The WAN data link layer defines how
data is encapsulated for transmission to remote sites. WAN data-link
protocols describe how frames are carried between systems on a single
data path.
Figure shows the common
data-link encapsulations associated with WAN lines, which are:
- Frame Relay -- By using simplified encapsulation with no error correction
mechanisms over high-quality digital facilities, Frame Relay can
transmit data very rapidly compared to the other WAN protocols.
- Point-to-Point Protocol (PPP) --
Described by RFC 1661, PPP was developed
by the IETF. PPP contains a protocol field to identify the
network-layer protocol.
- ISDN -- A set of digital services that
transmits voice and data over existing phone lines.
- Link Access
Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used
to encapsulate packets at Layer 2 of the X.25 stack. It can also be
used over a point-to-point link if the link is unreliable or there is
an inherent delay associated with the link, such as in a satellite
link. LAPB provides reliability and flow control on a point-to-point
basis.
- Cisco/IETF -- Used to encapsulate Frame Relay traffic. The
Cisco option is proprietary and can be used only between Cisco
routers.
- High-Level Data Link Control (HDLC)
-- An ISO standard, HDLC
might not be compatible between different vendors because of the way
each vendor has chosen to implement it. HDLC supports both
point-to-point and multipoint configurations.
|