At 10:54 AM 12/7/01, Michael Williams wrote: >Richard, > >Here's the damnest thing tho. I've asked many networking professionals, >including two CCIEs (not candidates), and no could seem to know 100% about >whether serial links were half or full duplex. Sad, eh?
It's not sad. Most serial, WAN technologies have been full duplex for so long that the question just doesn't come up. Each side has a dedicated transmit circuit (pair of wires), separate from the receive circuit. On a T1 link, to use your example, each side can transmit 1.544 Mbps. The two sides can do this simultaneously. Old style WAN links used a single cable, like the string between two cans in the game we played as kids. They were half duplex and required a station to send a Go Ahead message to tell the other side it was its turn. You sill see remnants of this in protocols used today. Telnet has a Go Ahead message for example. (It's not used anymore, though.) The LAN people stole the "full duplex" term to refer to Ethernet switched, point-to-point links, which are no longer really CSMA/CD. They aren't multiple access (MA) and since there's no chance of someone else using your transmit circuit, there's no need to do CS either. Each side has its own dedicated transmit pair. Big deal. That's been the case on WANs since like the 1940s or something. Well, maybe the 1970s. Old-style Ethernet used coax cable. There wasn't a separate transmit and receive pair. All devices shared the cable. Think about what a coax cable looks like: single, center copper core, surrounded by cladding. Bits sent by one station radiate outwards to all other stations who have no choice but to receive them. This isn't really "half duplex" and nobody called it "half duplex" until the time when the Ethernet people borrowed the term "full duplex" to refer to the point-to-point link between a device and its switch port. That's my take, anyway! Priscilla > I'm tempted to >setup a test with 2 PCs and 2 routers with a low bandwidth connection >(128Kbps or so) and try to jam traffic through both ways and monitor the >speeds and see what I get. But I've got that nagging in the back of my mind >that you're correct and serial links are indeed full duplex. > >Having said that, what are your thoughts on my question about a point to >point T1 link at 1.544 Mbps? Is that 768Kbps each way or 1.544each way? > >Mike W. > >richard beddow wrote: > > > > Mike, > > > > I have three final comments then I think enough has been said. > > > > 1. Ethernet has always been a half duplex standard until recent > > times, FDX operation is always quoted therefore to make the > > distinction from the default. > > > > 2. Serial lines, however, since the late seventies-early > > eighties have been by default a full duplex offering. > > Therefore FDX is assumed unless otherwise stated. > > > > 3. My comments were not ment to offend but supposed to be > > funny. Humour on a forum such as this is often missread and I > > should have learnt my leason along time ago but I just can't > > help it. > > > > Charles do you have your answer??????? > > > > > > RB ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=28458&t=28270 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
