Chapter 2: Direct Link Networks (continued)

1
Chapter 2 Direct Link Networks (continued)

• Slide Set 5

2
Reminder Announcements

• Quiz January 27th.
• Chapters 1 and 2.1 to 2.6 (Upto this set of
  slides)
• Bring Scantron sheets, calculators

3
In this set..

• Reliable Transmissions -- retransmissions.
• Ethernet

4
Why retransmissions ?

• Error correction although feasible, is not enough
  to handle all kinds of errors -- especially burst
  errors.
• Corrupt frames cannot be deciphered and are
  therefore dropped.
• Retransmissions needed to provide reliability.

5
ACKs and Time-outs

• When frames are sent piggyback an acknowledgement
  (ACK) for received packets onto sent packets.
• If no ACK received up to a preset time-out,
  resend frame.
• Called ARQ -- Automatic Repeat request.

1
100
Ack 1
6
Stop and Wait

• Allow only one outstanding packet at any given
  time.
• If ACK not received within time-out, send again.

7
How efficient is Stop and Wait ?

• Consider a 1.5 Mbps link with 45 ms RTT.
• BW - Delay product 67.5 Kb 8KB.
• You can fill 8 Kilo bytes of data prior to
  receiving an ACK.
• However, if your frame size is 1 KB, you are
  using only 1/8 of the capacity.
• Inefficient.

8
Sliding Window

• What we really like is that the 9th frame be
  transmitted when ACK for the first frame arrives
  ).
• Say we do this -- A window of packets sent -- as
  ACKs are received window slides i.e., more
  packets sent.
• Now what do we need in addition ?
• Need to know which packets have been received and
  which have not.
• Packets labeled using sequence numbers.

9
Some definitions

• We have a window of packets sent -- Send Window
  Size or SWS.
• Last Acknowledgement received is denoted LAR.
• LFS represents the last frame sent.
• NOTE LFS - LAR lt SWS.

10
Sender Functions

• When an ACK is received, the LAR moves to the
  right.
• This allows for the transmission of an
  additional frame.

11
Receiver functions
Notation RWS -- Receive Window Size LAF --
Largest Acceptable Frame. LFR -- Last Frame
Received.

• When frame with Seq_Num arrives
• If Seq_Num lt LFR or Seq Num gt LAF, discard.
• Frame is outside window.
• If LFR lt Seq_Num lt LAF, accept frame.
• Frame within window.
• Let Seq_Num_to_Ack be the largest sequence number
  yet to be acked. This implies, all frames lt
  Seq_Num_to_Ack have been received.
• LFR Seq_Num_to_Ack
• Adjust LAF LFR RWS.

12
An Example

• Let LFR 5 and RWS 4.
• This implies LAF 9
• If packets 7 and 8 arrive (not 6), they are
  buffered.
• Note that they are out of order.
• Typically, Receiver will resend an ACk for packet
  5.
• When 6 arrives, it can cumulatively ACK all
  buffered packets i.e., it ACKs 8 and moves LFR to
  8 and LAF to 12.

13
Other possibilities

• Send NAK (negative acknowledgement) for lost
  packets -- example for 6, when 7 is received.
• Duplicate ACKs -- send an ACK for 5 again when 7
  is received to trigger retransmission of 6.
• Selective ACKs Explicitly ACK frames that are
  received -- more complex.

14
Setting the Window Size

• SWS -- Set considering the BW product.
• RWS -- Receiver can set it to something
  appropriate -- may depend on buffering resources.
• If RWS 1 what happens to out of order frames ?

15
Sequence number wrapping

• Sequence numbers are finite -- thus there is a
  need to reuse -- called wrap around.
• What is the relationship between the SWS and
  MaxSeqNum ?

16
Maximum Sequence Number and SWS

• Should it not be SWS lt MaxSeqNum 1 ?
• Let us consider an example
• Sender has eight Seq Nums from 0 to 7.
• SWS RWS 7.
• Sender transmits frames 0-6
• Receiver gets them, ACKs but ACKs get lost.
• Receiver expects 7 and next 0...5. But sender
  sends the previous 0..5.
• When the receiver gets these, he cannot
  distinguish.
• Thus, when RWS SWS, SWS lt (MaxSeqNum1)/2.
  (Verify that system works).

17
Sequence Numbers and SWS

• For other cases i.e., when RWS is not equal to
  SWS, other rules may apply.
• Depends on the specific case.
• A different way with TCP -- we will see later.
• Easy solution -- large Sequence number space.

18
Flow Control

• Feedback control that allows the receiver to
  throttle the sender.
• Informs sender not only about what frames it has
  received but also how many more frames it can
  receive.
• Need this in order to ensure that receiver buffer
  does not overflow.
• Read rest of the parts on Sliding Window --
  implementation etc.

19
Multiple Access

• Nodes send and receive frames over a shared
  direct link network.
• One way -- contention based i.e., nodes contend
  to send -- example Ethernet.
• A second way is scheduled access -- everyone
  knows when and how to send -- i.e., they take
  turns -- example Token Ring.
• Decentralized operations needed.

20
CSMA - CD

• Stands for Carrier Sense Multiple Access with
  Collision Detection.
• Carrier sensing -- nodes can distinguish between
  an idle and a busy line.
• Collision detection -- nodes can detect when a
  transmission is interfered with.
• CSMA-CD can work on a bus architecture as an
  example.
• Forms the basis for Ethernet.

21
The Ethernet Standard

• Specified by IEEE.
• Referred to as the 802.3 standard.
• Initially started out as a standard for a bus
  topology -- 10 Mbps Ethernet.
• Today, we have switched Ethernet -- Fast Ethernet
  with 100 Mbps and Gigabit Ethernet with 1000 Mbps.

22
Ethernet on a Coax

• We consider the 10 Mbps Ethernet.
• Ethernet implemented on a coax cable of up to
  500m.
• Hosts tap into this cable and must be at least
  2.5 meters apart.
• Transceiver which is attached detects idle line
  and drives signal.

23
Repeaters

• Multiple Ethernet segments can be joined using
  repeaters.
• Repeaters amplify and forward signals.
• No more than four repeaters between any two
  hosts.
• This ensures that maximum reach of Ethernet
  2500 meters.
• We will see why this is needed later.

24
Ethernet -- Signal broadcasts

• Signal broadcast over the multiple-access link
• Broadcast means, sent to everyone.
• Each node propagates signal in both directions.
• Repeaters forward on all outgoing segments.
• Note Ethernet uses Manchester encoding.

25
Cables

• Use of a 10base2 cable -- 10 Mbps, base --gt it is
  baseband -- frequency shifting -- No more than
  200 m (actually only 185 m).
• 10base T -- T stands for twisted pair.

26
Ethernet Frame

• Nuggets (Other details refer book)
• 64 bit preamble for synchronization.
• 48 bit hardware or Ethernet address.
• Ethernet frame size up to 1500 bytes of data
  and at least 46 bytes --gt padding may be
  necessary.
• Note -- Body carries IP datagram -- length of IP
  header specified packet size -- can determine
  padding.

27
Ethernet Address

• Ethernet address unique for every host.
• Technically address belongs to adaptor and not
  host -- burned into ROM.
• 6 bytes -- represented as six numbers separated
  by colons 802be4b12.
• Each number corresponds to a pair of hexadecimal
  units -- one for each of the nibbles in the byte.

28
Addressing (cont)

• Uniqueness Each manufacturer (e.g. AMD) is
  allocated a different prefix -- 802b --gt 24
  bits.
• Every frame transmitted on the Ethernet is
  received by every adaptor on the network -- the
  address is recognized and only the particular
  adaptor recovers frame.
• All 1s implies broadcast -- every adaptor picks
  it up.
• An address that has the 1st bit set to 1 but is
  not the broadcast address is used for multicast.

29
Transmitter Algorithm

• Intelligence mainly on transmitter side --
  receiver dumb.
• Algorithm
• If line idle, transmit frame immediately no
  negotiation with other adaptors.
• If line busy, wait for the line to be idle and
  either
• Transmit immediately (1-persistent version)
• Transmit with a probability p (p-persistent
  version). With a probability q1-p, defer the
  transmission for a later time.

30
Why p-persistency?

• Deference is needed since multiple transmitters
  may be waiting to transmit.
• If for example, p 0.25, one could have up to 4
  transmitters on average -- only one would
  transmit when the line is free.
• If multiple transmitters send simultaneously, a
  collision will result.

31
Waiting time

• Each node looks at a slot time.
• Each slot is of a duration equal to that of a
  frame.
• When node has something to send, in each idle
  slot, it sends with a probability p, and defers
  to the next slot with a probability q and so on.
• Just a note 1-persistency is good if nodes are
  lightly loaded -- has been found effective.

32
Upon Collision Detection

• When an adaptor detects a collision, it transmits
  a 32 bit jamming sequence and stops transmission.
• Minimal amount of data sent 64 bit preamble 32
  jamming sequence 96 bits.
• How does a node detect a collision ? As nodes
  send, they monitor the line for other
  transmissions.

33
Runt frame

• If two hosts that simultaneously transmit are
  very close to each other, they can detect a
  collision quickly.
• In this case, enough only the 96 bits are sent --
  such a frame is called a runt frame.

34
Effects of distance on CD

• What if nodes are far apart ? Let us consider the
  case where propagation time is d.
• Let A transmit at time t.
• As transmission jams that of node B which just
  begins transmission at td.
• It then sends out a jamming sequence -- this
  takes another d seconds to reach A.
• Thus, A knows of the collision at t2d.
• A has to be transmitting at this time in order to
  realize the collision i.e., it has to be
  transmitting for 2d seconds.
• Note that 2d RTT.
• So transmission time should be at least equal to
  RTT.

35
Transmission time and RTT

• To repeat Transmission time at least equal to
  RTT.
• Now since RTT is at most 51.2 ms given that
  maximum reach is 2500 meters.
• Thus, Transmission time at least 51.2 ms. On a 10
  Mbps link, transmitter needs to send at least 512
  bits.
• Thus, we have a minimum size of 46 bytes for the
  body (header makes frame 512 bits).

36
Ethernet back-off

• Once adaptor detects collision, it waits before
  trying again.
• First delay -- selected randomly from the pair 0,
  (21-1)51.2 ms 0, 51.2 ms.
• If a second collision is experienced, choose
  back-off time from 0, (22-1)51.2 ms i.e, from 0,
  51.2, 102.4 and 153.6 ms.
• Thus, after N collisions choose a back-off time
  from 0.. (2N-1)51.2 ms.
• In reality, Ethernet gives up after a maximum
  number of attempts which is 16. However, maximum
  value of N is fixed typically N 10.

37
Key properties

• Ethernet works best when loads are light -- small
  number of active nodes.
• Typically utilization is less than 30 .
• Easy to administer and maintain inexpensive.
• Currently, switched Ethernet.

38
Next time

• Token Ring,
• FDDI,
• Wireless LANs.