II.7.3 PACKETS SCHEDULING MECHANISM
Packet scheduling is the mechanism that selects a packet for
transmission from the packets waiting in the transmission queue. It decides
which packet from which queue and station are scheduled for transmission in a
certain period of time. Packet scheduling controls bandwidth allocation to
stations, classes, and applications.
As shown in Figure 17, there are two levels ofpacket scheduling
mechanisms:
1. Intrastation packet scheduling: The packet scheduling
mechanism that retrieves a packet from a queue within the same host.
2. Interstation packet scheduling: The packet scheduling
mechanism that retrieves a packet from a queue from different hosts.
Figure 17: Packet Scheduling
Packet scheduling can be implemented using hierarchical or flat
approaches.
> Hierarchical packet scheduling: Bandwidth is allocated to
stations--that is, each station is allowed to transmit at a certain period of
time. The amount of bandwidth assigned to each station is controlled by
interstation policy and module. When a station receives the opportunity to
transmit, the intrastation packet scheduling module will decide which packets
to transmit.
> This approach is scalable because interstation packet
scheduling maintains the state by station (not by connection or application).
Overall bandwidth is allocated based on stations (in fact, they can be groups,
departments, or companies). Then, stations will have the authority to manage or
allocate their own bandwidth portion to applications or classes within the
host.
> Flat packet scheduling: Packet scheduling is based on all
queues of all stations. Each queue receives individual service from the
network.
Packet scheduling mechanism deals with how to retrieve packets
from queues, which is quite similar to a queuing mechanism. Since in
intrastation packet scheduling the status of each queue in a station is known,
the intrastation packet scheduling mechanism is virtually identical to a
queuing mechanism.Interstation packet scheduling mechanism is slightly
different from a queuing mechanism because queues are distributed among hosts
and there is no central knowledge of the status of each queue. Therefore, some
interstation packet scheduling mechanisms require a signaling procedure to
coordinate the scheduling among hosts.
Because of the similarities between packet scheduling and
queuing mechanisms ,there is introduction of a number of queuing schemes (First
In First Out [FIFO], Strict Priority, and Weight Fair Queue [WFQ]) and briefly
discuss how they support QoS services.22
22 Aura Ganz, Zvi Ganz, Kitti Wongthavarawat, Multimedia
Wireless Networks: Technologies, Standards, and QoS, Prentice Hall, page
56,57
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