3.1 Serial Point-to-Point Links  
  3.1.5 HDLC encapsulation  
Initially, serial communications were based on character-oriented protocols. Bit-oriented protocols were more efficient but they were also proprietary. In 1979, the ISO agreed on HDLC as a standard bit-oriented data link layer protocol that encapsulates data on synchronous serial data links. This standardization led to other committees adopting it and extending the protocol. Since 1981, ITU-T has developed a series of HDLC derivative protocols. The following examples of derivative protocols are called link access protocols:
  • Link Access Procedure, Balanced (LAPB) for X.25
  • Link Access Procedure on the D channel (LAPD) for ISDN
  • Link Access Procedure for Modems (LAPM) and PPP for modems
  • Link Access Procedure for Frame Relay (LAPF) for Frame Relay

HDLC uses synchronous serial transmission providing error-free communication between two points. HDLC defines a Layer 2 framing structure that allows for flow control and error control using acknowledgments and a windowing scheme. Each frame has the same format, whether it is a data frame or a control frame.

Standard HDLC does not inherently support multiple protocols on a single link, as it does not have a way to indicate which protocol is being carried. Cisco offers a proprietary version of HDLC. The Cisco HDLC frame uses a proprietary ‘type’ field that acts as a protocol field. This field enables multiple network layer protocols to share the same serial link. HDLC is the default Layer 2 protocol for Cisco router serial interfaces.

HDLC defines the following three types of frame, each with a different control field format:

  • Information frames (I-frames) – Carry the data to be transmitted for the station. Additional flow and error control - data may be piggybacked on an information frame.
  • Supervisory frames (S-frames) – Provide request/response mechanisms when piggybacking is not used.
  • Unnumbered frames (U-frames) – Provide supplemental link control functions, such as connection setup. The code field identifies the U-frame type.

The first one or two bits of the control field serve to identify the frame type. In the control field of an Information (I) frame, the send-sequence number refers to the number of the frame to be sent next. The receive-sequence number provides the number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers.

 

Web Links

Configuring Synchronous Serial Ports

http://www.cisco.com/en/US/products/sw/ iosswrel/ps1828/ products_configuration_ guide_ chapter09186a0080087283.html