Method and apparatus for arbitrating bandwidth in a communications switch
A network switch includes at least one port processor and at least one switch element. The port processor has an SONET OC-x interface (for TDM traffic), a UTOPIA interface (for ATM and packet traffic), and an interface to the switch element. In one embodiment, the port processor has a total I/O bandwidth equivalent to an OC-48, and the switch element has 12×12 ports for a total bandwidth of 30 Gbps. A typical switch includes multiple port processors and switch elements. A data frame of 9 rows by 1700 slots is used to transport ATM, TDM, and Packet data from a port processor through one or more switch elements to the same or another port processor. Each frame is transmitted in 125 microseconds; each row in 13.89 microseconds. Each slot includes a 4-bit tag plus a 4-byte payload. The slot bandwidth is 2.592 Mbps which is large enough to carry an E-1 signal with overhead. The 4-bit tag is a cross connect pointer which is setup when a TDM connection is provisioned. The last twenty slots of the frame are reserved for link overhead. Thus, the frame is capable of carrying the equivalent of 1,680 E-1 TDM signals. For ATM and packet data, a PDU (protocol data unit) of 16 slots is defined for a 64-byte payload. The PDUs are self-routed through the switch with a 28-bit routing tag which allows routing through seven switch stages using 4-bits per stage. Bandwidth is arbitrated among ATM and Packet connections while maintaining TDM timing.
1. A method for arbitrating bandwidth in a communications switch, comprising:
a) generating a repeating data frame having a plurality of rows;
b) making requests during row N for space in row N+1; and
c) granting the requests through an out-of-band link.
2. A method according to claim 1 , wherein:
each request includes through-the-switch routing information and priority level information.
3. A method according to claim 2 , wherein said switch is a multistage switch, said method further comprising:
d) buffering the request at each stage of the switch;
e) discarding low priority requests when the buffer reaches a threshold.
4. A method according to claim 3 , wherein:
said step of granting requests includes returning requests which have not been discarded before reaching the egress of the switch.
5. A method according to claim 1 , wherein:
each request for space is for a 52-byte chunk of space.
6. A method according to claim 5 , wherein bandwidth is arbitrated among ATM cells and variable length packets, said method further comprising:
d) segmenting each packet larger than 52-bytes into a plurality of 52-byte chunks.
7. A method according to claim 6 , wherein:
each request includes through-the-switch routing information and priority level information.
8. A method according to claim 7 , wherein said switch is a multistage switch, said method further comprising:
e) buffering the request at each stage of the switch;
f) discarding low priority requests when the buffer reaches a threshold.
9. A method according to claim 8 , wherein:
said step of granting requests includes returning requests which have not been discarded before reaching the egress of the switch.
10. A method according to claim 9 , further comprising:
g) discarding requests for all following segments of a packet when a request for one segment of the packet has been discarded.
11. A method according to claim 1 , wherein:
said requests are made in-band.
12. A method according to claim 1 , wherein:
said requests are made out-of-band.
13. An apparatus for arbitrating bandwidth in a communications switch, comprising:
a) means for generating a repeating data frame having a plurality of rows;
b) means for making requests during row N for space in row N+1; and
c) means for granting the requests through an out-of-band link.
14. An apparatus according to claim 13 , wherein:
each request includes through-the-switch routing information and priority level information.
15. An apparatus according to claim 14 , further comprising:
d) means for buffering the request at each stage of the switch; and
e) means for discarding low priority requests when the buffer reaches a threshold.
16. An apparatus according to claim 15 , wherein:
said means for granting requests includes means for returning requests which have not been discarded before reaching the egress of the switch.
17. An apparatus according to claim 13 , wherein:
each request for space is for a 52-byte chunk of space.
18. An apparatus according to claim 17 , wherein bandwidth is arbitrated among ATM cells and variable length packets, said apparatus further comprising:
d) means for segmenting each packet larger than 52-bytes into a plurality of 52-byte chunks.
19. An apparatus according to claim 18 , wherein:
each request includes through-the-switch routing information and priority level information.
20. An apparatus according to claim 19 , further comprising:
e) means for buffering the request at each stage of the switch; and
f) means for discarding low priority requests when the buffer reaches a threshold.
21. An apparatus according to claim 20 , wherein:
said means for granting requests includes means for returning requests which have not been discarded before reaching the egress of the switch.
22. An apparatus according to claim 21 , further comprising:
g) means for discarding requests for all following segments of a packet when a request for one segment of the packet has been discarded.
23. An apparatus according to claim 13 , wherein:
said requests are made in-band.
24. An apparatus according to claim 13 , wherein:
said requests are made out-of-band.