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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