Ethernet

Ethernet Goals

• Simplicity• Low Cost• Compatibility• Address flexibility• Fairness

– All nodes have equal access to the network

• High speed• Stability

Ethernet Non-Goals

• Full Duplex– At an given time, there can be only one source station.

• Error Control– Limited to detection of bit errors in the physical channel, and

detection and recovery from collisions• Security

– No data encryption is employed• Speed flexibility• Priority

– Does not provide any priority mechanism. • Hostile user

– There is no attempt to protect the network from a malicious user at the date link level.

Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

Preamble is a 64-bit pattern consisting of 7 bytes of 10101010 followed by one byte of 10101011. Data + Pad should be at least 46 bytes.

Typical MAC Address 8:0:2b:e4:b1:2

00001000 00000000 00101011 11100100 10110001 00000010

Ethernet MAC Sublayer Protocol

Collision detection can take as long as 2 .At 10 Mbps, each bit takes 100 nsec The round-trip time (max 2500 meters and 4 repeaters) is about 500 bit-time.So, each frame should take at least 500 bit-timeSlot time= 512 bit time512 bit times = 64 bytes at 10Mbps; which is the minimum frame length

Carrier Sense in Ethernet

• Whenever the channel is busy, it defers to the passing frame. When the last bit of the passing frame is gone by, that is, the medium is sensed idle, it continues to defer for another 9.6 micro seconds (12 bytes @ 10 Mbps) to provide proper interframe spacing

• Question: What “stuffing” technique does Ethernet use?

Collision Handling in Ethernet

• When a collision is detected during a frame transmission, the sending station does not terminate transmission immediately. Instead, the transmission continues until at least 32 (but not more than 48) additional bits have been transmitted from the time that collision is detected. This is called the jamming sequence. It ensures that all transmitting station would be able to detect the collision.

Collision Handling in Ethernet…

• The jamming sequence can be anything except the 32-bit CRC value corresponding to the partial frame transmitted prior to the jamming sequence.

• Question: How would the receiving stations detect collision?

Backoff and retransmission

• In case of a collision, transmission is tried for up to 16 times (1 original + 15 retries).

• At the end of the jamming sequence, the station delays before attempting to retransmit. The delay is an integral multiple of the slot time. The number of slot times to delay before the n-th attempt is chosen as a uniformly distributed random integer r in the range

0 ≤ r < 2k where k = min{n,10}.

• This is called truncated binary exponential backoff policy.

Truncated binary exponential backoff

• For the 1st retransmission:– k = min{1,10}, this give 0 ≤ r < 21. Thus, the station

waits either 0 or 1 slot time.

• For the 2nd retransmission:– k = min{2,10}, this give 0 ≤ r < 22. Thus, the station

waits either 0, 1, 2, or 3 slot times.

• On the average, the station will backoff

(2n – 1)/2 time slots for the n-th retry; n ≤ 10.

A Summary of 10Mbpb Ethernet Specs

• Date Rate 10 Mbps = 107 bits per second• Smallest frame = 8 bytes of preamble + 64 bytes of

frame = 72 bytes, taking 57.6 μsec.• Largest frame = 8 bytes of preamble + 1518 bytes of

frame = 1526 bytes, taking 1220.8 μsec.• FCS is computed for all fields using the following

generating polynomial:

.1

)(2457810

11121622232632

xxxxxxx

xxxxxxxxG

Performance Analysis of Ethernet

• We will do a simple analysis of CSMA part.

one way propagation time

2 slot time

time required to complete a successful

transmission

message length

: Each station transmit during an

available slot time

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Performance Analysis of Ethernet…

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Performance Analysis of Ethernet…

• Next, we need to get hold of the average contention time, that is, on the average, how long would it take for a typical station to seize the channel. We express that in terms of A, the probability of no collusion

Performance Analysis of Ethernet…

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Performance Analysis of Ethernet…

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Ethernet Performance

• Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

Fast Ethernet

• Ethernet Cablings

Gigabit Ethernet…

• Features– Frame bursting; a station can send more than one

frame at a time– Uses 8B/10B codes– Uses five different voltage levels– Transmits four symbols in parallel in each clock

period (125MHZ)– Each symbol carries two bits, for a total of 8 bits per

clock period to get 8x125MHz=1Gbps.– Flow control, using a PAUSE frame to slow down