WAN ENCAPSULATION

Data from the network layer is passed to the data link layer for delivery on a physical link, which is normally point-to-point on a WAN connection. The data link layer builds a frame around the network layer data so the necessary checks and controls can be applied. Each WAN connection type uses a Layer 2 protocol to encapsulate traffic while it is crossing the WAN link. To ensure that the correct encapsulation protocol is used, the Layer 2 encapsulation type used for each router serial interface must be configured. The choice of encapsulation protocols depends on the WAN technology and the equipment. Most framing is based on the HDLC standard.

HDLC framing gives reliable delivery of data over unreliable lines and includes signal mechanisms for flow and error control. The frame always starts and ends with an 8-bit flag field, the bit pattern 01111110. Because there is a likelihood that this pattern will occur in the actual data, the sending HDLC system always inserts a 0 bit after every five 1s in the data field, so in practice the flag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. When frames are transmitted consecutively the end flag of the first frame is used as the start flag of the next frame.

The address field is not needed for WAN links, which are almost always point-to-point. The address field is still present and may be one or two bytes long. The control field indicates the frame type, which may be information, supervisory, or unnumbered:

• Unnumbered frames carry line setup messages.
• Information frames carry network layer data.
• Supervisory frames control the flow of information frames and request data retransmission in the event of an error.

The control field is normally one byte, but will be two bytes for extended sliding windows systems. Together the address and control fields are called the frame header. The encapsulated data follows the control field. Then a frame check sequence (FCS) uses the cyclic redundancy check (CRC) mechanism to establish a two or four byte field.

Several data link protocols are used, including sub-sets and proprietary versions of HDLC. Both PPP and the Cisco version of HDLC have an extra field in the header to identify the network layer protocol of the encapsulated data.