an input and being transmitted by the output, and this delay varies, depending on how many other packets are already queued in the buffer. A similar situation can occur at the output of a host (in the NIC) when multiple packets have the same or similar release times, and this delay can usually be modelled as a delay in a virtual output buffer. For variable length packets, where the amount of water added by a given packet is proportional to its length, τ can't be seen as a limit on how full the bucket can be when a packet arrives, as this varies depending on the packet size. However, the time it takes to drain from this level to empty is still how much earlier a packet can arrive than is expected, when packets are transmitted at the bandwidth limit. Thus, it is still the maximum variation in transfer delay to the point where the conformance test is being applied that can be tolerated, and thus the tolerance on maximum delay variation. Maximum burst size The limit value or delay variation tolerance also controls how many packets can arrive in a burst, determined by the excess depth of the bucket over the capacity required for a single packet. Hence MBS is also a measure of burstiness or jitter, and it is possible to specify the burstiness as an MBS and derive the limit value τ from this or to specify it as a jitter/delay variation tolerance/limit value, and derive the MBS from this. A burst or clump of packets can arrive at a higher rate than determined by the emission interval T. This may be the line rate of the physical layer connection when the packets in the burst will arrive back-to-back. However, as in ATM, the tolerance may be applied to a lower rate, in that case the Sustainable Cell Rate (SCR), and the burst of packets (cells) can arrive at a higher rate, but less than the line rate of the physical layer, in that case, the Peak Cell Rate (PCR). The MBS may then be the number of cells needed to transport a higher layer packet (see segmentation and reassembly), where the packets are transmitted with a maximum bandwidth determined by the SCR and cells within the packets are transmitted at the PCR; thus allowing the last cell of the packet, and the packet itself, to arrive