The effects of user mobility on usage parameter control (UPC) in wireless ATM systems Page: 3 of 14
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parameters are cell-based ones such as Cell Transfer Delay (CTD), Cell Delay Variation (CDV) and Cell
Loss Ratio (CLR).
Different applications may need different QoS levels from an ATM network. For example, voice
applications need bounded end-to-end delay since conversation becomes difficult once the connection's
end-to-end CTD exceeds a few hundred milliseconds. Other non-interactive applications, such as
broadcast video distribution, can tolerate much larger end-to-end CTD. However, they still require
bounded delay-variation (or CDV) if the receiving terminal must provide a constant bit-rate input to the
user's display device. Finally, voice is relatively tolerant of cell-loss, since the receiving terminal can use
error-masking techniques. Indeed, an ATM CLR of le-3 may be acceptable for many voice applications.
However, TCP/IP-based file transfers may require much lower cell-loss rates. While TCP does guarantee
error-free end-to-end transmission, it also interprets cell-loss (and hence TCP segment loss) as network
congestion. This causes the TCP protocol to temporarily reduce its transmission rate. As such, a large
CLR within an ATM network can greatly reduce the end-to-end TCP performance, or "goodput".
If the user requests a given QoS, or traffic contract, from an ATM network then the user must
also supply the traffic characteristics for that connection to the network. The network then does Call
Admission Control (CAC) based on the network's CAC algorithm, the requested QoS, those traffic
characteristics and the contracted QoS for other existing connections. If the network can provide the
requested QoS, without violating the contracted QoS for the existing connections, then it usually accepts
the new connection. Otherwise, it typically rejects that connection. While the ATM Forum's (ATMF's)
Private Network-Network Interface (PNNI) [2] does specify a Generic Call Admission Control (GCAC)
algorithm, each network's CAC algorithm is typically network-specific. In any event, specific CAC
algorithms are outside this paper's scope.
If an ATM network does accept a new connection then it may need to "police" that connection's
traffic contract. Policing involves monitoring the connection to determine if it abides by its traffic
contract and also possibly taking action if it does not. Public networks often do policing for two reasons.2
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Sholander, P.; Martinez, L.; Tolendino, L. & Mah, B. A. The effects of user mobility on usage parameter control (UPC) in wireless ATM systems, article, July 1, 1997; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc675584/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.