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Data-Link Framing
Point-to-point dedicated DS0 and higher PSTN-based communications links require
a data-link protocol to format ULP data for transmission. The data-link protocol is
responsible for the encapsulation of ULP data into transmission frames, as well as
their transmission and delivery over the synchronous or asynchronous transport
link.
For serial data transmissions using Universal Synchronous Asynchronous
Receiver/Transmitter (USART) and Universal Asynchronous Receiver/Transmitter
(UART) based transmission devices, there are two commonly used data-link
framing protocols: the ISO's HDLC and the open standard, PPP. Both are derivatives
of IBM's Synchronous Data-Link Control (SDLC), which was developed in the 1970s
to provide serial data transport between IBM mainframes and terminal controllers.
It is perhaps more accurate to say that PPP is a variation of the IP serial transport
protocol, Serial Line Internet Protocol, which is built on a subset of the HDLC
protocol. SLIP at one time was the basis for all serial-based IP transport (and still is
among IP diehards). Today, however, it has been largely replaced by PPP, which
supports multiprotocol transport.
HDLC
HDLC provides multiprotocol framing over synchronous full-duplex transport links.
HDLC is the default framing mechanism employed by most router serial interfaces,
including Cisco products. It operates using one of three primary/secondary
control/transfer schemes:
• Normal Response Mode (NRM)—This is the SDLC operational model, where
the primary station must grant permission to the secondary stations before
they can transmit data to the primary station.
• Asynchronous Response Mode (ARM)—This mode supports unrestricted data
transmission between the primary and secondary nodes.
• Asynchronous Balanced Mode (ABM)—With ABM, nodes are hybrids, which
can act either as the primary or the secondary. Hybrid nodes also operate in
an unrestricted transmission mode.
The primary station is responsible for establishing, managing, and terminating the
transmission link. HDLC supports point-to-point (one primary and one secondary)
and multipoint (one primary and multiple secondary) configurations.
HDLC transmission is a two-phase process. The first phase is the assembly of the
frame, and the second is the transmission of the medium-dependent frame. The