Data-Link Protocols

WAN data-link protocols used on point-to-point serial links provide the basic function of data delivery across that one link. The two most popular WAN data-link protocols are High-Level Data Link Control (HDLC) and PPP. Each of these WAN protocols has the following functions in common:

  • HDLC and PPP provide for the delivery of data across a single point-to-point serial link.
  • HDLC and PPP deliver data on synchronous serial links; PPP also supports asynchronous serial links.

Each synchronous serial data-link protocol uses the concept of framing. Each data-link protocol defines the beginning and end of the frame, the information and format of a header and trailer, and the location of the packet between the header and trailer. In other words, synchronous WAN data-link protocols are frame-oriented, just like LAN data-link protocols.

Synchronous WAN links require the CSU/DSUs on each end of the link to operate at the exact same speed. Essentially, the CSU/DSUs on each side of the WAN link agree to use a certain clock rate, or speed, to send and receive bits. It is expensive to build CSU/DSUs that can truly operate at exactly the same speed, so after they agree to a particular speed, both CSU/DSUs try to operate at that speed. Then one CSU/DSU watches for small differences in clock rates between itself and the other CSU/DSU and makes small adjustments.

Synchronous CSU/DSUs adjust their clocks by noticing when the electrical signal received on the physical line changes. When the routers send data frames across the link, plenty of signal transitions are made, because the transitions are typically used to imply either a 1 or a 0. However, if no traffic were sent across the link, there would be no transitions in the electrical signal, and clock synchronization would be lost. So synchronous data-link protocols send frames continuously, even sending idle frames when there is no end-user data to be sent over the link. By always at least sending frames with no data in them, the routers ensure that there are plenty of electrical signal transitions on the line, allowing the clock adjustments required for synchronization. So HDLC and PPP define idle frames, called “Receiver Ready”, implying that the sender has nothing to send other than idle frames to allow for continued synchronization.

Unlike asynchronous links, in which no bits are sent during idle times, synchronous data links define idle frames. These frames do nothing more than provide plenty of signal transitions so that clocks can be adjusted on the receiving end, consequently maintaining synchronization.


Synchronous protocols allow more throughput over a serial link than asynchronous protocols. However, asynchronous protocols require less-expensive hardware, because there is no need to watch transitions and adjust the clock rate. For links between routers, synchronous links are typically desired and used. When you use a PC with a modem to dial up an Internet provider, you use an asynchronous link.

Another comparison criteria is error recovery. Do not confuse error recovery with error detection. Almost all data-link protocols, PPP and HDLC included, perform error detection. All the data-link protocols described here use a field in the trailer, usually called the frame check sequence (FCS), that verifies whether bit errors occurred during transmission of the frame. If so, the frame is discarded. Error recovery is the process that causes retransmission of the lost or errored frame(s). Error recovery can be performed by the data-link protocol or a higher-layer protocol, or it might not be performed at all. Regardless, all WAN data-link protocols perform error detection, which involves noticing the error and discarding the frame.

Here are the framing details of HDLC and PPP, showing the proprietary HDLC Protocol field and the standardized PPP Protocol field:

HDLC and PPP frames

The following table lists the important comparison points between HDLC and PPP.

Protocol Error Correction Archytected Type Field Other
HDLC No No HDLC serves as Cisco’s default on serial links. Cisco uses a Proprietary Type field to support multiprotocol traffic. Supports synchronous links only.
PPP Supported but not enabled by default Yes PPP was meant for multiprotocol interoperability from its inception, unlike HDLC. PPP also supports asynchronous communication.