ATM to Frame Relay Interworking

Document ID: 10439

Contents

Introduction
Before You Begin
      Conventions
      Prerequisites
      Components Used
What Is FRF.8?
      ATM Header Fields
      Frame Relay Header Fields
Mapping Loss Priority and Congestion Indication Values
Setting Mapping Policies on Cisco Routers
Other FRF.8 Requirements
Related Information

Introduction

ATM and Frame Relay are commonly used protocols for wide-area networks. Both protocols use two different headers at layer 2:

• Upper-layer protocol encapsulation header - Communicates the protocol encapsulated and transported in the frame or cell. Defined by Request for Comments (RFC) 1490 and Frame Relay Forum (FRF) specification 3.2 for Frame Relay, and RFC 1483 and RFC 2684 for ATM.

• Address header - Communicates the layer 2 address (data-link connection identifier [DLCI] or virtual path identifier/virtual channel identifier [VPI/VCI]) as well as loss priority and congestion indication values. Defined by Q.922 (typically two bytes) for Frame Relay and a five-byte cell header for ATM.

This document focuses on the address header. It explains how the loss priority and congestion indication bits are mapped between the ATM endpoint and the Frame Relay endpoint of an FRF.8 service interworking connection.

Before You Begin

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites

There are no specific prerequisites for this document.

Components Used

This document is not restricted to specific software and hardware versions.

What Is FRF.8?

The Frame Relay Forum publishes implementation agreements to standardize Frame Relay and encourage its use over wide-area networks. FRF.8 defines service interworking between a Frame Relay endpoint and an ATM endpoint.

An interworking (IWF) device, such as a WAN switch or a Cisco router, converts user data between frames and cells. Depending on the configuration, the IWF also maps specific bits in the Frame Relay header to equivalent bits in the ATM cell header. One set of bits defines the loss priority of the payload, and one set of bits indicates whether the network was congested while the payload was being transmitted through it.

Before explaining the bit-mapping procedures, it is important to understand the values in each header.

ATM Header Fields

All ATM cells are prepended with a five-byte header. The format of this header is illustrated in the following diagram.

The Payload Type (PT) field uses the second bit of a three-bit field to indicate whether a cell containing user data experienced congestion as it moved through the network. This bit is also known as the Explicit Forward Congestion Indication (EFCI) bit.

The Cell Loss Priority (CLP) bit communicates the loss priority of the payload. A value of zero indicates that the cell should not be discarded if it encounters congestion as it moves through the network. A value of one indicates that the cell is eligible to be dropped by the network.

Frame Relay Header Fields

The International Telecommunications Union (ITU-T) defines the format of a Frame Relay address header and overall frame structure in the Q.922 standard.

The core Q.922 frame includes a two-byte Address field with Frame Relay congestion indication and loss priority bits.

As their name implies, the Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) bits communicate network congestion to the endpoints. Forward means in the direction of traffic flow downstream to the receiving endpoint. Backward means in the direction opposite of the traffic flow.

The Discard Eligibility (DE) bit communicates the loss priority. A marked frame with a DE bit value of one is considered less important than other frames in transmission and should be discarded before frames with a DE bit value of zero.

Mapping Loss Priority and Congestion Indication Values

Depending on the configuration, the network switch or Cisco router performing the FRF.8 interworking function can extract the loss priority and congestion indication values on one end and map them into equivalent values on the other end. Sections 4.2 and 4.3 of FRF.8 define the mapping procedures.

Setting Mapping Policies on Cisco Routers

Several Cisco router series can perform the FRF.8 functions. These routers include the 2600 and 3600 series, the MC3810, and the 7200 series with a PA-A3.

Note: Bit mapping on the PA-A3 will be introduced in Cisco IOS^® Software Release 12.2 via Cisco Bug ID CSCdu01717.

Cisco routers use the following bit mapping policies by default:

7200-2.4#show connect name CISCO
FR/ATM Service Interworking Connection: CISCO 
  Status - UP 
  Segment 1 - Serial4/3 DLCI 50 
  Segment 2 - ATM5/0 VPI 5 VCI 50 
Interworking Parameters - 
   no service translation 
   efci-bit 0 
   de-bit map-clp 
   clp-bit map-de

Use the following commands in FRF.8 configuration mode to change the mapping policies from their defaults:

Router(config)#connect connection-name FR-interface FR-DLCI ATM-interface ATM-vpi/vci  service-interworking 
Router(config-frf8)#clp-bit {0 | 1 | map-de} 
Router(config-frf8)#de-bit {0 | 1 | map-clp} 
Router(config-frf8)#efci-bit {0 | map-fecn} 

Other FRF.8 Requirements

FRF.8 specifies that the IWF device also perform the following:

• Map the Frame Relay command and response (CR) field to the least significant bit of the common part convergence sublayer user-to-user information field (CPCS-UU) in the ATM adaptation layer 5 (AAL5) trailer.

• Map a single Frame Relay DLCI to a single ATM VPI/VCI pair.

• Start ATM segmentation and reassembly (SAR) only when it has received and translated the entire Frame Relay frame.

Related Information

• Frame Relay to ATM Network Interworking (FRF.5) Sample Configuration
• Frame Relay to ATM Service Interworking (FRF.8 - Translation Mode) Sample Configuration
• Configuring ATM-Frame Relay Service Interworking
• More ATM Information
• Technical Support & Documentation - Cisco Systems