Cell Networking Carey Williamson Department of Computer Science

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

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 2

Rationale for Cell Networks l Simpler interfaces l Simpler buffering l Simpler switches » slotted system » synchronous stages l 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) 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 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 Job 2 departs at time 7 7 (assumes jobs are equal priority)

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 8 (assumes job 2 is higher priority)

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 9

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 10

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 11

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) 12

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 13

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 14

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 15

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!! 16