Frame Relay & ATM

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Lecture 7

Paul Flynn

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Virtual Circuit SwitchingVirtual Circuit Switching

Digital Line Connection Identifier (DLCI)

Three Phases

Data Transfer Phase

Setup Phase

Teardown Phase

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Virtual circuit wide area network

Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.

Frame Relay operates only at the physical and data link layers.

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A 'local identifier' between the DTE and the DCE, it identifies the logical connection that is multiplexed into the physical channel.

Value that specifies a PVC in a Frame Relay network.

In the basic Frame Relay specification, DLCIs are 'locally significant'.

In the LMI extended specification, DLCIs are 'globally significant' (DLCIs specify individual end devices).

The FR Switch maps the DLCIs between each pair of routers to create a PVC.

DLCI values are typically assigned by the Frame Relay service provider

DLCI(Data-link Connection Identifier)

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Frame Relay Interface types

• UNI: User-|Network Interface• NNI: Network-Network Interface

Frame Relaynetwork

Frame Relaynetwork

Frame Relaynetworkuser user

UNI UNINNI NNIPVC segment

Multi-network PVC

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Frame Relay Functions

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VCI

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VCI phases

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Switch and table

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Frame RelayLocal addressing

• DLCI (Data Link Connection Identifier) - identification of a virtual circuit• DLCI - of local (for a given port) meaning• there can be max. 976 VCs on an interface user-network

• DLCI values: 0 - LMI channel, 1-15 - reserved, 16-991 - available for VCs, 992-1007 - layer 2 management of FR service, 1008-1022 - reserved, 1023 - in channel layer management

A

B

C

To A: DLCI 121To B: DLCI 243

To A: DLCI 182To C: DLCI 121

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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 RelayFrame Relay

Architecture

Frame Relay Layers

FRAD

VOFR

LMI

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Frame Relay network

VCIs in Frame Relay are called DLCIs.

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Frame Relay layers

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Frame Relay frame

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Frame RelayGlobal addressing

• Extension proposed by “Group of Four”• Each end user access device FRAD is assigned a unique

DLCI number - a global addressTransmission to a given user goes over VC identified by a unique DLCI

• Current DLCI format limits number of devices to less than 1000

• Another addition to the standard - extended DLCI addresses

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Three address formats

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LAPF Frame – Address Field

6-bits

4-bits

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FRAD

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Frame RelayFlow and congestion control

• There is no explicit flow control in FR; the network informs a user about congestion

• Congestion: FR frames are discarded from overflowed buffers of switching devices

• Congestion information:– FECN - Forward Explicit Congestion Notification– BECN - Backward Explicit Congestion Notification

• There are recommendations for access devices what to do with FECN and BECN (usually not implemented)

Transmission direction

BECN FECNFRAD

FRAD

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Frame Relay Concepts

Queue

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Frame Relay Concepts

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Frame Relay Concepts

DLCI-identifies logical connections on the Frame Relay switch to which the customer is attached

BECN-tells sending DTE device to reduce the rate of sending data.

FECN-tells receiving DTE device to implement congestion avoidance procedures

FRAMES

BECN FECN

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Frame RelayParameters of a UNI interface

• Physical speed - just clock rate• Guaranteed bandwidth parameters

– CIR: Committed Information Rate– BC: Committed Burst Size

• Extended bandwidth parameters– EIR: Extended Information Rate– BE: Extended Burst Size

• TC: Measurement IntervalUsertraffic

192kbps

64kbps

EIR

CIR

256kbps

time

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Frame RelayCIR and EIR - how does it work

• BC = TC * CIR

• BE = TC * EIR

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5Within CIR Within CIR Marked DE Marked DE Discarded

Bits

BC+BE

BC

T0

T0+TC

Time

CIR

CIR + EIR

Clock rate

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CIR (Committed Information Rate - The rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics.

Local access rate - The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network.

Committed Burst (Bc) - The maximum number of bits that the switch agrees to transfer during any Committed Rate Measurement Interval (Tc).

Excess Burst - The maximum number of uncommitted bits that the Frame Relay switch will attempt to transfer beyond the CIR. Excess Burst is dependent on the service offerings available by your vendor, but is typically limited to the port speed of the local access loop.

Glossary

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ECN (Forward explicit congestion notification) - When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device indicating that congestion has occurred.

BECN (Backward explicit congestion notification) - When a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router instructing the router to reduce the rate at which it is sending packets.

DE (Discard Eligibility indicator) - When the router detects network congestion, the FR switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic; that is, the traffic that was received after the CIR was met.

More Terms

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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

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Frame Relay Local Management Interface - LMI

• LMI - a signaling protocol used on an interface: end user - network (UNI)

• Implementation optional (everybody implements it...)• Usage:

– notification about: creation, deletion, existence of PVCs on a given port– notification about status and availability of PVCs– periodic checks of integrity of physical connection

• Planned extensions:– dynamic (SVC) channel creation and deletion– congestion notification

• Also planned: LMI for network-network interface (NNI)

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A signalling standard between the CPE device and the FR Switch that is responsible for managing the connection and maintaining "status" between the devices.

Set of enhancements to the basic Frame Relay specification. LMI includes support for:

• 'keepalive mechanism', which verifies that data is flowing; • 'multicast mechanism', which provides the network server

with its local DLCI and the multicast DLCI; • ‘global addressing', which gives DLCIs global rather than

local significance in Frame Relay networks; • 'status mechanism', which provides an on-going status

report on the DLCIs known to the FR Switch.

LMI(Local Management Interface)

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The main purpose for the LMI process is: (management of the connection) – PVC status - What is the operational status of the various PVCs that the

router knows about? – Transmission of 'keepalive' packets - Insure that the PVC stays up and

does not shut down due to inactivity.

Three types of LMIs are supported: 1. cisco - LMI type defined jointly by Cisco, StrataCom, Northern Telecom,

and DEC (frame relay forum) 2. ansi - Annex D defined by ANSI standard T1.617 3. q933a - ITU-T Q.933 Annex A

LMI encapsulation types: – IETF Encapsulation Type – Cisco Encapsulation Type

LMI

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Local Management Interface (LMI)

• 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

Frame Relay Map The term map means to “map” or bind a

Layer 2 address to a Layer 3 address. An ARP table maps MACs to IPs in a LAN In ISDN, we use the dailer-map command to map

SPIDs to IP addresses In Frame Relay, we need to map the data link

layer’s DLCI to the IP address We use the frame-relay map command

Frame Relay Map The Frame Relay switch builds a table of

incoming/outgoing ports and DLCIs. The router builds a Frame Relay Map through Inverse

ARP requests of the switch during the LMI exchange process.

The Frame Relay Map is used by the router for next-hop address resolution.

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Frame RelayIARP

• FRADs know DLCIs of available PVCs (through LMI), but don’t know IP addresses of other ends

• IP addresses for given DLCIs are obtained automatically; mapping IP-DLCI is generated - dynamic mapping

• IARP can be switched of; static maps have to be generated by FRAD user

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Configuring Basic Frame Relay

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Configuring a Static Frame Relay Map

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Reachability Issues with Routing Updates in NBMA

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Reachability Issues with Routing Updates in NBMA

By default, a Frame Relay network provides nonbroadcast multiaccess (NBMA) connectivity between remote sites. An NBMA environment is treated like other multiaccess media environments, where all the routers are on the same subnet.

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Frame Relay Subinterfaces

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Configuring Point-to-Point Subinterfaces

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Verifying Frame Relay

• The show interfaces command displays information regarding the encapsulation and Layer 1 and Layer 2 status. It also displays information about the following:

The LMI type The LMI DLCI The Frame Relay data terminal equipment/data circuit-terminating equipment (DTE/DCE) type

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The show interface Command

LMI Type

LMI DLCI

LMI Status

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The show frame-relay lmi Command

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The show frame-relay pvc Command

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The show frame-relay map Command

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Troubleshooting Frame Relay The debug frame-relay lmi Command

PVC Status0x2 – Active0x0 – Inactive0x4 – Deleted

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ATMATM

Design Goals

Problems

Architecture

Switching

Layers

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Multiplexing using different frame sizes

A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-sized

block of information.

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Multiplexing using cells

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ATM multiplexing

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Architecture of an ATM network

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TP, VPs, and VCs

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Example of VPs and VCs

Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

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Connection identifiers

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Virtual connection identifiers in UNIs and NNIs

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An ATM cell

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Routing with a switch

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ATM layers

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ATM layers in endpoint devices and switches

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ATM layer

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ATM headers

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AAL1

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AAL2

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AAL3/4

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AAL5