Other Pointto Point WAN Data Link Protocols

WAN data-link protocols can be compared relative to two main attributes. First, some protocols do support multiprotocol traffic by virtue of having a defined protocol type field. Also, some protocols actually perform error recovery—so when the receiving end notices that the received frame did not pass the FCS check, it causes the frame to be resent. Protocols that were developed more recently tend to have a protocol type field and do not perform error recovery. Instead, they expect a higher-layer protocol to perform recovery. Table 4-6 lists the protocols, with comments about each.

Table 4-6 List of WAN Data-Link Protocols

Correction?

Type Field?

Other Attributes

Synchronous Data Link Control (SDLC)

Yes

No

SDLC supports multipoint links. It assumes that an SNA header occurs after the SDLC header.

Link Access Procedure Balanced (LAPB)

Yes

No*

LAPB is used mainly with X.25.

Link Access Procedure on the D Channel (LAPD)

No

No

LAPD is used by ISDN lines for signaling to set up and bring down circuits.

Link Access Procedure for Frame Mode Bearer Services(LAPF)

No

Yes

This is a data-link protocol used over Frame Relay links.

High-Level Data Link Control (HDLC)

No

No*

HDLC serves as Cisco's default on serial links.

Point-to-Point Protocol (PPP)

Supported but not enabled by default

Yes

PPP was meant for multiprotocol interoperability from its inception, unlike all the others.

*Cisco's implementation of LAPB and HDLC includes a proprietary Protocol Type field.

*Cisco's implementation of LAPB and HDLC includes a proprietary Protocol Type field.

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