Let's say we have a WAN. Two routers - point-to-point, using HDLC. The
serial link goes out of the router, then to CSU/DSU and then hits the
WAN switch. My question is- how does a telco (like AT&T or SBC) know
(if there are more clients than one on one WAN switch) how to later
route/switch the packet across the network. Basically the packets from
different customers look same once they hit the switch - how does
telco recognize them ?

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Clarification of Question by wanderer83-ga on 02 Feb 2005 09:24 PST

Are there any encapsulation types used only by Telco ?

(The switch I am talking about is a WAN switch on the Telco side)

Various layers of networking are at play here. Yes there are
encapsulation types used by the Telco. You could have your router
connect with the V.35 block to the CSU/DSU that there connects to say
a copper wire to the telco. That copper wire would be for example a T1
or T3 line. Over that T1-T3 line you could be running something such
as Frame-Relay, ATM?, and other type of TELCO protocols. Those
Protocols will have information either provided by the telco or
depending on the CSU/DSU will be automatically assigned. The Telco can
create PVC or SVC (Permanent Virtual Circuit) (Switched Virtual
Circuit) this predates the internet.

Some time ago, I would have say a office in New York, and an office in
LA. I'd get a T1 Line in New York to the telco, and a T1 line in LA.
YOU connect to the Telco. From there the Telco will create a circuit
to connect those two points because this is CHEAPER than A) Satelite
Link, B) running a cable yourself!

Telco will provide you a way to Transmit DATA from one point to
another. Its a Link, almost Layer 1 and a bit into layer 2 in terms of
OSI networking.

If your'e curious consider picking up this chart:
http://www.javvin.com/map.html

It has all sorts of protocls and their perspective position in the
terms of the whole communication processes.

Hope this helps ;)

Thank you for your comment !

I guess what I am still unclear about is how telco distinguishes
signals/data coming from diffrent customers that are connected on the
same switch?

Let's say there are two companies - A and B. They have offices A1 and
A2 and B1 and B2. A1 is close (geographically) to B1 (although they
are 2 diffrent companies). Same is true for offices A2 and B2. Since
they both want leased lines (company A - office A1 to A2 and B - B1 to
B2) telco will most likely put A1 and B1 on the same switch (because
of their physical proximity) and B2 and A2 on another switch. Now -
the data that they send out is encapsulated in let's say HDLC - when
it hits the telco switch it looks the same - Telco can't tell by the
packet where it is coming from/where it is going to. Is there anything
like programming the telco WAN switch to tell it: "on this port there
is a circuit id XYZ and it should have a PVC with a circuit id UWI on
the switch in Alabama or wherever (I am not picking on Alabama)" ?

Are there then any protocols (perhaps very low level ones) that are
used to establish the connection? Or is that (what I am talking about)
a part of the PVC/SVC?

I guess the part I am wondering about is the one where you mentioned
"...From there the Telco will create a circuit to connect those two
points..." - how does telco do that ? is it by programming the
switches ?

I really, really do appreciate your help  !

:)

Yes it does get programed into the switches. They're not the same type
of switch that you would use for Ethernet. Ethernet switches are
actually multi-port bridges. Bridges learn what is on each port by
listening, basically it learns. When machine A has ip address of
10.0.0.2 and a MAC address of say A10000000000, and wants to talk to
machine B of ip address 10.0.0.3 and MAC address of say
B20000000000... machine A will check its ARP table to see if it has
the B MAC address, if it doesn't have the MAC address machine A does
an ARP packet. The arp packet is sent in an ethernet frame with a
BROADCAST MAC for a destination, which is all F's FFFFFFFFFF's. When
an ethernet switch see's the all F's.. it forwards this to every port,
and eventually B will receive this and say Oh... you want me so it
replies to the arp, and then A learns. This is LAN switching, it
learns. MAC stands for Media Access Control. Media being the signaling
on the wire, another aspect of this is LLC which is Layer Link Control
lets the computer know that it is connected, like the link lights.

Telco switching everything is entered in, if you compare it it is
similar to having the MAC addresses hardwired. You can have multiple
leased lines so company A1, B1, and C1 could all talk to each other.
These can be refered to point-to-point and point-to-multipoint. They
are encapsulated in something similar to an ethernet frame, but in the
telco world they call them cells. You can have Frame Relay as
encapsulation for telco switching, ATM is another, and an old
workhorse is x.25

CISCO provides some documentation that may help explain the switching,
and how it works.

X.25:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/x25.htm
Frame Relay:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm
ATM:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm

You can encapsulate ethernet frames within X.25, Frame Relay and ATM.

You couldn't create so called fake cells to try and hack the system
and send data to some place it shouldn't go. The switch will see a
cell with an improper destination and simply toss it. The circuit is
designed to allow data to travel in only certain paths.

Hope that helps.

This is exaclty what I was concerned about. (especially that part with
malforming packets that are sent :) )

Just to make sure - ethernet framing would be stripped before the IP
packet is "piggybacked" over other protocol, wouldn't it ? I always
thought that on a router with let's say an ethernet interface and
serial link whatever comes on the ethernet is deprived of MAC framing
and later the bare IP packet is encapsulted with DLCI and all that
stuff for Frame Relay for example.

I really apprecaite your help

I have asked this question to many of my co-workers and friends, but
most either don't care about that or don't know.

Thousand thanks !

(by this question I mean the one regarding the telco switching)