Chris, FYI - taken from
http://www.lanart.com/learning/whitepapers/gig_wp.htm
In Ethernet, the smallest packet size allowed is 64 bytes (8 bits per byte =
512 bits). The purpose of establishing a minimum packet size was to ensure
that a station could detect collisions at the furthest point of the network,
allowing the CSMS/CD portion of the protocol to deal with it appropriately
(referred to as the 512 bit-time rule). As speed increases by factors of 10
(10 Mbps to 100 Mbps to 1 Gbps), the distance that you can transmit and
still properly detect collisions is decreased by a factor of 10.
Consequently, at Gigabit speeds in a shared Ethernet environment, you are
limited to about 20 meters over UTP.
The Gigabit standard addresses this distance limitation issue by a method
known as "carrier extension." Carrier extension effectively increases the
packet size to 512 bytes (4096 bits), by adding "extension symbols" to
increase the size of the packet to a size that can be detected by all
devices on a Gigabit link up to 100 meters away. The end device then strips
this additional data or "extension symbols" off when it is received. The
problem is that increasing the packet size (adding 448 bytes of extension
symbols) means that you have actually decreased the throughput to about 100
Mbps Fast Ethernet speed. (Sending larger amounts of data down a larger pipe
nets you no significant gain.)
To deal with this reduction in throughput, a method known as "packet
bursting" is used in conjunction with carrier extension. Packet bursting
improves the efficiency of carrier extension by decreasing the inter-packet
gap when multiple packets are transmitted. (Reducing the amount of data you
send down a larger pipe nets you a nominal gain.) However, even when both
methods are used, throughput in half-duplex Gigabit remains hindered and
never achieves full 1 Gbps speed. The bottom line is that half-duplex is
possible but not recommended in Gigabit environments.
Carrier detection and packet bursting are not required in a full-duplex
Gigabit environment.
Regards
Giles
-----Original Message-----
From: Burnham, Chris [mailto:[EMAIL PROTECTED]]
Sent: Friday, June 01, 2001 12:26 PM
To: [EMAIL PROTECTED]
Subject: Gigabit Ethernet Signal propagation. [7:6739]
Does Gig Ethernet conform to the same minimum frame sizes as general
Ethernet ie 64 bytes.If so how does it accomodate late collisions when
running over distances of 2km or more.???????????
Or is this not relevant as it will be point to point full duplex and
effectively collision free?????????
Please help it is giving me brain strain
Chris Burnham,
Systems Engineer,
Delphis Consulting Plc.
Tel: +(44) 020 7916 0200
Mob: +(44) 07799403576
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