Chapter Fundamentals of WAN

OSI Layer 2 for Point to Point WANS

Wan protocols used on point to point serial links provide the basic function of data delivery accross that one link. The 2 most used protocols are HDLC and PPP.

HDLC
The work done by HDLC : determine if the data passed the link without any errors; if errors occured then the frame is discarded, determine the type of packet to let the receiving device know what type of packet it receives -- therefore HDLC defines framing.
The frame contains an address field and protocol type field and also frame check sequence (FCS) in the trailer.
Standard HDLC frame : 1 flag, 1 address, 1 control, data, 4 FCS.
Cisco HDLC frame : 1 flag, 1 address, 1 control, 2 type, data, 4 FCS.
The identification process of frame type was done the same way as with ethernet process.
Later on, IETF created PPP that is identical with Cisco HDLC protocol. So, when connecting Cisco routers to another vendor routers, PPP is the protocol of choice. Nowadays, it's the wan-rich famous point to point link wan protocol.

WAN key terminologies
Synchronous : the imposition of time ordering in a bit stream. Every 2 devices which are communicating will try to use the same speed, but by examining voltage states, they will notice slight variations with the clock and can adjust the speed accordingly.
Clock source : the device to which the other device adjust its speed
CSU/DSU : Channel Service Unit/Data Service Unit, used as an interface on a digital link to a telephone company. Usually a router is attached to it using a short cable.
Four-wire circuit : a line from telco that is consisted of two twisted-pair wires, allowing to send in one direction and full-duplex communication.
T1 : 1.5 Mbps
E1 : 2 Mbps