598 Chapter 8: WAN Protocols and Design
Table 8-40 lists the point-to-point data link protocols and their attributes.
Note: Be careful about confusing LAPB and LAPD. The D can help remind you that it is for an ISDN D channel, but
don't let that make you think that the B is for an ISDN B channel.
T/1
A line from the telco that allows transmission of data at 1.544Mbps. This can
be used with a T/1 multiplexor.
T/1 mux
A multiplexor that separates the T/1 into 24 different 64kbps channels. In the
United States, one of every 8 bits in each channel can be used by the telco so
that the channels are effectively 56kbps channels.
E/1
Like a T/1, but in Europe. It uses a rate of 2.048Mbps and 32 64kbps
channels.
Table 8-40
Point-to-Point Data Link Protocol Attributes
Protocol
Error Correction?
Architected
Type Field?
Other Attributes
Synchronous Data
Link Control (SDLC)
Yes
None
SDLC supports multipoint links;
it assumes that an SNA header
occurs after the SDLC header.
Link Access
Procedure Balanced
(LAPB)
Yes
None
Spec assumes a single
configurable protocol after
LAPB. This is used mainly with
X.25. Cisco uses a proprietary
type field to support
multiprotocol traffic.
Link Access
Procedure on the D
channel (LAPD)
No
No
LAPD is not used between
routers. It is used on a D channel
from the router to the ISDN
switch for signaling.
High-Level Data Link
Control (HDLC)
No
No
HDLC is Cisco's default on serial
links. Cisco uses a proprietary
type field to support
multiprotocol traffic.
Point-to-Point
Protocol (PPP)
Allows user to choose
whether error
correction is
performed; correction
uses LAPB
Yes
PPP was meant for multiprotocol
interoperability from its
inception, unlike all the others.
PPP also supports asynchronous
communication.
Table 8-39
WAN Terminology (Continued)
Term
Definition
ch08.fm Page 598 Monday, March 20, 2000 5:17 PM