base5 (10Mb/s, baseband, coax, 500m)n 10Base-T (10Mb/s, baseband, twisted pair)n 100Base-TX (100Mb/s, baseband, 2 pair)n 100Base-FX (100Mb/s, baseband, fiber)


Ethernet Physical Layer



Hub: Single Collision Domain

MAC Protocol: Wait until silent (carrier sense)TransmitIf collision, wait random time & repeatCSMA/CD



Switch: No Collisions

Multiple transmissions are possibleSwitch stores packets that wait for same output


Ethernet Frame

7 byte preamble: alternating 1/0 combination producing 10Mhz square wave [@ 10Mbps] for 5.6 µsec; used for receiver synchronization

1 byte SFD (start of frame delimiter) 10101011


Ethernet Frame

Length/Type field:§ Type (Ethernet)

§ indicates type of data contained in payload§ issue: what is the length?

§ Length field (802.3)§ type info follows frame header

§ So, is it the type or length?§ “Ethernet”: types have values above 2048 (RFC894 for IP)§ 802.3: length (RFC1042 for IP)

§ If length, next headers are LLC & SNAP (for IP)§ LLC (3 bytes): DSAP, SSAP, CTL§ SNAP (5 bytes): org code, type (above)


Ethernet IEEE 802.3u 100 Mb/s

“Fast Ethernet” (1995) adds:n 10x speed increase (100m max cable

length retains min 64 byte frames)n replace Manchester with 4B/5B (from

FDDI)n full-duplex operation using switchesn speed & duplex auto-negotiation


Ethernet IEEE 802.3{z,ab} 1000 Mb/s

“Gigabit Ethernet” (1998,9) adds:n 100x speed increasen carrier extension (invisible padding...)n packet bursting


Ethernet 10base5 - Definition

Will discuss “classical” Ethernet primarilySingle segments up to 500m; with up to 4 repeaters gives 2500m max lengthBaseband signals broadcast, Manchester encoding, 32-bit CRC for error detectionMax 100 stations/segment, 1024 stations/Ethernet


Ethernet 10base5 – Collision Detection

CD circuit operates by looking for voltage exceeding a transmitted voltageWant to ensure that a station does not complete transmission prior to 1st bit arriving at farthest-away stationTime to CD can thus take up to 2x{max prop. delay}

A B

CD

time


Ethernet 10base5 – minimum frame size

Speed of light is about 4µs/km in copperSo, max Ethernet signal prop time is about 10 µsec, or 20µsec RTTWith repeaters, etc. 802.3 requires 51usec, corresponding to 512 bit-timesThus, minimum frame size is 512 bits (64 bytes); also called slot time


Ethernet 10base5 – max. frame size

1500 byte limitation on maximum frame transmission sizeLater we will call this the MTUlimits maximum buffers at receiverallows for other stations to sendn also requires 96 bit Inter-Packet-Gap (IPG)


Ethernet 10base5 - Transmitter

When ready & line idle, await IPG (96 bit times) and send while listening (CD)If CD true, send max 48-bit jamming sequence and do exponential backoffJamming sequence used to inform all stations that a collision has occurred


Ethernet 10base5 – Exponential Backoff

For retransmission N (1<=N<=10)n choose k at random on U(0..2^N-1)n wait k * (51.2µsec) to retransmitn send on idle; repeat on another collisionn for (11<=N<=15), use U(0..1023)n if N = 16, drop frameLonger wait implies lower priority (strategy is not “fair”)


Ethernet 10base5 – Capture Effect

Given two stations A & B, unfair strategy can cause A to continue to “win”Assume A & B always ready to send:n if busy, both wait, send and colliden suppose A wins, B backs offn next time, B’s chances of winning are

halved


Ethernet Efficiency of CSMA/CD

Typical Sequence of Events:

StartTransmitting

Collision

Wait RandomTime

Successful Transmission

Average = L/R secondsL = average packet length

Average = 5TT = max.prop.time

between 2 nodes

Efficiency = L/RL/R + 5T

= 1/(1 + 5a)

a = T/(L/R) = RT/L


Ethernet Efficiency of CSMA/CD

a impacts what happens during simultaneous transmission:n a small early collision detection

Efficient

n a large late detectionInefficient

Example 1: 10Mbps, 1000m => T = (1km)(4µs/km) = 4µs; RT = 400 bits

L = 4000 bits, say5a = 2000/4000 = 0.5 => efficiency = 66%

Example 2: 1Gbps, 200m=> T = (0.2km)((4µs/km) = 0.8µs; RT = 800 bits

L = 4000 bits; 5a = 4000/4000 = 1 => efficiency = 50%

a = RT/Leff = 1/(1 + 5a)


Ethernet Efficiency of CSMA/CD - Analysis

Model: Slot = 2T

N stations compete by transmitting with probability p, independently

If success => transmit L bitsIf failure (idle or collision), try next slot

P(success) = P(exactly 1 out of N transmits) = Np(1 – p)N-1

Indeed: N possibilities of station that transmitsP(one given station transmits, others do not) = p(1 – p)N-1


Ethernet Efficiency of CSMA/CD - Analysis

Slot = 2T

P(success) = Np(1 – p)N-1

Assume backoff algorithm results in best p = 1/N=> P(success) ≈ 1/e = 0.36

Average time until success: A = 0.36x0 + 0.64x(2T + A)

=> A = 1.28T/0.36 = 3.5TIn practice, backoff not quite optimal => 5T

success

Average = A


Ethernet Addressing

48 bit Ethernet/MAC/Hardware AddressesPrefix assigned per-vendor by IEEEUnique per-adapter, burned in ID PROMMulticast & Broadcast (all 1’s) addressesMany adapters support promiscuousmode


Ethernet Addressing: Multicast

Each vendor assignment supports 2^24 individual and group (multicast) addressesEach adapter supports multiple group “subscriptions”n usually implemented as hash tablen thus, software may have to filter at higher

layer


Ethernet Perspective

Ethernet is wildly successful, partly due to low cost (compare with FDDI or Token Ring--- see text book)Some issues:n nondeterministic servicen no prioritiesn min frame size may be large

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