There are a number of different data link layer protocols that can be implemented on a point-to-point WAN.
WAN data-link protocols used on point-to-point serial links provide the basic function of data delivery
across that one link. The two most popular WAN data-link protocols are High-Level Data Link Control
(HDLC) and PPP. Both of these protocols provide for the delivery of data across a single point-to-point
serial link and deliver data on synchronous serial links. In addition, PPP also supports asynchronous serial
links.
Each synchronous serial data-link protocol is frame-oriented, with each data-link protocol defining the
beginning and end of the frame, the information and format of a header and trailer, and the location of the
packet between the header and trailer. Data-link protocols also send idle frames. This is because
synchronous WAN links require that the CSU/DSUs on each end of the link operate at the exact same speed.
To accomplish this, the CSU/DSUs on each side of the WAN link agree to use a certain clock rate, or speed,
to send and receive bits. After they agree to a particular speed, both CSU/DSUs try to operate at that speed.
One CSU/DSU is responsible for monitoring the clock rates between itself and the other CSU/DSU by
noticing changes in the electrical signal received on the physical line. When a change occurs, the CSU/DSU
monitoring the clock rates responds by adjusting its clock speed. If no traffic was sent across the link, there
would be no electrical signal and clock synchronization would be lost. Therefore synchronous data-link
protocols send idle frames when there is no end-user data to be sent over the link. The idle frames are called
Receiver Ready. This need to monitor and adjust the clock rates for synchronous protocols requires more
expensive hardware than asynchronous protocols. However, synchronous protocols allow more throughput
over a serial link than asynchronous protocols. For links between routers, synchronous links are typically
desired and used.
Almost all data-link protocols, including PPP and HDLC, perform error detection. These protocols use a
field in the trailer called the frame check sequence (FCS) for this purpose. The FCS is used to verify
whether bit errors occurred during transmission of the frame. If bit errors occurred, the frame is discarded.
However, error recovery, which is the process that causes retransmission of the lost or errored frame, is not
guaranteed. Error recovery can be performed by the data-link protocol or a higher-layer protocol, or it might
not be performed at all.
PPP was defined much later than the original HDLC specifications. As a result, PPP includes many new
features that are not implemented in HDLC. For this reason, PPP has become the most popular WAN data
link layer protocol.
PPP uses a protocol that offers features regardless of the Layer 3 protocol used, and a protocol to support
each Layer 3 protocol supported on the link. The PPP Link Control Protocol (LCP) provides the core
features for PPP that operate regardless of the Layer 3 protocol used, while a series of PPP control protocols,
such as IP Control Protocol (IPCP), provide features related to a specific Layer 3 protocol. Thus, PPP uses
one LCP per link and one Control Protocol for each Layer 3 protocol defined on the link. If a router is
configured for IPX, AppleTalk, and IP on a PPP serial link, the router configured for PPP encapsulation
automatically tries to bring up the appropriate control protocols for each Layer 3 protocol. Cisco routers also
use a PPP CP for supporting CDP traffic, called CDPCP.
LCP provides a variety of optional features for PPP. These are:
• Error detection, which is provided by Link Quality Monitoring (LQM). The router can be configured
to take down the link after a configured error rate has been exceeded. By taking down a link that has
many errors, you can cause packets to use an alternative path that might not have as many errors but this
is only useful when you have redundant routes in the network.
• Looped link detection, which is provided by magic numbers. Using different magic numbers, routers
send messages to each other. If a router receives its own magic number, it knows that the frame it sent
has been looped back. If configured to do so, the router can take down the interface through which the
frame was sent, and effectively close the loop. This will improve convergence.
• Multilink support, which is provided by Multilink PPP and allows PPP to load-balance fragments of
packets across multiple links.
• Authentication, which can be provided by Password Authentication Protocol (PAP) or Challenge
Handshake Authentication Protocol (CHAP) and allows for the exchange of names and passwords so
that each device can verify the identity of the device on the other end of the link. CHAP is the preferred
method because it uses a Message Digest 5 (MD5) one-way hash to encode the password while PAP
sends passwords in clear-text.