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

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

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

4. 4 Delay Example

5. 5 Delay Example (Cont’d) Job 1, Size 10, arrives at time 0 Job 2, Size 2, arrives at time 3 + = Job 1 departs at time 10 Job 2 departs at time 12 Server Depart

6. 6 Delay Example (Cont’d) l 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 l FIFO service, non-preemptive

7. 7 Delay Example (Cont’d) Job 1, Size 10, arrives at time 0 Job 2, Size 2, arrives at time 3 + = Job 1 departs at time 12 Job 2 departs at time 7 Server Depart (assumes jobs are equal priority)

8. 8 Delay Example (Cont’d) Job 1, Size 10, arrives at time 0 Job 2, Size 2, arrives at time 3 + = Job 1 departs at time 12 Job 2 departs at time 5 Server Depart (assumes job 2 is higher priority)

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

10. 10 Summary l 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) l That is why ATM uses cells

11. 11 Why 53 bytes? l The smaller the cell, the better (in terms of delay guarantees) l Need to design for traffic with the most stringent delay requirements l Considerations for voice traffic were an overriding concern

12. 12 Why 53 bytes? (Cont’d) l The ATM cell size was chosen by the CCITT international standards committee (now called ITU) l Influenced by voice traffic requirements and existing telco equipment in place at the time (e.g., echo cancellation)

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

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

15. 15 Why 53 bytes? (Cont’d) l 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. 16 Summary l 53 bytes is now the international standard for ATM cell size l “Only a standards committee could come up with a packet size that is a prime!’’ (Raj Jain, 1993) l Live with it; everyone else is!!