Cell Networking

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Cell Networking

• Carey Williamson

Department of Computer Science University of
Calgary
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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

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Rationale for Cell Networks

• Simpler interfaces
• Simpler buffering
• Simpler switches
• slotted system
• synchronous stages
• Better control of delay
• Better control of delay jitter

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Delay Example
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Delay Example (Contd)
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
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Delay Example (Contd)

• In the 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

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Delay Example (Contd)
Job 1, Size 10, arrives at time 0

Job 2, Size 2, arrives at time 3
Server

Depart
Job 1 departs at time 12 Job 2 departs at time 7
(assumes jobs are equal priority)
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Delay Example (Contd)
Job 1, Size 10, arrives at time 0

Job 2, Size 2, arrives at time 3
Server

Depart
Job 1 departs at time 12 Job 2 departs at time 5
(assumes job 2 is higher priority)
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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

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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

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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

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Why 53 bytes? (Contd)

• 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)

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Why 53 bytes? (Contd)

• 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

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Why 53 bytes? (Contd)

• 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

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Why 53 bytes? (Contd)

• 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 _at_ 64 kbps 6 msec
• video probably wants big(ger) packets

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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!!