It's tempting to say that the terms DTE and DCE only apply to WANs, but, in fact, IEEE uses the term DTE to refer to stations, nodes, hosts that reside on LANs. When talking about "interconnect" devices, they call them by their actual name, such as repeater or bridge. I don't think they ever use the term DCE, although IEEE documents don't have indexes, so it's hard to say for sure. Argh.
Does Cisco Networking Academy spell out DCE correctly (per ITU-T) as Data Circuit-Terminating Equipment? If so, that might help explain the difference. In LANs, we have end-to-end virtual circuits (with TCP, etc.) The "terminating" devices are the end points themselves. (Make those students learn a little TCP while you're at it?? ;-) One really couldn't claim that a LAN repeater, bridge, switch, or router terminates a circuit (with the exception of kludges like local ACK with LLC2.) Well, I'm getting beyond Cisco Academy here. ;-) With WANs, there is an actual circuit that implements the user-network interface between the DTE and DCE. Consider what happens when you make a phone call. The local phone switch at the Central Office provides dial-tone, accepts dialed numbers, etc. That switch makes a network-network connection to another switch, which has a local connection to the called party. This is how WANs work also. They have a telephony legacy, after all. LANs don't really behave like this, (although you could say that the initial ARP to find the MAC address of the default gateway is analogous to the call setup protocols in WANs.) In general, however, a LAN device makes a "call" to another LAN device across the network. The devices in the middle just act as relays. They don't communicate directly with the end devices; they just forward packets between end devices. ARP, ICMP, and IGMP are exceptions to this, but I wouldn't mention that right away when teaching academy students. Howard Berkowitz has a "grand unifying theory" that compares and contrasts LANs and WANs and uses the terminology in a unifying and elegant way. See the first chapter of his book WAN Solutions Guide for a summarized view of his grand theory. He may have written it up in a Group Study response in the past also. See the archives. Hey, how did you get the CCAI gig? You seem very well qualified for it. Good luck with it. Priscilla At 07:47 AM 1/23/02, Phil Barker wrote: >Hi group, > >I have just started teaching as a CCAI and was asked >the following question yesterday in class. > >One student pointed out that the Sem 2 curriculum >states that all devices on the LAN side of a >connection are DTE's. > >I disagree with this statement. > >If we take a switch and a PC(DTE), this patches to the >switch via a straight through cable. I conclude >therefore that the switch port must be a DCE. (Is this >safe thinking ?) > >If I uplink the switch to a Router(DTE) I use a >straight through cable. My conclusion again is that >the Switch port is DCE. > >If I link a PC(DTE) directly to a Router port(DTE) I >use a crossover cable since they are both DTE's. > >Can I conclude therefore that an internal switch will >in general be a DCE just as an ISP acts as DCE to a >LAN. > >Regards, > >Phil. > >__________________________________________________ >Do You Yahoo!? >Everything you'll ever need on one web page >from News and Sport to Email and Music Charts >http://uk.my.yahoo.com ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=33001&t=32924 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
