specifications define a field in the header that identifies the type of packet contained in the
data portion of the frame.
Synchronous protocols allow more throughput over a serial link than asynchronous protocols
do. 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. All the protocols covered in this section support synchronous links.
"OSI Reference Model & Layered Communication," but a brief review is in order here. All the
data link protocols described here use a field in the trailer, usually called the frame check
sequence (FCS), that is used to verify 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 frame(s); error recovery may be performed by the data link protocol or a higher-layer
protocol, or error recovery may not be performed at all. Regardless, all WAN data link protocols
perform error detection, which involves noticing the error and discarding the frame.
comparison. As described in more detail in Chapter 3, each data link protocol that supports
multiple network layer protocols needs a method of defining the type of packet encapsulated
inside the WAN data link frame. If such a field is part of the protocol specification, it is
considered "architected"--in other words, specified in the protocol. If Cisco must add some
other header information to create a Protocol Type field, then that type field is not considered
to be architected.
Protocol Type field, refer to Chapter 3.)
Correction?
Type Field?
Link Control
(SDLC)
assumes that the SNA header occurs
after the SDLC header.
Procedure
Balanced (LAPB)
protocol after LAPB. LAPB is used
mainly with X.25. Cisco uses a
proprietary type field to support
multiprotocol traffic.
Procedure on the D
channel (LAPD)
but is used on the D channel from
router to ISDN switch for signaling.