Traffic Parameters - ZyXEL Communications IES-1000 User Manual

IES-1000 User's Guide
Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections. Connections that use the Variable
Bit Rate (VBR) traffic class can be grouped into real time (rt-VBR) or non-real time (nrt-VBR) connections.
The rt-VBR (real-time Variable Bit Rate) type is used with bursty connections that require closely controlled delay
and delay variation. An example of an rt-VBR connection would be video conferencing. Video conferencing
requires real-time data transfers and the bandwidth requirement varies in proportion to the video image's changing
dynamics.
The nrt-VBR (non real-time Variable Bit Rate) type is used with bursty connections that do not require closely
controlled delay and delay variation. An example of an nrt-VBR connection would be non-time sensitive data file
transfers.
Available Bit Rate (ABR)
The Available Bit Rate (ABR) ATM traffic class is used for bursty data transfers. ABR gives a subscriber a set
amount of bandwidth and allows the use of more if it is available. End devices using ABR get feedback from the
network and can use flow-control to dynamically adjust transmission rates.
ABR uses RM (Resource Management) cells to send feedback information from the connection's destination
and/or intervening network switches to the connection's source. A source generates forward RM cells, which the
destination returns to the source as backward RM cells. Along the way, network switches can adjust the fields in
the RM cells depending on network conditions. Number of Resource Management (NRM) is the maximum
number of cells a source may send for each forward Resource Management cell.
Minimum Cell Rate (MCR) applies with the abr traffic class.
Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is similar to the ABR traffic class for bursty data transfers.
However, while ABR gives subscribers a set amount of bandwidth, UBR doesn't guarantee any bandwidth and only
delivers traffic when the network has spare bandwidth.
13.4.2

Traffic Parameters

These are the parameters that control the flow of ATM traffic.
Peak Cell Rate (PCR)
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This parameter may be lower (but
not higher) than the maximum line speed. 1 ATM cell is 53 bytes (424 bits), so a maximum speed of 832Kbps
gives a maximum PCR of 1962 cells/sec. This rate is not guaranteed because it is dependent on the line speed.
13-4 Virtual Channel Management