Introduction to Frame Relay

8/7/2019 Introduction to Frame Relay

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1. Introduction to Frame Relay:

Frame Relay is a high-performance WAN protocol that operates at the physical and data link

layers of the OSI reference model. Frame Relay is an example of a packet-switched technology.

Packet-switched networks enable end stations to dynamically share the network medium and the

available bandwidth.

Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust

capabilities, such as windowing and retransmission of last data that are offered in X.25. Frame

Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network

layer) as well. This enables Frame Relay to offer higher performance and greater transmissionefficiency than X.25, and makes Frame Relay suitable for current WAN applications, such as

LAN interconnection.

1.1.Frame Relay Devices

Devices attached to a Frame Relay WAN fall into the following two general categories:

Data terminal equipment (DTE)

Data circuit-terminating equipment (DCE)

DTEs generally are considered to be terminating equipment for a specific network and typically

are located on the premises of a customer. Examples of DTE devices are terminals, personal

computers, routers, and bridges.

DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to provide

clocking and switching services in a network, which are the devices that actually transmit data

through the WAN.

1.2.Frame Relay Virtual Circuits

Frame Relay provides connection-oriented data link layer communication. This means that a

defined communication exists between each pair of devices and that these connections are

associated with a connection identifier. This service is implemented by using a Frame Relay

virtual circuit, which is a logical connection created between two data terminal equipment (DTE)

devices across a Frame Relay packet-switched network (PSN).

Virtual circuits provide a bidirectional communication path from one DTE device to another and

are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits

can be multiplexed into a single physical circuit for transmission across the network. This

capability often can reduce the equipment and network complexity required to connect multipleDTE devices. A virtual circuit can pass through any number of intermediate DCE devices

(switches) located within the Frame Relay PSN.

Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and

permanent virtual circuits (PVCs).SVCs are not used nowadays and hence not concentrated much in this article.

1.3.Permanent Virtual Circuits


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Permanent virtual circuits (PVCs) are permanently established connections that are used for

frequent and consistent data transfers between DTE devices across the Frame Relay network.

PVCs always operate in one of the following two operational states:

Data transferData is transmitted between the DTE devices over the virtual circuit.

IdleThe connection between DTE devices is active, but no data is transferred. PVCs will not

be terminated under any circumstances when in an idle state.

DTE devices can begin transferring data whenever they are ready because the circuit is

permanently established.

1.4.Data-Link Connection Identifier

Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). DLCI

values typically are assigned by the Frame Relay service provider. Frame Relay DLCIs have

local significance, which means that their values are unique in the LAN, but not necessarily in the

Frame Relay WAN.

1.5.Frame Relay Local Management Interface

The Local Management Interface (LMI) is a set of enhancements to the basic Frame Relayspecification.

The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI)

values global rather than local significance. DLCI values become DTE addresses that are unique

in the Frame Relay WAN.

LMI virtual circuit status messages provide communication and synchronization between Frame

Relay DTE and DCE devices.

2 Introduction to ATM

ATM is a cell-relay technology that divides upper-level data units into 53-byte cells for

transmission over the physical medium. It operates independently of the type of transmission

being generated at the upper layers AND of the type and speed of the physical-layer medium

below it.

This allows the ATM technology to transport all kinds of transmissions (e.g, data, voice, video,etc.) in a single integrated data stream over any medium, ranging from existing T1/E1 lines, to

SONET OC-3 at speeds of 155 Mbps, and beyond.

The basic network structure is as shown on the following page.


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Three types of interfaces exist in this diagram:

1. User-to-Network Interface (UNI)

2. Network-to-Network Interface (NNI)

3. Inter-Carrier Interface (ICI)

The UNI exists between a single end user and a public ATM network, between a single end user

and a private ATM switch, or between a private ATM switch and the public ATM network of an

carrier. It is denoted as 1 in the above diagram.

The NNI exists between switches in a single public ATM network. NNIs may also exist between

two private ATM switches. Denoted as 2 in the diagram

The ICI is located between two public ATM networks (an RBOC and an interexchange carrier).

Denoted as 3 in the diagram.

All of these interfaces are very similar. The major differences between these types of interfaces

are administrative and signalling related. The only type of signalling exchanged across the UNI is

that required to set up a VIRTUAL CHANNEL for the transmission.

Communication across the NNI and the ICI will require signalling for virtual-path and virtual-

channel establishment together with various exchange mechanisms for the exchange of

information such as routing tables, etc.

The network functions as follows: End User 1 wishes to transfer a data file to End User 2. A

virtual channel is created and a virtual path is established from switch to switch within the public


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ATM network in ATM Network 1 which in turn, establishes contact with the public ATM

network 2.

ATM Network 2 also establishes a virtual path from switch to switch within the network and with

the Private ATM Switch at the destination. The private ATM network completes the virtual pathby establishing a virtual channel with End User 2.

At each interface in this network, a unique virtual path identifier (VPI) and virtual channel

identifier (VCI) are established for this transmission. These identifiers are of local significance

ONLY: the identifier is significant only for a specific switch and the two nodes adjacent to it in

the virtual path.

End User 2 encapsulates the file in 53-byte cells, each with its unique VPI/VCI "destination

address" in the header. These cells are streamed and sent across the UNI to the ATM network

switch. This switch reads the ATM header, consults the routing table created during the virtual

path setup, changes the VPI/VCI as necessary, and sends each cell in the stream out of the

appropriate port and across the NNI to the next switch in the virtual path.

The last switch within the virtual path for ATM Network 1 repeats this process and sends the cellout through the ICI to ATM Network 2.

ATM Network 2 continues the process in a similar manner until the cell is carried through the

UNI to the Private ATM Switch which, in turn, sends the cell to End User 2. End User 2 thenreconstructs the file from the sequential cells, stripping the header from each cell.

3 ATM & Frame Relay in ISP Environment

In ISP environment, only the provider end switches are either ATM or FR switch to satisfy thecustomer needs. But the connectivity within the core is mostly via ATM only.

In core Cisco BPX, MGX & IGX switches are used. The switches are physically connected with

STM-1 or STM-3 links. BPX is ATM switch and IGX and MGX are FR / ATM Switch.


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Customer locations which fall under the same PoP need not cross the core. So the connectivity

can be implemented using only FR or ATM depending on customer need. It is denoted as blue

line in the diagram. However the customer locations under different PoP locations need to cross

the provider core. The link between the customer end and POP can be either ATM or FR PVC.

But the PVC which connects the ISP PoPs use ATM PVC. The connectivity between the

customer locations under different PoPs is mentioned using Red line in the diagram.

4 Switches used in ISP:

4.1 Cisco BPX switch:

The BPX is an ATM switch that can support upto 19.2 Gbps across the backplane, capable of

switching upto 20 million cells per second.

It supports multiple broadband service interfaces ranging from T3 upto OC12 that can receiveand transmit standard UNI or NNI ATM cells. Cells are policed at ingress and forwarded across

the netword as CBR, VBR, ABR or UBR PVCs.

It user hardware base switching technique.

Following are the connection types in BPX switches

Constant Bit Rate (CBR): Typically used for time dependent traffic such as uncompressed voice

and video.


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Variable Bit Rate (VBR): Typically of a bursty nature, this type of connection may be used for

compressed voice, video or data.

Available Bit Rate (ABR): This type of traffic is similar to VBR but with the addition of ratecontrol.

Unassigned Bit Rate: This traffic is a best effort service with no guarantees.

Frame Relay to ATM: The BPX can transport or terminate the ATM cells on an interworking

connection. However, it cannot do any Network Interworking conversion, this will be carried out

by other Cisco products such as IGX or MGX switches.

Reliability Features

The BPX processor cards have the option of built in redundancy by using two cards, once will be

active, the other in a hot standby mode.

4.2 Cisco IGX switches:

The IGX node is a multiservice switch that can support up to 1.2 Gbps across the backplane using

a multiplexed bus architecture.

The IGX node encapsulates ingress voice, data and frame traffic into FastPackets for transport

across the backplane. ATM traffic may remain as ATM, or be segmented using a cell forwarding

gateway into FastPackets, for transport across the network.

The IGX reliability features are similar to those of BPX with the exception that there is no

support as yet for Sonet APS.

4.3 Cisco MGX switches

The MGX nodes support both ATM & Frame Relay.

MGX provides reliability, any detected failure of the active card will cause the switch to the

standby.

5 Hardware Types

5.1 BPX:

The BPX service node is a 15-card slot rack-mounted chassis. It has a backplane with connectors

on the front and back to attach the front and back card modules.


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5.1.1 Broadband Control Card:

The BCC is the central processor of the BPX switch and stores system software and all

configuration information. The BCC houses the crosspoint switch, which is the heart of the BPX

cell switching function. The BCC also communicates with other nodes in the network and

network management platforms.

The BCCs installed in card slots 7 & 8. One BCC is always active, the other is a hot standby.

5.1.2 Alarm Service Monitor:

The ASM is responsible for measuring local environmental conditions and reporting local and

network alarms using LEDs and an external alarm relay. The ASM is always installed in card slot

15.

5.1.3 Broadband Network Interface:

BNI cards support only T3, E3 and OC3 connections. The BNI card provides two or three

network trunks between the BPX service node and any of the following devices:

Another BPX switch with a BNI trunk card.

A Cisco MGX ATM interface shelf. A Cisco IGX multiservice cell relay switch.

5.1.4 ATM Service Interface:


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The ASI card provides two ATM line interfaces to accept user traffic from an attached user

device. The ASI line can terminate at any of the following devices:

An ATM router

An ATM switch

An ATM access device Any device that transmits standards-compliant ATM UNI or NNI cells.

The ASI card receives ATM cells, polices them, queues them, and passes them to the crosspoint

switch for transport across the network.

5.1.5 Broadband Switch Module:

The BXM module card supports both lines and trunks. As a trunk interface, the BXM can support

broadband trunks to the following equipment:

A BPX service node with BXM trunk A Cisco MGX interface shelf with BNM trunk card Any ATM UNI 3.1 standard interface

A line interface on a BXM card can terminate on any of the following devices:

An ATM router An ATM switch

An ATM access device Any device that transmits ATM UNI or NNI cells.

5.1.6 Broadband Multicast Engine:

The BME supports multicast connections on the BPX switch. A connection originating from anASI or BXM line card terminates on the BME and is then multicast from the BME to multiple

terminating ASI or BXM cards. The BME does not interface to any end-user equipment.

5.2 MGX:

The MGX 8250 chassis consists of 2 main bays, an upper and lower bay, which can

accommodate up to 32 half height cards.

The upper and lower bays are separated by a divider which may be removed to convert card slots

from half to full height.

The back-plane design connects the front and back card sets together and also contains several

buses for data, control and clocking.


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There are two common core cards namely the PXM and SRM

The PXM is responsible for managing the shelf and holds the Hard Disk which stores

configuration information and file images.

It has a shared memory switch which is used to switch cells locally or to / from the broadband

interface.

The SRM is used for Bert testing, T1 line distribution and 1:N redundancy.

The PXM and SRM work such that is the PXM in slot 7 is active then SRM s in slots 15 & 31

would also be active if installed. Similarly, the cards in slots 8, 16 & 32 would also form a logical

group.

5.2.1 Processor Switch Module:

Controls the switching of cells between local cards or between cards and the broadband

uplink. All the SRM activity is managed from the PXM.

It has external interfaces for network management devices. The PXM has an in built stratum4 oscillator that can be used as a network clock source. It

can also be configured to accept a clock from an external source.


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The PXM is responsible for monitoring all environmental and reporting failure if any.

5.2.2 Service Resource Module:

When the failure of an active card in a group is detected by the ASC, the SRM is ordered

to invoke 1:N redundancy for that group. With the SRM T1E1, only a single failure I

supported. SRM 3T3 can support multiple group failures. SRM 3T3 supports T1 distribution to service modules. This feature allows large

numbers of T1 lines to be supported over three T3 lines rather than over individual T1

lines. SRM can support BERT on a port or line and includes the facility to place loopbacks on

these interfaces.

5.2.3 Circuit Emulation Service Module:

The CESM segments and reassembles up to 8 T1/E1 lines using AAL1.

Two card sets are supported:

8T1 Has RJ48 connectors on the back card.

8E1 Has a choice of RJ48 or SMB connectors on the back card.

Can monitor the CAS signaling bits for on / off hook states for individual 64K

connections. This saves bandwidth by suppressing ATM cells during the on hook state. Will monitor the line for physical layer alarms such as loss of signal and framing.

5.2.4 Frame Relay Service Module:

The FRSM segments reassembles frame relay using AAL5 SAR. The FRSM also polices

incoming traffic and can carry out the optional ForeSight traffic flow feature.

Supports the following cards,

FRSM 8T1, FRSM 8E1, FRSM HS1, FRSM HS2, FRSM 2CT3 and FRSM 2T3/E3

Segmentation and Reassembly is carried out using AAL5 SAR processes. Supports LMI, ELMI, Annex A and Annex D signaling protocols.

It polices all frames at ingress Can carry out Network Inter Working (NIW) and Service Inter Working (SIW) in

accordance with FRF.5 and FRF.8 respectively. Can process legacy frame based traffic such as X.25, SDLC and HDLC but is limited to a

single connection per port. Will monitor the line fro physical layer alarms such as loss of signal and framing.

5.2.5 ATM UNI Service Module:

AUSM processes incoming ATM cells which may be either of UNI or NNI format.

It supports 8T1 and 8E1 cards.


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Supports UNI or NNI cell headers.

Supports all four types of ATM services CBR, VBR, ABR and UBR.

Supports VPI and VCI connections.

Multiple T1 or E1 lines can form a single ATM port using ATM inverse multiplexing.

Supports ILMI signaling protocol.

Will monitor the line fro physical layer alarms such as loss of signal and framing.

5.2.6 Route Processor Module:

The RPM provides layer 3 functionality on the MGX 8250

RPM is a 7200 series router with a 150/300 MHz processor Interfaces with the MGX cell bus via an internal 155 Mbps ATM deluxe port adapter.

5.3 IGX:


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The IGX chassis comes in three main sizes, 8, 16, and 32 slots. The first two slots in each

chassis type are reserved for processor cards

5.3.1 FRAME

The IGX backplane operates at 1.2 GBPS and is TDM based.

Four different Cards in Frame group:


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FRM D - this card supports 4 serial lines with V.35 or X.21 interfaces.

FRM E - this card supports a single E1/T1 channelised Line.

The above cards convert FR into FastPackets and have a throughput limit of up to 2.048

Mbps.

UFM U - this card can support 4 serial HSSI interfaces, or 12 serial V.35/X.21 interfaces.

UFM C this card can support either 4 or 8 channelised T1/E1 lines.

Assembles and disassembles Frames

All frame relay signaling mechanisms are supported, (E)LMI, Annex A and AnnexD

The card will monitor the physical layer for any alarms or errors.

5.3.2 ATM

The UXM card group maps incoming UNI/NNI cells to the backplane for either local switching

or for transmission across packets/cell based trunks.

There are three different cards in the group - T1/E1, T3/E3 and OC3.

Map incoming CBR, VBR, ABR and UBR cells to the backplane for either network or

local connectivity. All ATM signaling mechanisms are supported, LMI & ILMI Monitors physical layer for any alarms or errors.

6 Commands in IGX/BPX and MGX

6.1 Creating Frame relay PVC on IGX:

1 Before we create PVC, we need to add the port in which we want the PVC. The port

may be chanalised (logical) or unchanalised according to the need.

Syntax: addport .

addport 1.2 1.1-8 ( here the slot is 1, port is 2, line is 1 and used the timeslots 1 to 8

which will give the bandwidth of 512Kbps)

2. Now we need to configure the port as per the required parameters

Syntax: cnfport

cnfport 1.2 dce ** * d * * *


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If we use * in place of any configuration parameters, it will take the default value. In the

above example, default value will be taken for port-queue-depth, ecn-queue-threshold

and de-threshold and the signaling protocol will be Annex D, since we selected d.

3. Now we need to bring the port up.

Syntax: upport

upport 1.2

4. Once the port is up, then the PVC can be added.

Syntax: addcon

Note: In the remote side, the port should be active.

addcon 1.2.101 CHEN 3.2.102

Now a PVC is created to the node CHEN. The DLCI at CHEN end is 102 and thelocal DLCI is 101.

5. Now the PVC need to be configured as per the requirement.

Syntax: cnfcon

cnfcon 1.2.101 96/128 128/128 * * * * *

Now the connection is established and configured with CIR of 128Kbps and port speed of

512Kbps.

6.2 Deleting Frame Relay PVC:

1. PVC can be deleted by giving the following command

Syntax: delcon

delcon 1.2.101

Above command will delete the other end PVC also. No separate command required atthe remote end.

2. Now the port need to be made down before deleting it.

Syntax: dnport

dnport 1.2

3. Port can be deleted now using the following command


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Syntax: delport

delport 1.2

6.3 ATM PVC on IGX / BPX:

Before add the PVC, ensure that the line and port on both sides are configured. We can createthree type of PVCs. 1. CBR, 2. VBR, 3. ABRFST, 4. ABRSTD & UBR Normally in ISP

environment ABRFST is created.

Syntax: addcon .

Example:

addcon 12.1.1.100 CHEN 13.2.1.100 abrfst 20000 50 5000 15000 e d e * * * * *

upcon 12.1.1.100

To make the connection down,

downcon 12.1.1.100

delcon 12.1.1.100

6.4 PVCs in MGX

To create a PVC in MGX, first go to the particular slot where we need the PVC. PVC can be

created as master.

To add the port:

Syntax: addport

Example:

addport 5 1 4 1 4 1

In the above configuration,

5 Logical port1 Line number

4 No of timeslots (for 256K)1 Start timeslot

4 End timeslot

1 Framing type (1 - Framed, 2 Unframed)

To configure the port:

Syntax: cnfport "port_number percent_bandwidth min_VPI max_VPI max_rate_percent"


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Example:

cnfport 5 100 0 4095

cnfport 5 du n n * * * *

In the above configuration,

5 - Port number

du Annex A LMI

n Async

n Enhanced LMI

To add the PVC:

Syntax: addcon "port_num vpi vci conn_type service_type [Controller_Type] [mastership]

[remoteConnId]

Example:

addcon 5 1 100 1 1

In the above configuration;

5 Port number

1 VPI

100 VCI

1 CBR/VBR (1 CBR, 2 VBR)

1 Mastership

6.5 Interworking PVC:

To configure FR/ATM interworking PVC, it is better to configure from the FR side of the

PVC. Since the configuration parameters for FR is less compared to ATM, it will be easy to

add the interworking PVC from the FR end.

addcon 1.2.101 CHEN 3.2.1.100

where 1.100 is the VPI/VCI value in the remote node CHEN.

6.6 General Commands:

6.6.1 IGX / BPX:

1. dspcons: Displays all the connections on the node.

2. dspcon: Displays the details about the particular connection.

3. dspports: Displays all the ports in the node.

4. dspport: Displays the details about the particular port.


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5. dspclns: Displays the circuit lines.

6. dspcds: Displays the cards in the switch

7. dspchcnf: Displays the channel configuration.

8. dsplns: Displays the lines.

9. dspportcnf: Displays the port configuration.

10. dspportstats: This command shows the data traffic sent / received thru the port.

The traffic includes the traffic thru the all the PVCs in that port.

11. dspchstats: This command show the traffic sent / received thru the particular

PVC.

12. dsptrks: Displays the trunks

13. dsptrkstats: Displays the trunk statistics

6.6.2 MGX:

1. dspcons: Displays all the connections on the node.

2. dspcon: Displays the details about the particular connection.

3. dspports: Displays all the ports in the node.

4. dspport: Displays the details about the particular port.

5. dspsrcnt: This is same as dspportstats command used in IGX switches.

6. dspchancnt: This command is same as the dspchstats command used in IGX

switches.

7. dspalms: Displays the alarms

8. dspcds: Displays the cards in the switch

9. cnfbert: To provide loop in the particular port or line.

References:

Cisco IGX / BPX / MGX Student Guide.


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Presented By

Ramesh Somasundaram [email protected]( CE Config EMEA Chennai )

Venkatesh Selvaraj [email protected]

( CE Config EMEA Chennai
mailto:[email protected]:[email protected]:[email protected]:[email protected]

Documents

Introduction to Frame Relay