CALL ADMISSION CONTROL CAC 97
of K cells. The parameters for each connection are as follows: the peak bit rate is R; the average bit rate is ρ; and the mean duration of the on period is b. Note that the
quantity r defined above is related to ρ through the expression rR = ρ. In the numerical examples presented below, assume that all connections are identical with traffic parameters
R, ρ, b =
10 Mbps, 1 Mbps, 310 cells. Note that if connections are admitted, using their peak bit rate, then a maximum of
150 Mbps10 Mbps = 15 connections will be admitted. On the other hand, if connec- tions are admitted, using the average bit rate, a maximum of 150 Mbps1 Mbps = 150
connections will be admitted. These two values can be seen as the upper and lower range points on the number of connections that can be admitted using the equivalent
bandwidth method.
In Figure 4.10, the maximum number of connections that can be admitted is plotted as a function of the buffer size K. The buffer size was increased from 31 cells to 31,000
cells. For each value of K, the maximum number of admitted connections was obtained using expressions 4.1 and 4.2 with the cell loss rate fixed to 10
− 6
. For small values of K
, the maximum number of connections admitted by the equivalent bandwidth algorithm is constant. As K increases, the maximum number of admitted connections increases as
well, and eventually flattens out. In Figure 4.11, the maximum number of admitted connections is plotted against the cell
loss rate ε. The buffer size is fixed to 1236 cells i.e., 64 Kbytes. The maximum number of admitted connections is not very sensitive to the cell loss rate ε. In this particular
example, the buffer size is large enough so that the equivalent algorithm admits a large number of connections. In general, the equivalent bandwidth algorithm becomes more
sensitive to ε when the buffer size is smaller.
Finally, in Figure 4.12, the maximum number of admitted connections is plotted against r
, the fraction of time that a source is active, where r = ρR. Recall that r can be used to express the burstiness of a source see Section 4.1.1. The buffer size was fixed to 1236
cells and the cell loss rate ε to 10
− 6
. The maximum number of admitted connections depends on r. As r increases, the source becomes more bursty and requires more buffer
20 40
60 80
100 120
10 100
1000 10000
Buffer size log scale Max admitted connections
140
100000 160
Figure 4.10 Varying the buffer size K.
98 CONGESTION CONTROL IN ATM NETWORKS
40 60
80 100
120
Required cell loss rate log scale Maximum admitted connections
140 160
1e−09 1e−08 1e−07 1e−06 1e−05 1e−04 0.001 0.01
0.1
Figure 4.11 Varying the required cell loss rate ε.
20 40
60 80
100 120
Fraction of time source active r Max admitted connections
140 160
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Figure 4.12 Varying r.
space in order to maintain the same cell loss rate. As a result the maximum number of admitted connections falls sharply as r tends to 0.5.
4.6.3 The ATM Block Transfer ABT Scheme
A number of congestion control schemes were devised for bursty sources whereby each switch allocates bandwidth on demand and only for the duration of a burst. The main idea
behind these schemes is the following. At connection setup time, the path through the ATM network is selected, and each switch in the path allocates the necessary VPIVCI
labels and updates the switching table used for label swapping in the usual way. However, it does not allocate any bandwidth to this connection. When the source is ready to transmit
a burst, it notifies the switches along the path. Once notified, each switch allocates the necessary bandwidth for the duration of the burst.
These congestion control schemes are known as fast bandwidth allocation schemes. The ATM block transfer ABT scheme is a fast bandwidth allocation scheme; it is a
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standardized ATM transfer capability. ABT only uses the peak bit rate and is intended for VBR sources whose peak bit rate is less than 2 of the link’s capacity.
In ABT, a source requests bandwidth in incremental and decremental steps. The total requested bandwidth for each connection might vary between 0 and its peak bit rate. For
a step increase, a source uses a special reservation request cell. If the requested increase is accepted by all of the switches in the path, then the source can transmit at the higher bit
rate. If the step increase is denied by a switch in the path, then the step increase request is rejected. Step decreases are announced through a management cell. A step decrease is
always accepted. At the cell level, the incoming cell stream of a source is shaped, so that the enforced peak bit rate corresponds to the currently accepted peak bit rate.
A fast reservation protocol FRP unit was implemented to handle the relevant man- agement cells. This unit is located at the UNI. The protocol uses different timers to ensure
its reliable operation. The end device uses a timer to ensure that its management cells, such as step increase requests, sent to its local FRP unit are not lost. When the FRP unit
receives a step increase request, it forwards the request to the first switch in the path, which in turn forwards it to the next hop switch, and so on. If the request can be satisfied
by all of the switches on the path, then the last switch will send an ACK to the FRP unit. The FRP unit then informs the end device that the request has been accepted, updates the
policing function, and sends a validation cell to the switches in the path to confirm the reservation. If the request cannot be satisfied by a switch, the switch simply discards the
request. The upstream switches, which have already reserved bandwidth, will discard the reservation if they do not receive the validation cell by the time a timer expires. This timer
is set equal to the maximum round trip delay between the FRP unit and the furthermost switch. If the request is blocked, the FRP unit will retry to request the step increase after
a period set by another timer. The number of attempts is limited.
This mechanism can be used by an end device to transmit bursts. When the end device is ready to transmit a burst, it issues a step increase request with a requested bandwidth
equal to its peak bit rate. If the request is granted, the end device transmits its burst, and at the end it announces a step decrease with bandwidth equal to its peak bit rate.
In a slightly different version of the ABT protocol, the end device starts transmitting its burst immediately after it issues a reservation request. The advantage of this scheme
is that the end device does not have to wait until the request is granted. The burst will get lost if a switch in the path is unable to accommodate the request.
4.6.4 Virtual Path Connections
A virtual path connection can be used in an ATM network to create a dedicated connection between two switches. Within this connection, individual virtual circuit connections can
be set up without the knowledge of the network. Let us assume, for instance, that a permanent virtual path connection is established
between two switches switch 1 and switch 2. These two switches might not be adjacent, in which case, they can communicate through several other switches. A fixed amount
of bandwidth is allocated to the virtual path connection. This bandwidth is reserved for this particular connection and it cannot be shared with other connections, even when it
is not used entirely. An end device attached to switch 1 and wishing to communicate to an end device attached to switch 2, is allocated part of the bandwidth of the virtual
path connection using nonstatistical or statistical bandwidth allocation. The connection
