1. Peak Cell Rate (PCR) represented by R_p.
2. Cell Delay Variation (CDV) represented by = CDV Tolerance (CDVT).

3. Sustainable Cell Rate (SCR) represented by R_s (in cells/second).
   • Must have R_s < R_p.
   • SCR can be defined as the average allowable rate over time.
     

4. Burst Tolerance (BT) represented by _s.
   • This can be defined as the time scale for how often cell bursts can occur. Cell bursts can occur, for example, during a scene change in a video stream.

The above parameters can be combined into one quantity similar to the B quantity developed for PCR and CDV. This quantity is the Maximum Burst Size (MBS) defined by

MBS= floor(1+(_s/(T_s - T)))

This formula gives the maximum number of consecutive cells which can be transmitted at the peak cell rate.

To enforce the negotiated SCR and BT parameters, we use the GCRA(I,L) with the I=T_s and L=_s. You can think of the quantity B as that for peak rates and the parameter MBS as that for average rates.

Variable Bit Rate Source and Cell Bursts

• Constant Bit Rate (CBR) vs. Variable Bit Rate (VBR).
• Real Time (rt) vs. Non-Real Time (nrt).
• Connectionless vs. Connection-Oriented.

The ATM Forum Traffic Management Specification 4.0 [1996] focussed on only the first two of the criteria above for their classification. Based on these 2 criteria they have defined 5 classes as follows:

1. CBR
   This is the Constant Bit Rate service class.
2. rtVBR
   This is the real time Variable Bit Rate service class, e.g. interactive video (video conferencing).
3. nrtVBR
   This is the non-real time Variable Rate service class, e.g. video on demand.
4. UBR
   This is the Unspecified Bit Rate service class.
5. ABR
   This is the Available Bit Rate service class.

Constant Bit Rate

Real Time Variable Bit Rate

1. Cell Transfer Delay (CTD)
   This is the maximum end to end delay.
2. Cell Delay Variation (CDV)
   This is the variation allowed in the Cell Transfer Delay. This is like the fudge factor encountered before in other contexts. This is, for example, a measure of jitter in video.
3. Cell Loss Ratio (CLR)
   This gives the allowed probability of cell loss, e.g. 10^-7.

The way these factors are managed and controlled will be discussed later. It must be noted here that QoS guarantees cannot be given if the selection of packets to be dropped is done randomly, as was discussed in the Queueing Theory analysis.

rtVBR Subclasses

There are two subclasses that are being contemplated under the Real Time Variable Bit Rate service class. They are:

1. Peak Rate Allocated VBR (P-VBR). In this subclass, the peak rate is always guaranteed when needed by the user of a connection of this class. This is in contrast to the other classes where the peak rate is more of a limit on the user rather than a guarantee to them. The features of this sub class can be summarized as follows:
   • Peak rate is guaranteed.
   • Cell loss would be very rare.
   • Bandwidth left over from this class is used by the ABR and the UBR classes.
2. Statistically Multiplexed VBR (SM-VBR). In this subclass, multiple SM-VBR connections share a CBR connection, which could be a VPC for example. For this kind of connection, we can have:
   PCR > PCR of the VPC
   all
   VCC's

   Here we cannot have a very large buffer since a full large buffer would mean long delay which would eliminate the real time delivery capability. Thus, there is a possibility of loss of packets when all the connections on the VPC reach their respective PC's simultaneously (which is not true for the P-VBR subclass), but hopefully this is a rare event.

Non-Real Time Variable Bit Rate

• Policed by GCRA(SCR, BT).
• No max CTD, No CDV (since it's nrt).
• There is a mean CTD (guarantee of average delay).
• May be used for video on demand or data desiring bounded average delay.
• Not too much developed here yet.

UBR versus ABR

UBR is essentially a "Best Efforts" service like IP service in the Internet. There are no guarantees. It will try its best to deliver the packets, but beyond that there are no assurances.

ABR is, in contrast, a "Better Efforts" service. Here the assurance is that among the ABR connections currently active, the network will give every connection equal priority. It also "tries" to provide a low probability of loss. This is not a guarantee, but an assurance of effort. This class gets better service than the UBR, as it assures you that you will get a fair share of whatever bandwidth is available.