FRAME RELAY (FR)
INTRODUCTION:
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There were two main technologies in the 70’s and 80’s
3 factors necessitated development of new technologies
Need to handle higher data rates
Bursty LAN traffic
Cost
X.25 was designed for error prone links and wasted too mush resources in error checking
and flow control
Deployment of high quality links and fiber optic cables drastically reduced the need for
extensive error checking and flow control. Why check for errors every second when the
possibility of one occurring is only once in a week?
FR is a Fast Packet Switching technology very similar to X.25. It is really a stripped
down version of X.25
FR operates only over the Physical and Data Link Layer and leaves the responsibility for
error checking and flow control to higher level Protocols. It concentrates only on high
speed Data transfer
FR provides a PVC (Permanent Virtual Circuit Connection) between customers, that
appears to be a dedicated connection, but in fact it is a statistically multiplexed logical
connection.
FR provides BW on demand and therefore it is very suitable for bursty LAN traffic
whereby a customer transmits at variable data rates. Fig below shows a bursty traffic
pattern from a LAN.
FR is not suitable for constant data rate and time sensitive applications such as voice or
video.
Access rates range from fractional T1 to DS1 or even DS3
Pricing is based on access rate, # of configured PVCS and a CIR (Committed Information
Rate) assigned to the access line. In other words, the price is based on usage.
Very inexpensive for LAN-to-LAN connection over WAN as compared to T1 lines.
Widely available. Several LECS and IXCS such as Sprint, Bell, MCI provide FR service.
Although FR Standards support a Switched Virtual Connection also, but it is seldom used
in practice
Advantages of FR:
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Very inexpensive to implement. Sometimes only a software upgrade is required
Accommodates variable length frames
Provides BW on demand
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Cost effective for businesses as pricing is based on usage and therefore a customer pays
for only what he/she uses
High-speed data transfer.
Disadvantages:
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Not suitable for time sensitive applications such as voice, video and Multimedia.
FR Technology:
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Fig below shows a FR Network with associated components. Draw the picture in the
space provided. Fig 3-5/3-6 in the text p103.
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FRAD ( Frame Relay Assembler/Disassembler does the same job in a FR network what
PAD does for X.25. It converts user data into FR format.
FR Layers:
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FR implements only physical and Data Link Layers
FR accepts any existing physical layer protocol at the physical layer, such as RS-232,
422, 423, V.35 etc
At Data link Layer, it uses LAP-F CORE (link Access procedure for FR), which is a
subset of HDLC you learnt in Principles of Telecomm (DCCT 2010)
Frame Relay Frame Format:
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Fig below shows Frame relay frame format. Draw the picture in the space provided.
Explanation of Various fields:
Flag: same as HDLC – 7E ( 01111110). Marks the beginning and end of a frame
DLCI (Data Link Connection Identifier):
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A 10 bit number similar to Logical Channel Number in X.25, used for addressing.
Notice that in FR DLCI operates at link level and has only local significance. It has two
components – a 6 bit higher component and a 4 bit lower component.
A 10 bit DLCI allows a total of __________? DLCIS.
DLCI “0” is used for in-channel signaling
DLCI 1-15 are reserved for special purposes
16-991 can be assigned to users connection identification
FR allows 3-byte and 4- byte addressing as well. This is called extended addressing. In
that case, DLCI values will increase to allow a larger Network. Table 3-2, p105 Text
shows DLCI assignments for extended addressing.
C/R Bit: Command “1” , response “0”
EA: (Extended Address bit): used to indicate extended addressing. “1” if last byte of the
address, “0” otherwise.
DE (Discard Eligibility Bit): For frames going faster than the CIR, DE bit is set to “1” to
indicate to the Switches to drop them in the event of congestion. A brutal flow control
mechanism.
FECN and BECN bits: ( Forward/Backward Explicit Congestion Notification Bits):
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Used for flow control and congestion avoidance.
BECN bit is set to “1” in the frames going to Destination ( Receiver of the Frame) to tell
the Station that there is congestion in the direction it traveled in the hope that it will slow
down
FECN bit is set to “1” in the frames going back to transmitter to indicate that there is
congestion in the path.
Notice that FECN bit notifies the Station that caused the congestion in the first place i.e
it notifies the source of congestion whereas the BECN bit notifies that Receiver of the
Frames
FR Components and FR implementation:
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Fig below shows practical FR implementation.(Fig3-7,3-8,3-9 P106-108 Text Newman)
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FRADS: To access a FR network, a DTE needs a FR interface card and a FRAD is
required at each end of the FR Circuit. A FRAD , as mentioned earlier converts data
stream into FR frame format. FRAD function can be implemented in software in a
ROUTER or BRIDGE or a GATEWAY. Alternatively, it could also be a standalone
hardware device. Notice that the ROUTER acts as DTE and FRAD implements DCE
functions on the Network side (Equivalent to DSU/CSU functions in T1 circuits)
FR SWITCHES: FR Switches are usually located inside the cloud and perform Routing,
and High Speed Data transfer functions. Some times a small switch may also be located
at the customer premises. Cascade 9000 B-STDX is an example of a Carrier class Switch.
Do some research and study the specs for some of the commonly available carrier and
customer operated FR switches.
Network Interfaces:
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UNI ( User to Network Interface) : Defines procedures for connectivity between the
CPE and the Network Equipment. ANSI T-1.617 Annex D standards specify two
messages to verify Link Integrity. The CPE sends a Status Enquiry (SE) message to
determine the status of the PVC. The Network responds by sending the Status (S)
message specifying the status of the PVC. These messages are sent continuously at
regular intervals to make sure that the link is alive and well. In the LAB we shall
configure a PVC and Observe and record these messages.
NNI (Network to Network Interface): The NNI interface also sends SE and S
messages. However information exchanged between NNI is much more comprehensive
such as
Notification about the addition or deletion of a PVC
Verification of links between FR nodes located within the Cloud
Availability of a PVC
Diagnostics and Failure reports etc
These messages are enclosed in HDLC Unnumbered frames.
FR QoS parameters:
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FR uses 4 quantitative parameters to ensure QoS.
Access Rate
Committed Burst Size (Bc)
Committed Information Rate (CIR)
Excess Burst Size (Be)
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Access Rate: The maximum rate in bps that the physical line can carry. A user can never
exceed this rate. For example if the user is connected to the Network via a T1 line, his
rate can never exceed 1.544Mbps
Committed Burst Size (Bc) : The maximum number of bits in a specified time period
that the Network commits to transmit without dropping any frames. It is a cumulative
measurement. Suppose you buy a Bc of 400Kbps over a period of 4 s, then you can
transmit 100 kbps during each of the first and 2nd seconds, nothing during the 3rd and then
200Kbps in the 4rth second. Or 400Kbps during the first second and then nothing over
the next 3 seconds.
CIR (Committed Information rate: CIR=Bc/T. In other words, the customer is
committing to an average rate determine by the above equation and the Network is
guaranteeing the safe delivery of the user data as long as he does not exceed this rate.
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Excess Burst Size (Be) : The maximum number of bits in excess of Bc that a user can
send during a specified period of time. The Network will make a best effort to send these
bits, but no guarantees. In the event of congestion, the Network will drop those
frames that exceed Be. It will set the DE bit in those frames that exceed Be. See fig
below and draw the fig in the space provided.
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FR Applications :
LAN-to LAN Connectivity via WAN
Distance learning
Telemedicine
Eletronic tax filing
Transmitting high resolution graphics
Large file transfer
Business organization
CAD/CAM
Read detailed applications in the Text page 112 Newman
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Technology Alternatives: Which technologies, you think, competes with FR and what
are the advantages and disadvantages of these technologies as compared to FR