Point-to-Point Protocols and Links

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Chapter 2

• Point-to-Point Protocols and Links

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Section 2.1

• Introduction

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2.1 Introduction

• Physical communication links
• Data link control
• i.e. point-to-point protocols
• Physical links requires background in
• Linear system theory
• Random process
• Modern communication theory
• Recall
• Chapter 1 section 1.3.1 page 34

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Introduction
5
Introduction

• Our major problem in DLC correct bit error
• Error detection correction
• ARQ ( Automatic Repeat request)
• Header packet trailer gt frame

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Section 2.2

• Physical layer
• Channels modems

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2.2 Physical layerchannels modems

• Skip,will be discussed if necessary

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Section 2.3

• Error Detection

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2.3 Error Detection

• DLC layer is to provide error-free packets to
  next layer up

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2.3.1 Single Parity checks

• Parity checks bit is the sum, modulo 2 , of the
  bits in the original bit string
• Total number of 1s in an encoded string is
  always even
• Detect single bit error only.And , odd number of
  bit errors

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2.3.2 Horizontal Vertical Parity Checks
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Horizontal Vertical Parity Checks

• Common use for ASCII encoded characters
• Cannot detect four errors confined to 2 rows and
  2 columns

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2.3.3 Parity Check Codes
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Parity Check Codes

• Effectiveness of a code for error detection
• Minimum distance of the code
• Single parity check is 2
• Horizontal Vertical is 4
• Burst-detecting capability
• Single parity is 1
• Horizontal Vertical is 1length of row
• Probability that a completely random string will
  be accepted as error-free

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Parity Check Codes

• E.g.
• Minimum distance
• Single parity check is 2
• Horizontal Vertical is 4
• Burst-detecting
• Single parity check is 1
• Horizontal Vertical is 1 length of row

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Parity Check Codes
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Figure 2.15
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2.3.4 Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
L3 S(D)D21
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Cyclic Redundancy Checks(CRC)
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Section 2.4

• ARQ Retransmission Strategies

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2.4 ARQRetransmission Strategies

• 2 aspects of retx algorithms or protocols
• Succeed in releasing each packet,one oad only
  once without errors
• Efficiency releasing unnecessary waiting
  unnecessary retx

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ARQRetransmission Strategies

• We assume , all frames containing transmission
  errors are detected
• Delay is arbitrary
• Frame may be lost never arrive
• Frames arrive in the same order as transmitted

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ARQRetransmission Strategies
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2.4.1 Stop-and-wait ARQ

• Each packet has been received correctly before
  initiating tx of next packet
• If
• Error free
• Acknowledge , Ack
• Error frame
• Negative acknowledgement , NAK
• Ack NAK is protect with a CRC
• Ack lost or NAK
• Resend the old packet

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Stop-and-wait ARQ
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Stop-and-wait ARQ
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Stop-and-wait ARQ

• Avoid this problem,returns the number of next
  packet awaited
• Piggyback

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Stop-and-wait ARQ
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Fig 2.21
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Stop-and-wait ARQ

• The algorithm for A to B
• At node A
• SN?0
• Assign SN to the new packet
• Tx SN-th frame
• If receive from B with B RNgtSN,SN?RN,go to Step2
• If no received frame from B,timeout , go to Step
  3

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Stop-and-wait ARQ

• Continued
• At node B
• RN?0,repeat step2 3 forever
• If error-free frame received with SNRN,RN
• Within bounded delay after receiving error-free
  frame send a frame to A containing RN

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Stop-and-wait ARQ

• Correctness of stop-and-wait
• An algorithm is safe if it never produces an
  incorrect result
• An algorithm is live if it can continue forever
  to produce results
• Safety
• Initially, node B awaiting packet 0 , and only
  packet 0 is released.Subsequently,node B has
  released all packets in order,up to , but not
  including ,packet RN

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Stop-and-wait ARQ
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Stop-and-wait ARQ
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Stop-and-wait ARQ
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Stop-and-wait ARQ
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Stop-and-wait ARQ

• One trouble with Stop-and-wait
• SN become arbitrarily large with increasing time
• Given our assumption that frames travel in order
  on the link,SN modulus 2 is sufficient!

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Stop-and-wait ARQ
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2.4.2 Go Back n ARQ

• Several successive packets can be sent without
  waiting for the next packet to be requested
• Accept packets only in the correct order , and
  send RN back
• RN is to acknowledge all packets prior to RN and
  to request packet RN

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Go Back n ARQ
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Go Back n ARQ
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Go Back n ARQ
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Go Back n ARQ
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Go Back n ARQ
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Go Back n ARQ

• Go back n algorithm
• At node A
• SNmin ?0,SNmax?0 (SNmin to SNmax-1)
• Do steps 3,4,5 repeatedly in any order
• If SNmaxltSNminn and a packet is available,assign
  SNmax to it increment SNmax
• If an error-free frame is received from B
  containing RNgtSNmin,increase SNmin to RN
• If SNminltSNmax,and no frame is currently in
  transmission,choose some number SN,SNmin
  ltSNltSNmaxtransmit SN-th frame. At most a
  bounded delay is allowed between successive
  transmission of packet SNmin over intervals when
  SNmin does not change

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Go Back n ARQ

• Go back n algorithm
• At node B
• RN?0,repeat steps 23 forever
• When an error-free frame is received from A
  containing SNRN,increment RN
• At arbitrary times,but within bounded delay after
  receiving any error-free data frame from
  A,transmit a frame to A containing RN

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Go Back n ARQ

• Safety
• Same as stop-and-wait
• Liveness
• t1ltt3 ,t2ltt3

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Go Back n ARQ
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Go Back n ARQ
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Go Back n ARQ

• t2 could be ltt1(why? P.77)
• It t1ltt2 ,then RN(t)i for t1 ? t ? t2?
• packet i after t1 will be accepted and since
  t2ltt3 , node A will retx packet i until this
  happened
• ?t1 to t2 is finite
• No matter t1ltt2 or t1gtt2
• Node B transmit frames carrying RN ? i1 from
  time t2 until t3 since ggt0, t2 to t3 is
  finite?live!

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Go Back n ARQ with modulo mgtn
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Go Back n ARQ with modulo mgtn
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Go Back n ARQ with modulo mgtn
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2.4.3 Selective Repeat ARQ

• Even if unnecessary retx are avoided,go back n
  protocol must re-tx at least one round-trip-delay
  when a single error occurs in an awaited packet
• Idea of selective repeat ARQ is to accept out
  -of-order packet,and to request retx from A only
  for those packets that are not correctly received

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Selective Repeat ARQ

• RN lowest number packet not yet correctly
  received
• N window size
• Let p is probability of frame error,the expected
  number ? of packets delivered to B per frame
• ? ? 1-p ? throughput of ideal
  selective repeat

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Ideal go back n ARQ

• r ( 1-p ) p( 1 ? ? )
• r ????Frames???next awaited frame
• ( 1-p ) ?????
• p ??

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Selective Repeat ARQ modulo m?2n

• B accept RNRNn-1
• Feedback also contains which packets beyond RN
  have been correctly received
• Accepted out -of-order packets are saved until
  earlier packets are accepted released

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Selective Repeat ARQ modulo m?2n

• t1 frame was first generated at node A at time
  t1
• t2 frame is received at node B
• RN(t2)-n?SN?RN(t2)n-1
• If mod m ,and if packets are accepted in the
  range RN(t2)RN(t2)n-1
• ?it is necessary for B to distinguish values of
  SN in the entire range?m?2n

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2.4.4 ARPANET ARQ

• Using 8 stop-and-wait strategies in parallel
• Each incoming packet is assigned to one of eight
  virtual channels if one of eight is idle
• If all channels are busy,wait

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ARPANET ARQ
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ARPANET ARQ

• More overhead is required
• Sequence number modulo 2
• Each frame carries information for all 8 virtual
  channels
• ?ack information is repeated so often
• (Typically,only one retx is required in forward
  direction)
• Undesirable packets released at receiver in a
  different order

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Section 2.5

• Framing

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2.5 Framing

• Decide where successive frames start and stop
• Character-based framing use special control
  characters for idle fill and to indicate frame
  beginning and ending
• Bit-oriented framing with flagsuse special bit
  string(flag)
• Length countsin a field of the header

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2.5.1 Character-Based Framing

• SYN(synchronous idle) provides idle fill b/w
  frames
• STX(start of txt)
• ETX(end of text)

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Character-Based Framing
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Character-Based Framing

• If the packet to be transmitted in an arbitrary
  binary string,?packet might contain ETX character
  ,for example ?trouble!
• ?transparent modeDLE character is inserted
  before STX (Data Link Escape)
• DLE in dataDLE DLE ?DLE
• E.g. (x)DLE ETX ? end of frame
• (x)DLE DLE ETX ? DLE ETX ? Data

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Character-Based Framing
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Character-Based Framing

• Disadvantages
• Excessive use of framing overhead
• Each frame must consist of an integer number of
  characters
• What happens if error?
• Error in DLE ETX ending a frame won't detect
  end of frame
• Error in data causes DLE ETX interpret as end of
  frame
• Probability 2-L CRC undetected

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2.5.2 Bit-Oriented Framing Flags

• Another approach use a flag at the end of frame
• Bit stuffing is used to avoid confusing b/w data
  ad flag
• Difference b/w bit-oriented character-based
  framingbit-oriented can have any length of bits
• For example 0160 as a flag to indicate frame
  ending
• Rule insert (stuff) a 0 into data string of the
  frame proper after each successive 5 1s

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Bit-Oriented Framing Flags
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Bit-Oriented Framing Flags

• After stuffing never contains more than 5
  consecutive 1s.If a string of 5 1s is followed
  by a 1 ?frame end
• 016 frame end
• 016 0 normal frame termination
• 016 1 abnormal termination ? abort

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Bit-Oriented Framing Flags

• ??(fig 2.36)
• The first stuffed bit not necessary(????,??just
  start)
• Second stuffed bit necessary
• Third stuffed bit could be eliminated
• Fourth stuffed bit required(????0??stuffed)
• ?by modifying stuffing rule, but the reduction
  of overhead is negligible

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Bit-Oriented Framing Flags

• Consider the overhead
• Assume a frame(before bit stuffing) consists of i
  id random binary variables with equal probability
  0 or 1
• Assume for increased generality that the flag
  uses 01j for some j
• 01j0 normal termination
• 01j1 abnormal

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Bit-Oriented Framing Flags

• An insertion will occur at the i-th bit of the
  original frame (for i ? j)
• if the string from i-j1 to I is 01j-1
• ?probability 2-j
• An insertion will also occur (for i ? 2j-1)
• if string from i-2j2 to i is 012j-2(01j-11j-1)
• ?probability 2-2j1 (ignore this term,??)

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Bit-Oriented Framing Flags

• Bit j-1 occur an insertion if the first j-1 bits
  are all 1s
• ? probability 2-j1
• Recall
• Expected value of a sum of r.vs
• Expected number f insertions in a frame
• ?for k(original frame length) ? j-1
• Expected number of insertions
• 2-j1 (k-j1)2-j (k-j3)2-j

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Bit-Oriented Framing Flags

• Eoverhead (Ek-j3)2-jj1(2.33)
• j1 means termination string
• Since Ekgtgtj
• ?Eoverhead ? Ek2-jj1(2.34)

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Bit-Oriented Framing Flags

• Minimize(2.34) for a given Ek?find optimal j
• So, optimal j must
• Ek2-jj1 lt Ek2-j-1j2
• Ek2-j-1 lt1
• smallest j j ?log2Ek? (2.36)
• Eoverhead ? log2Ek 2 (2.37)
• For example
• if Ek 1000 bits , optimal j 9
• Eoverhead ? 12 bits
• for j 6 , Eoverhead ? 23 bits

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2.5.3 Length Fields

• Basic problem in framing
• Determine where each idle fill string
  ends(trivial, when pattern is broken) where each
  frame endsharder
• Include a length field in the frame header(e.g.
  DECNET) length field(overhead) must have at least
  ? log2Kmax? 1 bits where Kmax is maximum frame
  size
• Compared to 2.37?similar overhead
• Question
• Any other method requires smaller overhead

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Bit-Oriented Framing Flags

• Information theory
• By source coding theorem
• Given any probability assignment P(K) on frame
  lengths.Minimum expected number of bits that can
  encode such a length is at least entropy of that
  distribution
• (2.38)
• at least this many bits of framing overhead to
  specify length

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Bit-Oriented Framing Flags

• If uniform P(K) 1/Kmax for 1 ? K ? Kmax
• H log2Kmax
• If geometric distance on lengths
• H log2EKlog2e for large EK(2.37)-1/2
• Note geometric have largest entropy.
  i.e.requires more bits than any other distance

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Bit-Oriented Framing Flags

• If one does this for a geometric distance
  ?unary-binary encoding
• In particular , for a given j , K i2jr0?rlt2j
• Encoding for K i?0s followed by a 1 (unary
  encoding of i) followed by ordinary binary
  encoding of r (j bits)
• For example if j2 , k7 ? i1,r3
• so, K 01 11

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Bit-Oriented Framing Flags

• End of encoding occurring j bits after the
  first 1 .
• In general
• K ?(maps) bit string of length ?K/2j? 1j
• Eoverhead ? EK2-j1j (2.40)
• ?(2.34)??
• Again , minimize by choosing j ? log2EK?

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2.5.4 Framing with Errors

• Error in the flag ? will not detect frame-end
  until next flag is detected CRC check with 2
  frames (undetected probability 2-L)
• Error within the frame to change a bit string
  into the flag 0160
• 0100110111001(sent)
• 0100111111001(received)
• Probability ? 1/32 Kp , where p is bit error
  probability using CRC, undetected probability
  2-L

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Framing with Errors

• If length field has error ? causes receiver to
  look for CRC in wrong place ? 2-L

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Framing with Errors

• There are several partial solutions to the above
  problems , but none are without disadvantages
• Using a fixed-length header for each frame , and
  put length of frame in header , and header has
  its own CRC
• If error in length field , still got CRC. But
  have to resynchronize after such an error. ??will
  not know when next frame starts. 2 CRCs must be
  used ? inefficient some what!

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Framing with Errors

• Put length field of one frame into trailer of the
  preceding frame
• Avoid inefficiency , but still requires a special
  synchronizing sequence after each detected error
• Require a special header whenever length of next
  frame is unknown
• Use a longer CRC
• Reduce probability of falsely accepting a frame

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Framing with Errors

• Regard framing as being at a higher layer than
  ARQ
• Packets would be separated by flags
• Resulting sequence of packets flags would be
  divided into fixed-length frames. ( If a packet
  ended in middle of a frame ? idle fill)
• Because of fixed-length , CRC would always be
  found
• Disadvantage delay
• A packet could not be accepted until entire frame
  containing end of packet was accepted

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2.5.5 Maximum Frame(packet) Size

• Variable packet lengths
• Most existing packet networks
• Use very short frames with a fixed length
• ISDN,ATM53bytes

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Maximum Frame(packet) Size

• Variable frame length
• Fixed number V of overhead bits
• Let Kmax maximum length of a packet
• Assume each message is broken up into as many
  maximum-length packets as possible with last
  packet containing what is left over
• message of length M ? ?M/Kmax? packets
• Total bits transmitted M ?M/Kmax? V

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Maximum Frame(packet) Size

• Kmax????
• As Kmax ?, ?
• fraction overhead?
• Processing on a frame basis
• Kmax ? , processing ?

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Maximum Frame(packet) Size

• Kmax????
• Pipeline
• Assume a packet must be completely received over
  one link before starting transmission over the
  next
• If message is broken into several packets ,
  earlier packets may proceed along the path on
  first link ? reducing overall message delay

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Maximum Frame(packet) Size
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Maximum Frame(packet) Size(pipeline contd)

• Let's consider combined effects of overheads
  pipelining
• Assume message transmitted over j equal-capacity
  links and that network is lightly loaded(no
  waiting packets) and ignore propagation delay
• Total time T required to transmit message to
  destination
• T time for first packet to travel over the
  first j-1 links time for entire message to
  travel over the final link

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Maximum Frame(packet) Size (pipeline contd)

• Assume M?Kmax
• Let C be capacity of each link
• TC number of bit transmission times (Kmax
  V)(j-1) M ?M/Kmax? V
• Approximate E?M/Kmax? EM/Kmax1/2
• ETC ? (KmaxV)(j-1)EMEMV/Kmax V/2

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Maximum Frame(packet) Size (pipeline contd)

• to minimize ETC
• so, Kmax ?
• An overhead V ? , Kmax ?
• As path length j ? , Kmax ?
• In practice , see if delay is important

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Maximum Frame(packet) Size (pipeline contd)

• Delay for stream-type traffic(such as voice)
• delay from when a given bit enters the network
  until that bit leaves
• Consider the light leading case
• Assume arrival rate R
• Packet length K
• The first bit in a packet is held up for K/R
  waiting time for packet to be assembled
• Along the path , link capacities C1,C2(gtR)
• A given packet is delayed by (KV)/Ci on i-th
  link

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Maximum Frame(packet) Size (pipeline contd)

• Total delay
• (KV)/Ci ? K/Ri for each link i
• T? as K? until (KV)/Ci K/R for some link
  yields minimum delay (i.e. small K)
• Under heavy-loading
• Use long packets by some users increases delay
  for all users

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Maximum Frame(packet) Size

• Retransmission
• increase waiting time
• small maximum packet size
• Large frame have higher probability of error than
  small frames
• In WAN , frame size ?order of 1 a few K bits
• In LAN , much longer frame size , ??single multi
  access link

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Maximum Frame(packet) Size

• Fixed frame length
• Determine end of frame ?same
• Optimal packet length ? smaller

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Homework One

• 2.15
• 2.16
• 2.17
• 2.30
• 2.33
• 2.39
• ???

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Section 2.8

• Point-to-Point Protocols
• at Network Layer

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• Major issues
• Routing
• Flow control
• Here, smaller issue
• Transfer of packets between a pair of nodes
• Distinguish packets of one session from another
• Distinguish packets within same session.

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2.8.1 Session ID addressing

• Brute-force approach source destination ID
  allows different packets of same paths ? But
  considerable overhead
• If uses virtual circuit(VC) ?far less overhead
• Each link shared by a set of virtual channels
• When set up, a path is established,use one unused
  virtual channel to that session.

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Virtual Channel
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Session ID addressing

• Headervirtual channel
• ?because limited capacity of link.
• so upper bound on of sessions
• ?use a fixed of bits(overhead)

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Session ID in TRMNET 1970
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Session ID addressing

• Note data from multiple sessions are included in
  a single frame
• Go back n ARQ with modulo m8
• Virtual channel identify session
• ?statistical multiplexing link is multiplexed
  between virtual channels
• Sessions are served in round-robin
• Each frame takes some chars from each session

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Session ID in codex networks

• Again, multiple sessions transmitting chars
  within the same frame
• But, a more efficient encoding is used to
  represent VC
• Four bit nibbles

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Figure 2.49
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Figure 2.49

• Map char into one nibble(highly probable)
• Map char into two nibble(less likely)
• Map char into three nibble(least likely)
• Packet header1 nibble for start record
• 1 or more nibble for
  session
• -under heavy loadingheader 1byte

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2.8.2 packet numbering, Window Flow Control,Error
Recovery

• Reorder or re-tx packets?need packet numbering
  can be done in Network or Transport Layer
• This is necessary, because
• at DLC Layer
• 1. CRC could fail
• 2. If link fail, DLC dont know how many packets
  was sent
• 3. If node fail, packets stored at the node are
  lost

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Error Recovery(at Network or Transport Layer)

• Similar to ARQ at DLC Layer
• Each packet within a session is numbered modulo
  m2k
• Different between end-to-end and DLC
• 1. End-to-end involves 2 sites and subnet in
  between
• DLC2 nodes with a link in between
• 2. Sequence numbering for end-to-end error
  recovery involves only packets of a given
  sessions.
• DLCall packets using the link
• 3. frame stay in order on a link, packets might
  arrive out of order in a network.

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Error Recovery

• NoteVC would guarantee in order for same
  session,
• but,link fails?a new path is set up.
• Last packet on old path might arrive out of
  order,relative to first packet on new path.
• Packet retx, and arbitrary delay?earlier copy can
  remain in network until packet numbering wraps
  around?trouble.

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Error Recovery

• 1. insist use virtual circuits
• 2. Modulus large enough
• 3. Packets are destroyed after lifetime.

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Flow Control

• Use of either go back n or selective repeat
  ARQ,also provide flow controlwindow sizen
• Some limitation
• 1. If ACK not arrive because of error, packet
  should be re-tx.
• If ACK is delayed due to congestion, packet
  should not be re-tx.
• ?difficult to set time-out

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Flow Control

• Partial solution(at destination)
• 1. Slow down the source, but not delay ACK.
• 2. permittells source how many packets is
  prepared to receive.
• RNprovide ACK

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DiscussionError recovery at transport vs network
layer

• From practical standpoint, belongs to transport
  layer.
• Large of networks do not provide reliable
  service
• E.g. internet protocol
• Disadvantage for error recovery at transport
• ACK are slow due to congestion
• ACK was throw away
• ?no way to distinguish

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DiscussionError recovery at transport vs network
layer

• When congestion ,
• Packet retx?greater congestion
• More dropped packets?unstable

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2.8.3 X.25(self-reading)
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2.8.4 The Internet Protocol(IP)

• 2 transport layer protocols were developed along
  with IP
• TCP(transmission control protocol)reliable VC
  service
• UPP(user datagram protocol)simply where to sent
  datagram to destination

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2.8.4 The Internet Protocol(IP)

• Primary functions of IP
• 1.route datagram through internet
• 2.provide addressing to identify source
  destination
• 3.fragment datagram into smaller pieces
• Addresses in IP32 bits
• 1.class A(0)7 bit network ID, 24 bit host ID
• 2.class B(10)14 bit network ID,16 bit host ID
• 3.class C(11)22 bit network ID,8 bit host ID

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2.9 Transport Layer

• Functions
• 1. Breaking message into packets
• 2. error recovery
• 3. flow control
• 4. Multiplexing / Demultiplexing sessions
  together

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2.9.2

• In ICP, entire address of a source(or
  destination)is called a socket.
• (Network ID, host ID?in IP header, user/process
  ID within that node?port?in transport header)
• All session between same pair of hosts are
  multiplexed together at transport layer into a
  common lower-layer session

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2.9.3 Error recovery in TCP

• Enlarge modulus to allow out-of-order packet
  arrivals.
• Some complications
• 1. retx timeouts (retx due to congestion,)
• (if done at network layer, better estimate delay)

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2.9.3 Error recovery in TCP

• 2. Setting up tearing down connection
• Packet from an old connection entering a new
  connection
• Packet still exist at some nodes after loss of
  connection

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2.9.4 Flow control in TCP/IP

• Source monitor round-trip delay
• Provide measure of congestion
• Unfair
• good citizen session cut back under congestion,
• bad citizen do not cut back
• ?network resources should be shared fairly, need
  to have individual control over session.
• But, network layer does not know session.

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2.10.1 ATM(Asynchronous Transfer Mode)

• Cell 53 bytes 485(header)
• Because packetization delay of voice
• E.g. 64 kbps voice(by 8 bit sample each 125
  ?sec), time to collect 48 bytes6 ms.
• ATM uses VC
• Address
• (user-network interface) 24 bits?16 million
  sessions
• (Internal subnet )28 bits ?268 million.

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2.10.1 ATM(Asynchronous Transfer Mode)

• Address field is divided into 2 subfield
• VCI(Virtual channel identifier)
• VPI(Virtual path identifier)
• ?sessions sharing same path are assigned same VPI
  to be switched together

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2.10.1 ATM(Asynchronous Transfer Mode)

• Use CRC to correct errors(not just detect)
• Bit error rate(fiber)lt10-9
• Single error correction might remove most
  errors(note CRC checks only header)
• Why?(voice,videos, tx)
• Neither ARQ nor framing is required
• Bit error rate is quite low
• Voice, video.

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2.10.2 Adaptation layer

• Breaking incoming source data into ATM
  cells.(network layer)
• Used only on entry exit from the
  network(transport layer)
• Not yet well-defined
• Different types of input
• Class 1CBR64 kbps voice
• Class 2VBRpacketized voice or video
• Class 3connection-oriented data
• Class 4connectionless datadatagram

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2.10.2 Adaptation layer

• Class 3(connection-oriented) traffic
• Split into 2 sublayer
• Convergence segmentation/reassembly
• ?flow control error recovery

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2.10.2 Adaptation layer

• ?Class 4(connectionless) traffic
• Routed still by VC??Trouble
• Passes through adaptation layer, use a datagram
  switch
• Use permanent VC to carry all datagram between 2
  switches

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2.10.3 Congestion

• Congestion is still a problem for broadband
  networks
• Due to large data rate for video
  applications,etc.
• 3 mechanisms
• 1. At connection setup time, negotiate QoS,
  burstiness.
• 2. Monitor each connection
• 3. Use priority bit in ATM header

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2.10.3 Congestion

• For connectionless, no negotiation,
• Carefully controlled to avoid excessive delays
  for connection-oriented sessions.
• Usually, a higher-layer network handles this.
• For negotiation ?QoS guarantee

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2.10.3 Congestion

• ?For monitor regular ?e.g. leaky bucket
• (noteFor light loaded,allow excessive traffic )
• Discarded later when congestion
• For priorityby user to indicate low priority
  traffic
• E.g. video/voice compression.

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Session ID addressing