QUALITY OF SERVICE (QOS)
QUALITY OF SERVICE (QOS)
ATM Quality of Service Review
A basic performance measure for any digital data communication system is bit error rate
(BER). Well-designed fiber-optic links will predominate now and into the foreseeable
future. We may expect BERs from such links on the order of 1
and with end-
to-end performance better than 5
(Ref. 14). Thus other performance issues may
dominate the scene. These may be called ATM unique QoS items, namely:
Cell transfer delay
Cell delay variation
Cell loss ratio
Mean cell transfer delay
Cell error ratio
Severely errored cell block ratio
Cell misinsertion rate
Selected QoS Parameter Descriptions
Cell Transfer Delay
. In addition to the normal delay through network ele-
ments and transmission paths, extra delay is added to an ATM network at an ATM switch.
The cause of the delay at this point is the statistical asynchronous multiplexing. Because of
this, two cells can be directed toward the same output of an ATM switch or cross-connect
resulting in output contention.
The result is that one cell or more is held in a buffer until the next available opportunity
to continue transmission. We can see that the second cell will suffer additional delay. The
delay of a cell will depend upon the amount of traffic within a switch and thus the
probability of contention.
The asynchronous path of each ATM cell also contributes to cell delay. Cells can be
delayed one or many cell periods, depending on traffic intensity, switch sizing, and the
transmission path taken through the network.
Cell Delay Variation (CDV)
. By definition, ATM traffic is asynchronous,
magnifying transmission delay. Delay is also inconsistent across the network. It can be a
function of time (i.e., a moment in time), network design/switch design (such as buffer
size), and traffic characteristics at that moment in time. The result is cell delay varia-
CDV can have several deleterious effects. The dispersion effect, or spreading out,
of cell interarrival times can impact signaling functions or the reassembly of cell user
data. Another effect is called clumping. This occurs when the interarrival times between
transmitted cells shorten. One can imagine how this could affect the instantaneous network
capacity and how it can impact other services using the network.
There are two performance parameters associated with cell delay variation: 1-point cell
delay variation (1-point CDV) and 2-point cell delay variation (2-point CDV).
The 1-point CDV describes variability in the pattern of cell arrival events observed
at a single boundary with reference to the negotiated peak rate 1/
T as defined in ITU-T
Rec. I.371 (Ref. 13). The 2-point CDV describes variability in the pattern of cell arrival
events as observed at the output of a connection portion