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From: beach@ddnuvax.af.mil (beach)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Frame Relay?
Message-ID: <35495@boulder.Colorado.EDU>
Date: 30 May 91 11:03:19 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 30

Linda,
  The major prerequisite is to have a network which uses frame relay as
an access protocol.  Frame relay is more flexible than simply using
T-1 pipes to interconnect routers.  The reason is that frame relay
gives you a mechanism (albeit limited) to individually address multiple
end systems on the network while only requiring a single port on the
router, i.e. one connection to the network.  If you simply use big
pipes, then you have to get a separate port for each destination you
want to talk to.  That gives you a pretty limited fan-out per router,
and would increase tandem traffic on intermediate routers.  It would
also complicate the basic design of connectivity as you increase the
number of routers.
   Frame relay lets you preestablish (key word) what are essentially
permanent virtual circuit from one end point to some number > 1 other
end points.  Which PVC a "frame" uses is determined by an address
mechanism using a field in the frame header called the DLCI.  Normally
a particular DLCI refers to a particular PVC in the network.  This
isn;t always the case.
   At least one vendor will let you configure a DLCI to represent an
end system, instead of a particular circuit.  You can also use a
mechanism to discover the IP address of what's on the other end of a
DLCI dynamically, as well as find out what DLCIs are available to use,
again dynamically.

/darrel
---------
Darrel Beach	       	     beach@server.af.mil	snail-mail:
SAIC Montgomery
Network Systems Engineer     205-279-4075        	SSC/SSMT
USAF DDN Program Office	     AV: 446-4075       	Gunter AFB, AL 36114