CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Comparisons between synchronous and asynchronous links:
Synchronous Links Asynchronous Links
Have identical frequencies. Each individual Send digital signals without timing. They
character is encapsulated in control bits – the agree on the same speed, but there is no
Start / Stop bits, which designate the beginning checking or adjustment of the rates if they are
and ending of each character.
slightly different.
Send frame continuously. Send idle frames
called Receiver Ready when there is no data
transmission over the link. The purpose is to
maintain the clock synchronization.
Allow more throughputs (due to clock
synchronization).
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No bits are sent during idle times. Only 1 byte
is sent per transfer.
Require less expensive hardware (do not
require clock synchronization).
Normally used for links between routers. Normally used for dial-up links (modems).
Isochronous transmission allows asynchronous data transfer over a synchronous link. It
requires constant bit rate for reliable transport.
- Below list some other common WAN data link protocols:
Protocol
Error
Recovery?
Protocol
Type Field?
Other Attributes
Synchronous Data Link Yes No Developed by IBM. Supports
Control (SDLC)
multipoint links. It assumes an
IBM SNA header after the SDLC
header. Replaced by HDLC.
High-level Data Link
Control (HDLC)
Qualified Logical Link
Control (QLLC)
Link Access Procedure,
Balanced (LAPB)
Link Access Procedure
– D Channel (LAPD)
Link Access Procedure
for Frame-Mode Bearer
Services (LAPF)
Point-to-Point Protocol
(PPP)
No No [1] Developed by IBM. Default on
Cisco synchronous serial links.
Developed by IBM to transport
SNA traffic over X.25 networks.
Yes No [1] Mainly used in X.25 networks.
LAPB operates similar to HDLC,
but is restricted to be used only in
point-to-point links.
No No Mainly used on ISDN D channels
for signaling in establishing and
terminating ISDN circuits.
Evolved from LAPB.
No Yes Mainly used over Frame Relay
links (between a DCE and a DTE).
Similar to LAPD.
Yes but disabled
by default
Yes Designed for multiprotocol
interoperability.
[1] Cisco’s implementation of LAPB and HDLC includes a proprietary Protocol Type field.
- PPP and ATM can operate on synchronous and asynchronous links; while HDLC and Frame
Relay can only operate on synchronous links.
- PPP, HDLC, and LAPB are the data link layer protocols (or encapsulations) that can be used on
ISDN B channels. PPP uses HDLC to encapsulate and transmit packets over point-to-point links.
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com