Many switches today employ some sort of flow control, either in 802.3x form for full duplex or carrier-sense jamming for half duplex. In either case, it is important to understand the benefits - such as traffic preservation during congestion and more efficient use of switch resources - and be aware of compatibility issues and potential negative effects on network performance.
Flow control can ensure better throughput by throttling devices that send traffic when the switch is overloaded, thereby reducing potential frame loss when a switch gets congested. On the flip side, flow control can add latency and traffic to a potentially congested segment or, even worse, to an uncongested one.
When a congested full-duplex device sends a "slow down" frame to an adjacent device, that device holds the frame in its buffers, increasing latency. By contrast, a switch employing half-duplex carrier-sense jamming essentially raises the carrier sense when it requires an upstream switch to stop sending data, thereby causing collisions and buffered - or even lost - frames. Each approach increases latency, either by delaying frames or causing retransmissions of dropped frames. That's not to say that an increase in latency may not be better than protocol recovery from frames lost in congestion, however; unpredictable latency can negatively affect high priority or time-sensitive traffic.
RELATED LINKS
Switching grows up: Layer 3 switching completes a circle, giving us pause to relect on where switching has brought us and where it's headed. Network World, 5/4/98.
Switching's dark side: Why packet collisions can wreak havoc with LAN and ATM switch performance, but you can avoid them. Network World, 2/7/97.
Ethernet Switching: An Anixter Technology White Paper
Hot switches, cool features: HP, Cisco and FORE combine great performance and flexibility to lead tests of 100Base-T switches. Network World, 8/11/97.
