Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!lll-tis!helios.ee.lbl.gov!pasteur!agate!ig!uwmcsd1!leah!itsgw!batcomputer!cornell!rochester!udel!princeton!njin!aramis.rutgers.edu!athos.rutgers.edu!hedrick From: hedrick@athos.rutgers.edu (Charles Hedrick) Newsgroups: comp.dcom.lans Subject: Re: Ethernet bridges above 56Kb Message-ID: Date: 23 Jul 88 00:57:25 GMT References: <9392@dartvax.Dartmouth.EDU> Organization: Rutgers Univ., New Brunswick, N.J. Lines: 27 To: andyb@burcoat.uucp >All the bridges that I have seen either use an entire T1 circuit, or >use only 56Kb of it. I have never heard of a bridge that actually expects T1-formatted data. In general bridges and routers expect a standard synchronous line. If you have a 56K circuit, you just connect them to the modem, CSU/DSU, or whatever. If you have a T1 circuit, things are more complex. T1 is not just a 1.5 Mbps line. It has a fancy format of its own, which is really designed to multiplex multiple 56K subchannels. To use a whole T1 circuit with a bridge or router, you have to use a box that takes a generic 1.5 Mbps signal and puts it into the funny T1 format. I call such a box a T1-izer. If you want to do several different things with your T1 line, then instead of a simple T1-izer, you use a multiplexer. This can take several generic signals, whose bandwidth totals 1.5 Mbps or less, and multiplexes them onto a T1 line. In general the bridges and routers don't know and don't care exactly how fast a line is. They just lock onto the clock that comes from the T1-izer of multiplexer. You can use them with a T1-izer if you want to use the whole bandwidth of your line, or you can hook them to one channel of a T1 mux if you want to share the line. This certainly applies to Ungermann-Bass and cisco equipment, and I'm reasonably sure it applies to other vendors as well. Cisco's high-speed serial card will work at any speed from 9600 to 3Mbps (in theory, though I don't know of anyone who has the equipment to use it above T1 speeds). The only way to build a bridge that would work with a whole T1 but not with a piece of a T1 would be if the bridge dealt with the T1 formatting itself. I've never heard of such a beast.