Data Link Protocols
In addition to physical layer devices, WANs require data link layer protocols to establish the link across the communication line from the sending to the receiving device. This topic describes the common data link protocols that are used in today's enterprise networks to implement WAN connections.
Data link layer protocols define how data is encapsulated for transmission to remote sites and the mechanisms for transferring the resulting frames. A variety of different technologies, such as ISDN, Frame Relay, or ATM, are used. Many of these protocols use the same basic framing mechanism, HDLC, an ISO standard, or one of its subsets or variants. ATM is different from the others, because it uses small fixed-size cells of 53 bytes (48 bytes for data), unlike the other packet-switched technologies, which use variable-sized packets.
The most common WAN data-link protocols are:
ISDN and X.25 are older data-link protocols that are less frequently used today. However, ISDN is still covered in this course because of its use when provisioning VoIP network using PRI links. X.25 is mentioned to help explain the relevance of Frame Relay. As well, X.25 is still in use in developing countries where packet data networks (PDN) are used to transmit credit card and debit card transactions from retailers.
Note: Another data-link layer protocol is the Multiprotocol Label Switching (MPLS) protocol. MPLS is increasingly being deployed by service providers to provide an economical solution to carry circuit-switched as well as packet-switched network traffic. It can operate over any existing infrastructure, such as IP, Frame Relay, ATM, or Ethernet. It sits between Layer 2 and Layer 3 and is sometimes referred to as a Layer 2.5 protocol. However, MPLS is beyond the scope of this course but is covered in the CCNP: Implementing Secure Converged Wide-area Networks.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 that the necessary checks and controls can be applied. Each WAN connection type uses a Layer 2 protocol to encapsulate a packet 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. HDLC was first proposed in 1979 and for this reason, most framing protocols which were developed afterwards are based on it.