| The four repeater rule in Ethernet
states, that no more than four repeaters or repeating hubs can be
between any two computers on the network. To assure that a repeated
10BASE-T network will function properly, the following condition must
be true: (repeater delays + cable delays + NIC delays) x 2 <
maximum round-trip delay. Repeater delays for 10BASE-T are usually
less than 2 microseconds per repeater; cable delays are near 0.55
microseconds per 100 m trip; NIC delays are about 1 microsecond per
NIC; and the maximum round-trip delay (the 10BASE-T bit time of 0.1
microseconds times the minimum frame size of 512 bits) is 51.2
microseconds. For a 500 m length of UTP connected by 4 repeaters
(hubs) and 2 NIC delays the total delay would be well below the
maximum round-trip delay. Repeater latency, propagation delay, and NIC
latency all contribute to the 4-repeater rule. Exceeding the four
repeater rule can lead to violating the maximum delay limit.
When this delay limit is exceeded, the number of late collisions dramatically increase. A late
collision, is when a collision happens after the first 64 bytes of the
frame are transmitted. The chipsets in NICs are not required to retransmit
automatically when a late collision occurs. These late collision frames add delay referred
to as consumption delay. As consumption delay and latency
increase, network performance decreases. This Ethernet rule of thumb is also known as the 5-4-3-2-1 rule. Five sections of the network, four repeaters or hubs, three sections of the network are "mixing"
sections (with hosts), two sections are link sections (for link purposes), and one large collision domain.
|