NETE0510
Frame Relay
Dr. Supakorn Kungpisdan
supakorn@mut.ac.th
NETE0510: Communication Media and Data
Communications
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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Background
 The most technical innovation to come out of
standardization work on narrowband ISDN is
Frame Relay.
 Frame relay is a streamlined technique for
packet switching that operates at the data link
layer with much less overhead than packet
switching X.25
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Key features of the X.25
 Call-control packets, used for setting up and clearing
virtual circuits, are carried on the same channel and
same virtual circuit as data packets. In effect, inband
signaling is used.
 Multiplexing of virtual circuits takes place at layer 3
 Both layer 2 and layer 3 include flow-control and errorcontrol mechanisms
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Problems of X.25

Packet switching results in considerable overhead. For
a simple network of just three nodes between source
and destination
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Source data needs to be stored for possible retransmission
X.25 header is added to blocks of data to form a packet
Routing calculations are made
Packets enclosed in an LAPB frame by adding LAPB header
and trailer
At intermediate node, flow- and error-control are performed
The node removes data link layer field for routing purposes
The entire process is repeated at each hop across the network
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Problems of X.25 (cont’d)
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3
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• With highly reliable digital transmission technology, e.g. one used in ISDN,
with a low probability of error, the above approach is not the best.
• Moreover, such overheads degrade effective utilization of the links
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Background (cont’d)
 Frame relay eliminates overhead of X.25 by
having these characteristics:
Call control signaling is carried on a separate logical
connection from user data. Thus, intermediate nodes
need not maintain state tables or process messages
relating to call control on an individual per-connection
basis.
Multiplexing and switching of logical connections are in
layer 2.
 Eliminate one entire layer of processing
There is no hop-to-hop flow- and error-control
(performed at a higher layer)
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Frame relay operation
A single-user data frame is sent
from source to destination, and an
acknowledgement, generated at a
higher layer, is carried back in a
frame
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Background (cont’d)
 Advantage of frame relay is streamlining the
communication process.
 Protocol functionality required at user-network interface and
internal network processing are reduced.
 Thus, lower delay and higher throughput can be expected.
 Disadvantages of frame relay compared to X.25:
 Lost the ability to do link-by-link flow and error control.
 In X.25 multiple VCs are carried on a single physical link and
LAPB is available at link layer for providing reliable
transmission.
 However, with the increasing reliability of transmission and
switching facilities, this is not a major disadvantage
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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Frame Relay
 Frame Relay is a packet-switched, connection-oriented,
WAN service. It operates at the data link layer of the OSI
reference model.
 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 DTE,
and DCE at the edge of the WAN.
 Originally 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.
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Frame Relay Operation
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Frame Relay (cont’d)
 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.
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Frame Relay Switches
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Frame Relay Concepts
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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Virtual Circuits
 The connection through the Frame Relay network
between two DTEs is called a virtual circuit (VC).
 Two types of VCs:
 Switched virtual circuits (SVCs): VCs that are established
dynamically by sending signaling messages to the network.
 Permanent virtual circuits (PVCs): preconfigured by the carrier
are used.
 A VC is created by storing input-port to output-port
mapping in the memory of each switch and thus linking
one switch to another until a continuous path from one
end of the circuit to the other is identified.
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Virtual Circuits (cont’d)
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Virtual Circuits
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Local Significance of DLCIs
The data-link connection identifier (DLCI) is stored in
the Address field of every frame transmitted.
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Frame Relay Stack Layered Support
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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Frame Relay Bandwidth and Flow Control
 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 each VC. CIR may be less than port speed
 Sum of CIRs may be greater than port speed. Statistical multiplexing
accommodates the bursty nature of computer communications since
channels are unlikely to be at their maximum data rate simultaneously.
 The difference between the CIR and the maximum, whether the
maximum is port speed or lower (that the switch can handle), is
called the Excess Information Rate (EIR).
 Switch can accept the frames at the rate CIR+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).
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Frame Relay Bandwidth and Flow Control
(cont’d)
 Although the switch accepts frames in excess of the CIR,
each excess frame is marked at the switch by setting the
Discard Eligible (DE) bit to "1" in the address field.
 The switch maintains a bit counter for each VC. An
incoming frame is marked DE if it puts the counter over
Bc.
 An incoming frame is discarded if it pushes the counter
over Bc + Be.
 At the end of each Tc seconds the counter is reset. The
counter may not be negative, so idle time cannot be
saved up.
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Frame Relay Bandwidth and Flow Control
(cont’d)
 Frames arriving at a switch are queued or buffered prior
to forwarding.
 It is possible that there will be an excessive buildup of frames
at a switch. This causes delays.
 Delays lead to unnecessary retransmissions that occur
when higher-level protocols receive no acknowledgment
within a set time.
 This can cause a serious drop in network throughput.
 To avoid this problem, Frame Relay switches incorporate
a policy of dropping frames from a queue to keep the
queues short.
 Frames with their DE bit set will be dropped first.
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Frame Relay Bandwidth and Flow Control
(cont’d)
 When a switch sees its queue increasing, it tries to
reduce the flow of frames to it.
 It does this by notifying DTEs of the problem by setting
the Explicit Congestion Notification (ECN) bits in the
frame address field.
 The Forward ECN (FECN) bit is set on every frame that
the switch receives on the congested link.
 The Backward ECN (BECN) bit is set on every frame
that the switch places onto the congested link.
 DTEs receiving frames with the ECN bits set are
expected to try to reduce the flow of frames until the
congestion clears.
 The DE, FECN and BECN bits are part of the address
field in the LAPF frame.
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Frame Relay Bandwidth and Flow Control
(cont’d)
Queue
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Frame Relay Bandwidth and Flow Control
(cont’d)
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Frame Relay Bandwidth and Flow Control
(cont’d)
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Frame Relay Address Mapping and
topology
 WANs are often interconnected as a star topology. A
central site hosts the primary services and is connected
to each of the remote sites needing access to the
services
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Frame Relay Address Mapping and
topology (cont’d)
 In a hub and spoke topology the location of the
hub is chosen to give the lowest leased line cost.
 When implementing a star topology with Frame
Relay, each remote site has an access link to
the frame relay cloud with a single VC.
 The hub has an access link with multiple VCs,
one for each remote site.
 Because Frame Relay tariffs are not distance
related, the hub does not need to be in the
geographical center of the network.
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Frame Relay Address Mapping and
topology (cont’d)
Hub
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Frame Relay Address Mapping and
topology
 For large networks, full mesh topology is seldom
affordable. This is because the number of links required
for a full mesh topology grows at almost the square of
the number of sites.
 While there is no equipment issue for Frame Relay, there
is a limit of less than 1000 VCs per link.
 In practice, the limit will be less than that, and larger networks
will generally be partial mesh topology.
 With partial mesh, there are more interconnections than
required for a star arrangement, but not as many as for a
full mesh. The actual pattern is very dependant on the
data flow requirements.
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Frame Relay Address Mapping and
topology (cont’d)
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Frame Relay Address Mapping and
topology
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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LAPF Frame Format
EA
Extended Address field signifies up to two additional bytes in the Frame Relay
header, thus greatly expanding the number of possible addresses.
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LAPF Frame – Address Field
6-bits
4-bits
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Data Link Control Identifier
 The 10-bit DLCI associates the frame with its virtual circuit
 It is of local significance only - a frame will not generally be
delivered with the same DLCI with which it started
 Some DLCI’s are reserved
(Consolidated Link Layer Management)
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Consolidated Link Layer Management
 It may occur that there are no frames traveling back to
the source node which is causing the congestion.
 In this case, the network will want to send its own
message to the problematic source node.
 The standard, however, does not allow the network to
send its own frames with the DLCI of the desired virtual
circuit.
 To address this problem, ANSI defined the
Consolidated Link Layer Management (CLLM).
 The ANSI standard (T1.618) defines the format of the
CLLM message. It contains a code for the cause of the
congestion and a listing of all DLCIs that should act to
reduce their data transmission to lower congestion.
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Local Management Interface (LMI)
 Each DLCI corresponds to a PVC.
 It is sometimes necessary to transmit information about
this connection (e.g., whether the interface is still active)
the valid DLCIs for the interface and the status of each
PVC.
 This information is transmitted using the reserved DLCI
1023 (ANSI, ITU) or DLCI 0 (Cisco), depending on the
standard used.
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LMI (cont’d)
 There are several LMI types, each of which is incompatible with the
others.
 The LMI type configured on the router must match the type used by
the service provider.
 Three types of LMIs are supported by Cisco routers:
 Cisco — The original LMI extensions
 Ansi — Corresponding to the ANSI standard T1.617 Annex D
 q933a — Corresponding to the ITU standard Q933 Annex A
 LMI messages are carried in a variant of LAPF frames. This variant
includes four extra fields in the header so that they will be
compatible with the LAPD frames used in ISDN.
 Control, protocol discriminator, call reference, LMI message type
 The address field carries one of the reserved DLCIs.
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LMI Frame Format
Contains DLCI
1
Flag
2
Address
1
1
1
1
Control
PD
CR
MT
2
LMI Message
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FCS Flag
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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Inverse ARP
 If the router needs to map the VCs to network layer
addresses, it will send an Inverse ARP message on
each VC.
 The Inverse ARP message includes the network layer
address of the router, so the remote DTE, or router, can
also perform the mapping.
 The Inverse ARP reply allows the router to make the
necessary mapping entries in its address to DLCI map
table.
 If several network layer protocols are supported on the
link, Inverse ARP messages will be sent for each.
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Stages of Inverse ARP and
LMI Operation #1
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Stages of Inverse ARP and
LMI Operation #2
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Configuring Basic Frame Relay
The bandwidth value is used by IGRP, EIGRP,
and OSPF to determine the metric of the link.
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Configuring a Static Frame Relay Map
 The local DLCI must be statically mapped to the network
layer address of the remote router when the remote
router does not support Inverse ARP.
 This is also true when broadcast traffic and multicast
traffic over the PVC must be controlled. These static
Frame Relay map entries are referred to as static maps.
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Configuring a Static Frame Relay Map
(cont’d)
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Outline
 Background
 Frame Relay
 Virtual Circuits
 Frame Relay Bandwidth and Flow Control
 LAPF Frame Format
 Inverse ARP
 Non-broadcast Multi-access
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NBMA
 By default, a Frame Relay network provides nonbroadcast multi-access (NBMA) connectivity between
remote sites.
 An NBMA environment is viewed like other
multiaccess media environments, such as Ethernet,
where all the routers are on the same subnet.
 However, to reduce cost, NBMA clouds are usually built
in a hub-and-spoke topology.
 With a hub-and-spoke topology, the physical topology
does not provide the multi-access capabilities that
Ethernet does.
 The physical topology consists of multiple PVCs.
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NBMA Problems
A Frame Relay NBMA topology may cause
two problems:
Reachability issues regarding routing updates
The need to replicate broadcasts on each PVC
when a physical interface contains more than
one PVC
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Split Horizon and NBMA
 Split-horizon updates reduce routing loops by not
allowing a routing update received on one interface to be
forwarded out the same interface.
 If Router B, a spoke router, sends a broadcast routing
update to Router A, the hub router, and Router A has
multiple PVCs over a single physical interface, then
Router A cannot forward that routing update through the
same physical interface to other remote spoke routers.
 If split-horizon is disabled, then the routing update can be
forwarded out the same physical interface from which it
came.
 Split-horizon is not a problem when there is a single PVC
on a physical interface. This would be a point-to-point
Frame Relay connection.
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Split Horizon and NBMA (cont’d)
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Split Horizon and NBMA (cont’d)
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Replicating Broadcast Packets
 Routers that support multiple connections over a single
physical interface have many PVCs that terminate in a
single router.
 This router must replicate broadcast packets such as
routing update broadcasts, on each PVC, to the remote
routers.
 The replicated broadcast packets can consume
bandwidth and cause significant latency to user
traffic.
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Solving NBMA Problems
 It might seem logical to turn off split-horizon to
resolve the reachability issues caused by splithorizon.
However, not all network layer protocols allow splithorizon to be disabled and disabling split-horizon
increases the chances of routing loops in any network.
 One way to solve the split-horizon problem is to
use a fully meshed topology.
However, this will increase the cost because more PVCs
are required.
 The preferred solution is to use subinterfaces.
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Frame Relay Subinterfaces
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Configuring Point-to-Point
Subinterfaces
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Questions?
Next Lecture
Frame Relay
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