NETE0510 Frame Relay

NETE0510: Communication Media and Data Communications

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NETE0510Frame Relay

Dr. Supakorn [email protected]


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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 error-

control 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

1. Source data needs to be stored for possible retransmission2. X.25 header is added to blocks of data to form a packet3. Routing calculations are made4. Packets enclosed in an LAPB frame by adding LAPB header

and trailer5. At intermediate node, flow- and error-control are performed6. The node removes data link layer field for routing purposes7. The entire process is repeated at each hop across the network


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Problems of X.25 (cont’d)

• 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 operationA 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 i

nternal 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



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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



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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 DLCIsThe 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



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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 ac

commodates 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 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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Hub


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Frame Relay Address Mapping and topology For large networks, full mesh topology is seldom afforda

ble. 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 wil

l 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


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Outline



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LAPF Frame Format

EAExtended 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-bitsEA


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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

LMI MessageFlag FlagFCS

1 2 1

Address

2 1

Control

1

PD

1

CR

1

MT

Contains DLCI


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Outline



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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 andLMI 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



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NBMA

By default, a Frame Relay network provides non-broadcast 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 split-horizon. However, not all network layer protocols allow split-

horizon 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?

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Frame Relay

NETE0510 Frame Relay

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