Cell Networking

							Cell Networking

   Carey Williamson
Department of Computer Science
      University of Calgary


            1
            Definition

 The term “cell networking” means
  packet switching, but with fixed size
  packets (called cells)
 Contrast with “frame relay” which
  uses variable size packets
 ATM is an example of cell
  networking with 53-byte cells
                  2
Rationale for Cell Networks

 Simpler interfaces
 Simpler buffering
 Simpler switches
  » slotted system
  » synchronous stages
 Better control of delay
 Better control of delay jitter
                    3
Delay Example




     4
     Delay Example (Cont’d)

                           Job 1, Size 10,
                           arrives at time 0

+           Job 2, Size 2,
            arrives at time 3      Server
=
                                           Depart
    Job 1 departs at time 10
    Job 2 departs at time 12
                       5
   Delay Example (Cont’d)

 Inthe Internet, for example, a small
  packet (e.g., a telnet packet) that
  happens to get stuck behind a large
  packet (e.g., an ftp packet) can
  experience a large delay
 FIFO service, non-preemptive



                 6
     Delay Example (Cont’d)

                           Job 1, Size 10,
                           arrives at time 0

+           Job 2, Size 2,
            arrives at time 3      Server
=
                                             Depart
    Job 1 departs at time 12       (assumes jobs
    Job 2 departs at time 7        are equal priority)
                       7
     Delay Example (Cont’d)

                           Job 1, Size 10,
                           arrives at time 0

+           Job 2, Size 2,
            arrives at time 3      Server
=
                                              Depart
    Job 1 departs at time 12       (assumes job 2
    Job 2 departs at time 5        is higher priority)
                       8
      Advantages of Cells

 High   priority or delay-sensitive
  traffic will likely spend less time
  “stuck behind” other traffic
 The smaller the cell, the better
 Lower mean delay, and lower
  variation of delay
 Easier to provide performance
  guarantees to integrated traffic
                      9
            Summary

 In addition to the ease of
  implementation considerations,
  cell based networks offer a better
  framework for providing delay
  guarantees on integrated traffic
  flows (e.g., data, voice, video)
 That is why ATM uses cells

                  10
         Why 53 bytes?

 The   smaller the cell, the better
  (in terms of delay guarantees)
 Need to design for traffic with the
  most stringent delay requirements
 Considerations for voice traffic were
  an overriding concern

                  11
   Why 53 bytes? (Cont’d)

 The  ATM cell size was chosen by
  the CCITT international standards
  committee (now called ITU)
 Influenced by voice traffic
  requirements and existing telco
  equipment in place at the time
  (e.g., echo cancellation)

                 12
   Why 53 bytes? (Cont’d)

 European community   wanted 32
  bytes of data per ATM cell
 American community wanted 64
 Result: compromise!
  » (32 + 64) / 2 = 48
  » thus, 48 bytes of data per ATM cell
 Both   sides equally (un)happy
                   13
   Why 53 bytes? (Cont’d)

 European community   wanted 4
  bytes of header per ATM cell
 American community wanted 6
 Result: compromise!
  » (4 + 6) / 2 = 5
  » thus, 5 bytes of header per ATM cell
 48   + 5 = 53 bytes per ATM cell
                   14
   Why 53 bytes? (Cont’d)

 Equally  inefficient for all types of
 traffic (data, voice, video)
  » data networks want big packets
  » ATM overhead is 5/53 = 10% (too high!)
  » voice networks want small(er) packets
  » 48 bytes @ 64 kbps = 6 msec
  » video probably wants big(ger) packets


                    15
             Summary

 53 bytes is now the international
  standard for ATM cell size
 “Only a standards committee could
  come up with a packet size that is
  a prime!’’             (Raj Jain, 1993)
 Live with it; everyone else is!!


                   16