22.8 PPP Framing over Serial Links

PPP can be used over most Serial/WAN interfaces, with the only overriding requirement being that the link must be full duplex. For example, linking networks over Synchronous Point-to-Point connections (leased lines), or over Dial-Up Asynchronous links as would be the case for remote Internet access.

Most commonly, we find PPP used in environments requiring Higher Data Link Control (HDLC) like framing as described by RFC 1662. Other framing is possible, and these techniques are discussed in separate documents as indicated in Table 22-6. For our discussion, we shall consider only HDLC-like framing since this is the most common implementation over WAN links.

The Link Layer Frame now looks as shown in Figure 22-43.

The 8 bit Flag field is used to indicate the start (and end) of each frame. Only one Flag field is required between any two frames thus, as our diagram shows, the trailing Flag can be immediately followed by the Address field of the next frame. The Flag octet is a unique binary pattern (01111110 ₂, 7E ₁₆) that must be preserved at all cost. As such, a scheme known as zero-bit-insertion or bit stuffing, is used by the transmitter to insert an additional zero bit into the bit stream wherever five consecutive 1 bits exist.