Datasheet

812
Encapsulation Types
✦ Cell-switched connection: A type of point-to-point packet-switched
connection that uses digital circuits to transmit 53-byte packets called cells.
Asynchronous Transfer Mode (ATM) is an example of a cell-switched WAN.
Encapsulation Types
Encapsulation is the wrapping of data in a particular protocol header to suc-
cessfully transport the data. Because IP is a network layer protocol, it must
be encapsulated when traversing a WAN (data link/physical layer) link. Cisco
devices support many WAN encapsulation types. Each encapsulation type must
be manually assigned to a router’s serial interface, while identically matching
the corresponding point-to-point link on the other end of the connection.
The configured encapsulation type must be identical between both end-
points on a single link; otherwise, communications will not be possible.
The following sections describe the most popular encapsulation types used
on Cisco devices.
HDLC (High-Level Data Link Control)
Cisco’s default encapsulation type for serial interfaces, HDLC is used to
encapsulate packets into frames over established, dedicated, point-to-point
leased lines. HDLC packets are small and use low overhead, thus making
them quite efficient. HDLC also verifies link integrity and communications by
implementing keepalives and sequence numbering. A downside of HDLC is
that it provides no means for authentication.
The original HDLC specification outlined by the International Organization for
Standardization (ISO) did not provide simultaneous support for multiple net-
work layer protocols. Independent vendors took it upon themselves to make
improvements to the HDLC protocol, which introduced new functionality and
incorporated the use of multiple Layer 3 protocols. Cisco’s proprietary ver-
sion of HDLC (known as cHDLC) is a synchronous, bit-oriented protocol which
adds a new frame field in the packet header to allow for multi-protocol sup-
port. cHDLC uses a link keepalive method called Serial Line Address resolu-
tion Protocol (SLARP ) to verify connection availability and integrity.
HDLC encapsulation is the default assignment for serial interfaces. If a non-
HDLC encapsulation type is required for link establishment, it must be manu-
ally configured by the network administrator on both sides of the link.
PPP (Point-to-Point Protocol)
PPP encapsulates network layer packets and transmits frames over Layer 2
connections used by point-to-point dedicated circuits and asynchronous
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