Thank you for the information.
Neil
""Howard C. Berkowitz"" <[EMAIL PROTECTED]> wrote in message
news:p0500190eb64481a0727c@[63.216.127.98]...
> THe LAN data link protocols have source and destination addresses.
> WAN protocols usually have a destination address field only (see
> below).
>
> >If you look at the frame format for any LAN protocol you will see where
the
> >Destination and Source MAC address are.
> >If you look at
> >http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm you
> >will see that where the WAN technologies lay in the OSI model and
hopefully
> >this will also explain why serial lines don't have MAC addresses.
> >The reason that LAN protocols have a MAC address and WAN's don't is
because
> >LAN's are contention based where WAN's are always full-duplex. Even
though
> >layer 2 switching has been around for LAN's for a few years now the
> >protocols have stayed the same for backwards compatibility.
> >Neil
> >
>
>
> I'd disagree that WAN technologies are necessarily full-duplex.
> Polled, half-duplex operation was extremely common in SNA, as a means
> of sharing expensive dedicated lines (before frame relay and the
> like).
>
> Both SDLC (and its predecessors such as BSC) and LLC2 are
> deterministic/token-based rather than collision/contention protocols.
> The key difference between polled SNA and token ring, however, is
> control of the token. In SDLC, the token is centrally controlled (by
> the PU4 or PU5). In TR, control of the token is distributed.
>
> When control is centralized, and all traffic flows through the
> hub/mainframe, there's no need for a source address. The source
> address is always clear from context. There is a need for a
> destination address so a destination can know a poll is intended for
> it.
>
> So there is a need for destination addresses in WAN protocols
> intended for use in a point-to-multipoint environment. PPP,
> operating in point-to-point mode, never really needed any address
> field, but was designed with one because not to have one would have
> been incompatible with commercial data link chips of the time.
> Indeed, protocols such as SRP are being proposed for efficient POS
> applications, and these protocols have no address field because they
> don't need one.
>
> PS -- one thing that might be confusing about router serial lines
> having MAC addresses is that IPX and XNS will "borrow" a MAC address
> from a LAN interface in order to create the host part of a layer 3
> address.
>
>
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