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Problems with synchronization and clock recovery cause errors like...

Problems with synchronization and clock recovery on Ethernet cause errors like... using old exam questions that are no longer relevant to practical networks.

Clock recovery was an important consideration is the very early days of Ethernet, when marginal 10 Mb/s PLL circuits could produce alignment errors. This is no longer an issue of any real significance; in particular, Fast and Gigabit Ethernet all use continuous-signaling systems, where clock recovery is not performed on a frame-by-frame basis.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Reply to
Rich Seifert
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Hi.

"Problems with synchronization and clock recovery on Ethernet cause errors like..."

I've got a question in a test and can't find what that synchronization drifts can do. Packet loss? Something else, more specific?

Thanks in Advance, Chris.

Reply to
Krzysiek Orlikowski

Possibly I am thinking of something different than you are, but the IFG is not 64 bit times but rather 96 bit times. 48 bit times would be half of the IFG not 2/3.

64... that sounds like the -bytes- for the minimum ethernet packet, with 64 bytes minus 1 bit allowed for collisions.
Reply to
Walter Roberson

(snip)

As they are asynchronous, a station can start sending at any time.

The 2/3 rule removes any advantage to a device with a slightly faster clock. As each has its own crystal oscillator, they will never be exactly the same.

-- glen

Reply to
glen herrmannsfeldt

Rich Seifert napisa³(a):

Thanks. I know the test's old, but I didn't do it, I just got to pass it ;)

Another question is about the interframe gap in CSMA/CD Ethernet. Why would a station make a decision to start sending in exactly 2/3 of an IFG (64bit times)? The minimum is 47BT in this configuration so why

64BT? It's the minimum in Gigabit Ethernet, but here?

All I know is that 2/3 of IFG ist the time it still listens for a present carrier.

Chris.

Reply to
Krzysiek Orlikowski

Yep, IFG is 96 BT, all I know is to know what conditions are met when a station sends in 2/3 of the IFG (it's not written '64', I just calculated 2/3 of 96BT so I got 64BT)

Answer could be : "it's a Gigabit Ethernet and a hardware vendor used the smallest IFG possible - 64BT) to get highest data rates, but a) 1000Base harldy ever operates in half-duplex (that's what I've heard) and b) 99% of this test ist about 10Base and 100Base so my answer is probably wrong :/

Chris.

Reply to
Krzysiek Orlikowski

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