Background

This application note proposes a model for emulating Frame Relay behavior.

The paper is targeted at IT professionals who are familiar with the concept of a Frame Relay network and the way it functions.

Abbreviations

The following are used as abbreviations throughout this application note:

• AR – Access Rate
• BW – Bandwidth
• CIR – Committed Information Rate DE Discard Eligible
• EIR – Excess Information Rate FR Frame Relay
• FRAD – Frame Relay Access Device
• LMI – Local Management Interface
• MS – Milliseconds
• NNI – Network to Network Interface
• N/A – Not Applicable
• PVC – Permanent Virtual Circuit
• SLA – Service Level Agreement

Frame Relay Basics

Introduction

When a client buys a FR service, the carrier supplies the client with a single physical AccessLine per location and usually the needed end equipment (Frame Relay Access Device – FRAD) which is used to connect the user LAN to the FR network.

Access lines’ maximum bandwidth usually range from 64Kbps to 45Mbps, but can also go higher.

The carrier (almost always) creates a logical connection called Permanent Virtual Circuit– or PVC connecting different client sites through the frame relay cloud or a client site to the Internet.

Another aspect of the relationship is the Service Level Agreement – or SLA. It usually defines two major parameters – The access line speed (physical) and the CIR.

Frame Relay Terminology

The following terms appear in FR standards and in SLA’s:

• Committed Burst Size (Bc) The maximum amount of data (in bits) that the network ‘agrees’ to transfer, under normal conditions, during a time interval Tc.
• Committed Information Rate (CIR) – The committed rate (in bits per second) at which the ingress access interface trunk interfaces, and egress access interface of a frame relay network, transfer information to the destination frame relay end system under normal conditions. The rate is averaged over a minimum time interval Tc.
• Committed Rate Measurement Interval(Tc) – The time interval during which the user can send only Bc – committed amount of data and Be – excess amount of data is computed as Tc = Bc/CIR.
  • In general, the duration of Tc is proportional to the “burstiness” of the traffic (usually 1 second).
  • Tc is not a periodic time interval. Instead, it is used only to measure incoming data, during which it acts like a sliding window. Incoming data triggers the Tc interval, which continues until it completes its commuted duration.
• Excess Burst Size (Be) – The maximum amount of uncommitted data (in bits) in excess of Bc that a frame relay network can attempt to deliver during a time interval Tc. This data (Be) generally is delivered with a lower probability than Bc. The network treats Be data as discard eligible (DE).
  • The FRAD enforces the access rates of the line, so the user can transmit at most Bc+Be bits at interval Tc.
  • The probability of DE marked packet reaching the destination is not guaranteed. It is related to the overall FR Cloud load and the network architecture. Some carriers give information regarding the network performance in delivery of excess data.
• Excess Information Rate (EIR) – The excess rate (in bits per second) at which the ingress access interface trunk interfaces, and egress access interface of a frame relay network agrees to transfer information to the destination frame relay end system under normal conditions. The rate is averaged of Be over a minimum time interval Tc.
• Access Rate (AR) – The data rate of the user physical access line. The transmission speed at the access line, always matches the full line speed.

The Operation of a Frame Relay PVC

PVC Overview

The PVC is the ‘logical link’ connecting two communication ends through the FR cloud. The PVC defines the path, speed and other link characteristics.

At each Tc interval, the network accepts frames until the total bit count reaches Bc + Be. After that limit and until the end of this time slot, additional frames will be dropped immediately, or accumulated at the FRAD (Depending on the FRAD maker or settings). In the same interval, when the bit count passes Bc, any additional frames transmitted by the FRAD (until the Bc + Be limit) will be marked with DE=1.

Under normal conditions, the network will agree to pass information at this rate, while giving low and stable latency and close to zero frame loss for frames with DE=0. Frames with DE=1 have slightly higher loss chance.

Network Congestion Effect

Once in a while, the network becomes congested due to simultaneous clients transmitting above their CIR. At this stage the network will start sending congestion notifications.

These notifications are identified by the FRAD which (normally) responds by lowering the transmission rate down to as low as the CIR (or some other minimum value if CIR=0).

The minimum rate to which the FRAD will slow depends on the severity and duration of the congestion. That is – to what extent the congested network switch is utilized by all PVCs in its interface, and for how long this condition will go on.

The FRAD itself may implement different strategies for situations where the input rate is higher then the output rate (when input > Bc+Be or when throttling down on congestion). According to the configuration, the FRAD can either, drop what it cannot transmit right away – maximizing loss and optimizing latency, or, queue some or all frames – reducing loss but increasing latency.

At all times, transmission out of the FRAD is done in bursts, in each burst the bit transmission speed is according to the physical access line speed. This causes the time interval between frames to look like this:

!!!!!!!!….!!!!!……!!!!!!!!!

(burst-delay-burst-delay-burst).

Users that are transmitting below their CIR do not feel any effect of network congestion (unless the supplier overbooked the network with CIR).

When the network lowers the transmission rate due to congestion notifications, there is a queuing mechanism in effect. Frames are buffered by the FRAD or by the network switches, and transmitted out at a lower rate.

This behavior creates increased delay, which builds up to a threshold at which frames (with DE=1) are dropped. At that point the delay will stabilize on its maximum value. The maximum delay size is determined by the max size of the queue and the minimum transmission rate.

Note: Frames that are dropped at the FRAD (because they were above Bc + Be or due to queue overflow during congestion) are not calculated in the FR provider drop rate, since they did not enter the network.