Frame Relay is a WAN protocol that has enjoyed a wide
implementation since the late 1980s. Over the next few weeks, I
will be writing newsletters that concentrate on Frame Relay
topics as they pertain to the CCNA/CCNP exams. This week's
article offers a basic introduction to Frame Relay.
Frame Relay is a WAN protocol that has seen wide deployment
since 1991. Frame Relay has been described as a streamlined
version of the ancient X.25 protocol. Frame Relay (Frame) was
originally designed to be used with ISDN, and operates on the
physical and data link layers of the OSI model. Frame standards
are maintained by the Frame Relay Forum, which was originally
comprised of Cisco, DEC (Digital Equipment Corporation),
Northern Telecom, and Stratacom.
Frame is a packet-switched protocol, so Frame does not require
expensive leased lines to operate. Frame uses either a PVC
(permanent virtual circuit) or a SVC (switched virtual circuit)
to provide a data connection. As a packet-switched protocol,
Frame supports variable packet sizes and only uses bandwidth
when it has data to send.
Frame traffic is routed to the correct destination using an
address called a DLCI (Data-Link Connection Identifier). The
DLCI is a 10-bit number that is found in the Frame header, and
is used to identify the virtual circuit (VC). The DLCI is only
significant to the local network (a service provider may have
several customers that use the same DLCI on their networks).
Equipment on a Frame network can be divided into two categories:
DTE (data terminal equipment), and DCE (data communications
equipment). DTE equipment is usually customer premises
equipment such as workstations, servers, or routers. DCE
equipment provides the clocking and switching for the Frame
circuit. In most cases, the DCE equipment is a packet-switch
owned by the service provider.
Frame Relay has several integrated congestion control
mechanisms. FECN (forward-explicit congestion notification)
and BECN (backwards-explicit congestion notification) are both
single bits in the Frame header. If congestion is detected on
a virtual circuit, the DCE device sets the FECN bit if the
congestion is detected from source to destination, and the BECN
bit if the congestion is for the opposite direction. The DE
(discard eligibility) bit indicates a packet has lower
importance when set. If congestion occurs, packets with the DE
bit set are dropped.
That concludes the basic introduction to Frame Relay. Next
week, I will be writing about extensions to the original Frame
specification, and configuring Frame on Cisco routers.
Until then, have a great week.
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