the packet length, the maximum rate at which they can conform varies according to their length: the amount that the bucket must have leaked from full for a packet to conform is the amount the packet will add, and if this is proportional to the packet length, so is the interval between it and the preceding packet that filled the bucket. Hence, it is not possible to specify a specific emission interval for variable-length packets, and the bandwidth limit has to be specified explicitly, in bits or bytes per second. Delay variation tolerance It is easiest to explain what this tolerance is where packets have a fixed length. Hence, the first part of this description assumes this, and the implications of variable packet lengths are considered separately. The ITU-T defines a limit value, τ, that is less than the capacity of the bucket by T (the amount by which the bucket contents is incremented for each conforming cell), so that the capacity of the bucket is T + τ. This limit value specifies how much earlier a packet can arrive than it would normally be expected if the packets were arriving with exactly the emission interval between them. Imagine the following situation: A bucket leaks at 1 unit of water per second, so the limit value, τ and the amount of water added by a packet, T, are effectively in units of seconds. This bucket starts off empty, so when a packet arrives at the bucket, it does not quite fill the bucket by adding its water T, and the bucket is now τ below its capacity. So when the next packet arrives the bucket only has to have drained by T – τ for this to conform. So the interval between these two packets can be as much as τ less than T. This extends to multiple packets in a sequence: Imagine the following: The bucket again starts off empty, so the first packet to arrive must conform. The bucket then becomes exactly full after a number of conforming packets, N, have arrived in the minimum possible time for them to conform. For the last (the Nth) packet to conform, the bucket must have leaked enough of the water from the preceding N – 1 packets ((N – 1) * T seconds' worth) for it to be exactly at the limit value τ at this time. Hence, the water