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Cell Networking

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  1. Cell Networking Carey Williamson Department of Computer Science University of Calgary

  2. 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

  3. Rationale for Cell Networks • Simpler interfaces • Simpler buffering • Simpler switches • slotted system • synchronous stages • Better control of delay • Better control of delay jitter

  4. Delay Example

  5. 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

  6. Delay Example (Cont’d) • 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

  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 Job 2 departs at time 7 (assumes jobs are equal priority)

  8. 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 Job 2 departs at time 5 (assumes job 2 is higher priority)

  9. 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

  10. 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

  11. 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

  12. 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)

  13. 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

  14. 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

  15. 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

  16. 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!!

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