Frame relay

FRAME RELAY
BY
KALLURI VINAY REDDY

What is frame relay?
Frame relay is a standardized wide area
network technology that specifies the physical and
Data link layers of digital telecommunications
channels using a packet switching methodology.
Originally designed for transport
across Integrated Services Digital Network (ISDN)
infrastructure.
It may be used today in the context of many other
network interfaces.

Frame relay is a telecommunication service designed for
cost-efficient data transmission for intermittent traffic
between local area networks(LANs) and between end-points in
a wide area network(WANs).
Frame relay puts data in a variable-size unit called
a frame and leaves any necessary error correction
(retransmission of data) up to the end-points, which speeds
up overall data transmission.

For most services, the network provides a
permanent virtual circuit (PVC), which means that
the customer sees a continuous, dedicated
connection without having to pay for a full-time
leased line, while the service provider figures out
the route each frame travels to its destination and
can charge based on usage.

Frame relay is provided on fractional T-1 or
full T-carrier system carriers.
Frame relay complements and provides a mid-range service
between ISDN, which offers bandwidth at 128 Kbps, and
Asynchronous Transfer Mode (ATM), which operates in
somewhat similar fashion to frame relay but at speeds from
155.520 Mbps or 622.080 Mbps.

Frame relay is based on the older X.25 packet-switching
technology which was designed for transmitting analog data
such as voice conversations.
Unlike X.25 which was designed for analog signals, frame
relay is a fast packet technology, which means that the
protocol does not attempt to correct errors.
When an error is detected in a frame, it is simply
"dropped." (thrown away).
The end points are responsible for detecting and
retransmitting dropped frames.

Implementation
Network providers commonly implement frame relay for voice
(VoFR) and data as an encapsulation technique, used
between local area networks (LANs) over a wide area network
(WAN).
Each end-user gets a private line (or leased line) to a
frame relay node.
The frame relay network handles the transmission over a
frequently changing path transparent to all end-user
extensively used WAN protocols.

It is less expensive than leased lines and that is one
reason for its popularity.
The extreme simplicity of configuring user equipment in a
frame relay network offers another reason for frame relay's
popularity.

Architecture of frame relay
network

Physical Layer
No specific protocol is defined for the physical layer in
Frame Relay.
Instead, it is left to the implementer to use whatever is
available.
Frame Relay supports any of the protocols recognized by
ANSI.

Data link layer
At the data link layer, Frame Relay uses a simple protocol
that does not support flow or error control.
It only has an error detection mechanism.
The address field defines the DLCI as well as some bits
used to control congestion.

Protocol data unit
in frame relay
Flag field
Address Field
Information field
Frame check sequence

Flag field
The flag is used to perform high-level data link
synchronization which indicates the beginning and end of
the frame with the unique pattern 01111110.
To ensure that the 01111110 pattern does not appear
somewhere inside the frame, bit stuffing and
destuffing procedures are used.

Address field
Each address field may occupy either octet 2 to 3, octet 2 to 4, or
octet 2 to 5, depending on the range of the address in use.
A two-octet address field comprises the EA=ADDRESS FIELD EXTENSION BITS
and the C/R=COMMAND/RESPONSE BIT.
DLCI-Data Link Connection Identifier Bits. The DLCI serves to identify
the virtual connection so that the receiving end knows which
information connection a frame belongs to.
Note that this DLCI has only local significance. A single physical
channel can multiplex several different virtual connections.
FECN, BECN, DE bits. These bits report congestion:
◦ FECN=Forward Explicit Congestion Notification bit
◦ BECN=Backward Explicit Congestion Notification bit
◦ DE=Discard Eligibility bit

Information field
A system parameter defines the maximum number of data
bytes that a host can pack into a frame.
Hosts may negotiate the actual maximum frame length at
call set-up time.
The standard specifies the maximum information field size
(supportable by any network) as at least 262 octets.
Since end-to-end protocols typically operate on the basis
of larger information units, frame relay recommends that
the network support the maximum value of at least 1600
octets in order to avoid the need for segmentation and
reassembling by end-users.

Frame Check Sequence (FCS) Field.
Since one cannot completely ignore the bit error-rate of
the medium, each switching node needs to implement error
detection to avoid wasting bandwidth due to the
transmission of erred frames.
The error detection mechanism used in frame relay uses
the cyclic redundancy check (CRC) as its basis.

FRADs
To handle frames arriving from other protocols, Frame
Relay uses a device called a Frame Relay
assembler/disassembler (FRAD).
A FRAD assembles and disassembles frames coming from other
protocols to allow them to be carried by Frame Relay
frames.
A FRAD can be implemented as a separate device or as part
of a switch.

VOFR
Frame Relay networks offer an option called Voice Over
Frame Relay (VOFR) that sends voice through the network.
Voice is digitized using PCM and then compressed.
The result is sent as data frames over the network.
This feature allows the inexpensive sending of voice over
long distances.
However, note that the quality of voice is not as good as
voice over a circuit-switched network such as the telephone
network.
Also, the varying delay mentioned earlier sometimes
corrupts real-time voice.

LMI
Frame Relay was originally designed to provide PVC connections. Therefore, a
provision for controlling or managing interfaces.
Local Management Information (LMI) is a protocol added recently to the Frame
Relay protocol to provide more management features.
In particular, LMI can provide
1. A keep-alive mechanism to check if data are flowing.
2. A multicast mechanism to allow a local end system to
send frames to more than one remote end system.
3. A mechanism to allow an end system to check the status
of a switch (e.g., to see if the switch is congested).

Congestion control
The frame relay network uses a simplified protocol at each
switching node.
It achieves simplicity by omitting link-by-link flow-control.
As a result, the offered load has largely determined the
performance of frame relay networks.
When offered load is high, due to the bursts in some
services, temporary overload at some Frame Relay nodes
causes a collapse in network throughput.

Therefore, frame relay networks require some effective mechanisms to
control the congestion.
Admission control: This provides the principal mechanism used in frame
relay to ensure the guarantee of resource requirement once accepted
Committed information rate: The average rate (in bit/s) at which the
network guarantees to transfer information units over a measurement
interval T. This T interval is defined as: T = Bc/CIR.
Committed Burst Size (BC): The maximum number of information units
transmittable during the interval T.
Excess Burst Size (BE). The maximum number of uncommitted information
units (in bits) that the network will attempt to carry during the
interval.

Frame relay

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