Content Overview Frame Relay was
originally developed as an extension of Integrated Services
Digital Network (ISDN). It was designed to enable the
circuit-switched technology to be transported on a
packet-switched network. The technology has become a
stand-alone and cost-effective means of creating a WAN.Frame
Relay switches create virtual circuits to connect remote LANs
to a WAN. The Frame Relay network exists between a LAN border
device, usually a router, and the carrier switch. The
technology used by the carrier to transport the data between
the switches is not important to Frame Relay. The
sophistication of the technology requires a thorough
understanding of the terms used to describe how Frame Relay
works. Without a firm understanding of Frame Relay, it is
difficult to troubleshoot its performance. Frame Relay has
become one of the most extensively used WAN protocols. One
reason for its popularity is that it is inexpensive compared to
leased lines. Another reason Frame Relay is popular is that
configuration of user equipment in a Frame Relay network is
very simple. This module explains how to configure Frame Relay
on a Cisco router. Frame Relay connections are created by
configuring routers or other devices to communicate with a
Frame Relay switch. The Frame Relay switch is usually
configured by the service provider. This helps keep end-user
configuration tasks to a minimum. Students completing this
module should be able to: - Identify the components of a
Frame Relay network
- Explain the scope and purpose of
Frame Relay
- Discuss the technology of Frame
Relay
- Compare point-to-point and point-to-multipoint
topologies
- Examine the topology of a Frame Relay
network
- Configure a Frame Relay Permanent Virtual
Circuit (PVC)
- Create a Frame Relay Map on a remote
network
- Explain the issues of a non-broadcast
multi-access network
- Describe the need for
subinterfaces and how to configure them
- Verify and
troubleshoot a Frame Relay connection
Content
5.1 Frame Relay Concepts 5.1.1
Introducing Frame Relay Frame Relay is an International
Telecommunication Union Telecommunications Standardization
Sector (ITU-T) and American National Standards Institute (ANSI)
standard. Frame Relay is a packet-switched,
connection-oriented, WAN service. It operates at the data link
layer of the OSI reference model. Frame Relay uses a subset of
the high-level data link control (HDLC) protocol called Link
Access Procedure for Frame Relay (LAPF). Frames carry data
between user devices called data terminal equipment (DTE), and
the data communications equipment (DCE) at the edge of the WAN.
Originally Frame Relay was designed to allow ISDN equipment to
have access to a packet-switched service on a B channel.
However, Frame Relay is now a stand-alone technology.A Frame
Relay network may be privately owned, but it is more commonly
provided as a service by a public carrier. It typically
consists of many geographically scattered Frame Relay switches
interconnected by trunk lines. Frame Relay is often used to
interconnect LANs. When this is the case, a router on each LAN
will be the DTE. A serial connection, such as a T1/E1 leased
line, will connect the router to a Frame Relay switch of the
carrier at the nearest point-of-presence for the carrier. The
Frame Relay switch is a DCE device. Frames from one DTE will be
moved across the network and delivered to other DTEs by way of
DCEs. Computing equipment that is not on a LAN may also send
data across a Frame Relay network. The computing equipment will
use a Frame Relay access device (FRAD) as the DTE. Web
Links Frame Relay http://www.cisco.com/univercd/cc/ td/doc/
cisintwk/ito_doc/ frame.htm
Content 5.1
Frame Relay Concepts 5.1.2 Frame Relay
terminology The connection through the Frame Relay network
between two DTEs is called a virtual circuit (VC). Virtual
circuits may be established dynamically by sending signaling
messages to the network. In this case they are called switched
virtual circuits (SVCs). However, SVCs are not very common.
Generally permanent virtual circuits (PVCs) that have been
preconfigured by the carrier are used. The switching
information for a VC is stored in the memory of the switch.
Because it was designed to operate on high-quality digital
lines, Frame Relay provides no error recovery mechanism. If
there is an error in a frame it is discarded without
notification. The FRAD or router connected to the Frame Relay
network may have multiple virtual circuits connecting it to
various end points. This makes it a very cost-effective
replacement for a mesh of access lines. With this
configuration, each end point needs only a single access line
and interface. More savings arise as the capacity of the access
line is based on the average bandwidth requirement of the
virtual circuits, rather than on the maximum bandwidth
requirement. The various virtual circuits on a single access
line can be distinguished because each VC has its own Data Link
Channel Identifier (DLCI). The DLCI is stored in the address
field of every frame transmitted. The DLCI usually has only
local significance and may be different at each end of a VC.
Interactive Media Activity Drag and Drop: Frame Relay
Terminology When the student has completed this activity, the
student will be able to correctly identify frame relay
terminology. Web Links Frame Relay
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/frame.htm
Content
5.1 Frame Relay Concepts 5.1.3
Frame Relay stack layered support Frame Relay functions by
doing the following: - Takes data packets from a network
layer protocol, such as IP or IPX
- Encapsulates them
as the data portion of a Frame Relay frame
- Passes
them to the physical layer for delivery on the wire
The physical layer is typically EIA/TIA-232, 449 or 530, V.35,
or X.21. The Frame Relay frame is a sub-set of the HDLC frame
type. Therefore it is delimited with flag fields. The 1-byte
flag uses the bit pattern 01111110. If the Frame CheckSum (FCS)
does not match the address and data fields at the receiving
end, the frame is discarded without notification.
Content
5.1 Frame Relay Concepts 5.1.4
Frame Relay bandwidth and flow control The serial connection or
access link to the Frame Relay network is normally a leased
line. The speed of the line is the access speed or port speed.
Port speeds are typically between 64 kbps and 4 Mbps. Some
providers offer speeds up to 45 Mbps.Usually there are several
PVCs operating on the access link with each VC having dedicated
bandwidth availability. This is called the committed
information rate (CIR). The CIR is the rate at which the
service provider agrees to accept bits on the VC. Individual
CIRs are normally less than the port speed. However, the sum of
the CIRs will normally be greater than the port speed.
Sometimes this is a factor of 2 or 3. This statistical
multiplexing accomodates the bursty nature of computer
communications since channels are unlikely to be at their
maximum data rate simultaneously. While a frame is being
transmitted, each bit will be sent at the port speed. For this
reason, there must be a gap between frames on a VC if the
average bit rate is to be the CIR. The switch will accept
frames from the DTE at rates in excess of the CIR. This
effectively provides each channel with bandwidth on demand up
to a maximum of the port speed. Some service providers impose a
VC maximum that is less than the port speed. The difference
between the CIR and the maximum, whether the maximum is port
speed or lower, is called the Excess Information Rate (EIR).
The time interval over which the rates are calculated is called
the committed time (Tc). The number of committed bits in Tc is
the committed burst (Bc). The extra number of bits above the
committed burst, up to the maximum speed of the access link, is
the excess burst (Be). Although the switch accepts frames in
excess of the CIR, each excess frame is marked at the switch by