Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!hp-col!hpctdlb!franka
From: franka@hpctdlb.HP.COM (Frank Actis)
Newsgroups: comp.dcom.lans
Subject: Re: More questions regarding the LAN
Message-ID: <31710002@hpctdlb.HP.COM>
Date: 21 Feb 91 23:50:56 GMT
References: <2334@enuxha.eas.asu.edu>
Organization: Hewlett-Packard CTD, Colo. Spgs.
Lines: 49

>  From: gdrao@enuxha.eas.asu.edu (Giridhar Rao)
>  Subject: More questions regarding the LAN
>  
>  Hi netters,
>  
>  I have some more questions regarding the token ring ( IEEE 802.5 ). I wish to
>  thank those guys who responded to my previous queries.
>  
>  	Does a station in a token ring regenerate the token immediately
>  	after the last bit in the frame is sent  OR does it generate a 
>  	new token until its frame has gone all the way around and come
>  	back ? What does the IEEE 802.5 follow ?

This question has two different answers, depending on what you have.  
Normally, a station will wait until the frame that it has transmitted comes 
around the ring before the token is generated.

There is a feature called "early token release" which allows the
station to generate a token as soon as the frame has been transmitted.
With early token release, it is possible for several tokens to be on the
LAN at once.

>  
>  	What consequences arise if the destination station removes the
>  	frame instead  of the sender ? The IEEE 802.5 standard says that
>  	sender removes the frame ? Any advantages in that ? Isn't it a
>  	lot of overhead ?

The method of the sending station removing his (or her) own frames
is a simple method of making sure that the destination system received the
frame properly and that the ring is working.  If the destination (or anyone 
else for that matter) were to remove the frame before it came back to 
the sender, the sender would never regenerate the token, causing the
active monitor to purge the ring and send out a new token.

The sending station knows that the frame has been received by looking at
a couple of bits on the frame that has been set by the destination system.
These bits are the "address recognized" (I saw the frame sent to me) and 
the "frame copied" (I was able to pull the frame off the LAN and into my 
buffers) which tell the sender if the remote station is up and that it
received the frame.

One thing to keep in mind is that most rings are small enough that the sending
station will start receiving its frame while it is still sending it so stations
using early token release rarely get to send an early token.

Frank Actis
Hewlett-Packard
franka@hpctdlb.hp.com