Ethernet

1
Ethernet
EECS 325/425, Fall 2005 November 21
2
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

3
What should we know?

• Ethernet frame structure
• Ethernet algorithm
• Various Ethernet technologies

4
Ethernet

• dominant LAN technology
• cheap 20 for 100Mbs. cheaper now!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10, 100, 1000 Mbps,
  10Gbps ?

• In 1973 Xerox began the development of a bus
  topology LAN.
• In 1976, CS was added and Xerox built a 2.94
  Mbps CSMA/CD, 100 stations, 1km.
• In 1980 DEC, Intel, and Xerox released a
  standard for 10 Mbps Ethernet, IEEE 802.3
• In 1986 10Base2 in 1991 10BaseT in 1995 100
  Mbps in 1999 Gigabit in 2002 10Gigabit.

Metcalfes Ethernet sketch
5
Ethernet Frame Structure

• Sending adapter encapsulates IP datagram in
  Ethernet frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates.

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Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (e.g. ARP packet), it passes data in frame to
  network layer protocol
• otherwise, adapter discards frame
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

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Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send ACKs
  or NACKs to sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if application is using TCP
• otherwise, application will see the gaps (network
  and link layer do not retransmit. Leave it to
  upper layer, why?).

8
Ethernet uses CSMA/CD

• multiple nodes attached to the link share the
  channel, that is, multiple access
• no slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sensing
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access
• Similar effect if, with probability p, sends a
  frame
• Need to listen to the channel and detect
  collision, compared to ALOHA
• Measure the voltage levels before and during
  transmission

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Ethernet CSMA/CD algorithm

• 1. Adaptor gets datagram from the node and
  creates frame
• 2. If adapter senses channel idle ( 96 bit
  times), it starts to transmit frame. If it senses
  channel busy, waits until channel idle ( 96 bit
  times) and then transmits
• Likely collisions after wait?
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !
• Can collisions still occur?

• 4. If adapter detects another transmission while
  transmitting, aborts and sends 48-bit jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter
  chooses a K at random from 0,1,2,,2m-1.
  Adapter waits K512 bit times and returns to Step
  2

Similarity between this exponential backoff and
TCPs congestion control mechanisms (rate-halving
and backoff when consecutive timeout)
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Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time 0.1 microsec for 10 Mbps Ethernet
  e.g. 1K bit time is about 100 microsecs

• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer.
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times.
• Why 512 bit times?
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

Remember each node sends a frame with probability
p?
11
A fairness problem

• New node has initial m0, but the other 100 nodes
  are in exponential backoff (m 5,6,7).
• Unfairness, the new node usually grabs the
  channel first
• A selfish node always acts like a new node

12
Ethernet CSMA/CD efficiency

• tprop max prop between 2 nodes in LAN
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap
• Q1 Remember the more general CSMA/CD efficiency
  we calculated last lecture? Why different?
• Q2 bandwidth increases, but propagation delay
  does not decrease fast (speed-of-light). So
  consequence?

13
Ethernet Technologies 10Base2

• 10 10Mbps 2 under 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface
  to its other interfaces physical layer device
  only!
• has become a legacy technology, 3Com

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Star topology

• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices hub or switch (more later)

hub or switch
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10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Nodes connect to a hub star topology 100m max
  distance between nodes and hub
• Hubs are essentially physical-layer repeaters
• bits coming in one link go out all other links
• no frame buffering
• no CSMA/CD at hub adapters detect collisions
• provides network management functionality

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Gigabit Ethernet

• Star topology with a hub or switch at its center
• use standard Ethernet frame format
• allows for point-to-point links (using switches)
  and shared broadcast channels (using hubs)
• in shared mode, CSMA/CD is used short distances
  between nodes to be efficient
• but hubs are here Buffered Distributors
• full-duplex at 1 Gbps for point-to-point channels
• 10Gbps now. 100Gbps possible? E.g. Lucents
  recent 100Gbps Ethernet-over-optical.

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Wednesday

• We have a class
• LAN and ARP
• Homework 6 due