3com 1005 Manual De Usuario

DUA1840-0AAA01

Switch Operation and Features

Intelligent Flow Management

Intelligent Flow Management

(IFM)

is a congestion control mechanism

built into the Switch 1005. Congestion is caused by one or more

devices sending traffic to a Switch port which is already busy. The

Switch 1005 contains both input and output packet buffers and while

congestion is rare, IFM is designed to alleviate problems during those

moments when packet buffers in the Switch 1005 are full. IFM will

prevent packet loss by inhibiting the transmitting device from sending

any further packets until the port is no longer congested.

If a packet arrives at a conventional switch that does not operate IFM,

and that port is congested, the transmitting device is unaware of this

until it times out and decides that the receiving station is not going to

respond to the message. This can take as long as 30 seconds, and

depending on the protocol you are running, may not happen until

many packets have been sent. The transmitting device then has to

retransmit the packets, effectively wasting bandwidth.

Switch

modules

implementing IFM are aware of congestion, and

prevent packet loss by inhibiting the transmitting device from

transmitting the packet in the first place. It does this by forcing the

device to retransmit the packet later. This “back-off” and retransmission

happens very quickly (typically less than one second) and is much

faster than waiting for the transmitting device to time-out. There are

two benefits:

■

the packet is transmitted quickly and successfully

■

the packet is only transmitted once, thereby saving bandwidth.

IFM is designed to be enabled on ports connected to a single network

device. If IFM is enabled on a port connected to multiple devices

through a repeater, packet congestion within the Switch 1005 could

result in packet transmission between two devices connected to the

repeater being inhibited.