CCNA 28

Ethernet is not the only Layer 2 technology. When referring to a WAN link, we typically speak about other Layer 2 encapsulations like HDLC, Frame Relay and PPP. WAN Links define a new type of L1 and L2 connectivity. WAN allows links to the internet or other offices. There are many different types and prices.
There are many different styles of WAN connections:

Leased Lines: Dedicated bandwidth between locations
-T1 CAS (1.544Mbps, usually in US)
-E1 CAS (2.048Mbps, usually in Europe)

Circuit Switched: On-demand Bandwidth Between Locations
-Dial-Up
-ISDN

Packet Switched: Shared, but Guaranteed, Bandwidth Between Locations
-Frame Relay
-ATM

T1 is just a measure of speed, which is 1.544Mbps. T1 CAS is a type of connection. Frame Relay and ATM connects around through virtual circuits. Virtual circuits are actually paths carved into Frame Relay or ATM switches.

There are two major types of cards that we can slot into a WIC slot. The oldest type are WIC-1T cards and a DB60 connector plugs into it. The DCE end (can be many different kinds, such as V.35) plugs into a CSU/DSU which converts the DB60 into RJ-45 which plugs into the wall jack. The WIC-2T allows two serial connectors to plug into it, but uses a different serial interface known as the "Smart connector".

A WIC-1DSU-T1-V2 is a built-in CSU/DSU unit. This way, you won't need an external CSU/DSU. The router only needs to connect to the wall jack with an RJ-45 cable.

To check the encapsulation of a serial interface, use:
show int s0/0

By default it uses HDLC. HDLC stands for High-Level Data Link Control. Even though HDLC is an ISO standard, Cisco's implementation is proprietary. It requires no configuration to run for connectivity.

An alternative to HDLC is PPP. PPP is industry standard and Cisco's implementation follows it. To configure a port for PPP, go under the interface and type:
in s0/0
encap ppp

To suspend a telnet session, use CTRL+Shift+6 then "x". CTRL+Shift+6 can also stop pings and many other operations. When you change a serial link's encapsulation using a telnet session connecting through serial link, your prompt will stop coming back if the encapsulation no longer matches.

When you move over to the router, you'll see that the line protocol is down. Line protocol down means that the Data Link layer is down. If there is an encapsulation mismatch, the Data Link layer will not come up.

In a lab environment, typically null modem cables are used. A null modem will have DTE and DCE ends terminating with DB60, so the DCE side will have to set the clock. In a real-life scenario, you have a DTE end on your router, and the DCE end to the service provider.

To see what side you're connected to, type:
show control s0/0

The DCE side will have to set the clock rate through:
clock rate 64000

You can actually set the clock rate on both sides if you're not sure.