Реферат: Evaluating the GPRS Radio Interface for Different Quality of Service Profiles

In both schemes, the PDCHs are fairly shared by all packets in transfer up to a

maximum of 8 PDCHs per IP packet ("multislot mode") and a maximum of 8 packets

per PDCH [6].

The software architecture of the simulator follows the network architecture of the

GPRS Network [14]. To accurately model the communication over the radio

interface, we include the functionality of a BSC and a BTS. IP packets that arrive at

the BSC are logically organized in two distinct queues. The transfer queue can hold

up to Q n = ⋅ 8 packets that are served according to a processor sharing service

discipline, with n the number of physical channels that are potentially available for

data transfer, i.e. n = NGPRS under the static scheme and n = N under the dynamic

scheme. The processor sharing service discipline fairly shares the available channel

capacity over the packets in the transfer queue. An arriving IP packet that cannot enter

the transfer queue immediately is held in a first-come first-served (in case of one

priority) access queue that can store up to K packets. The access queue models the

BSC buffer in the GPRS network. Upon termination of a packet transfer, the IP

packet at the head of the access queue is polled into the transfer queue, where it

immediately shares in the assignment of available PDCHs. For this study, we fix the

modulation and coding scheme to CS-2 [14]. It allows a data transfer rate of 13,4

kbit/sec on one PDCH. Figure 1 depicts the software architecture of the simulator.

Figure 1. Software Architecture of GSM/GPRS Simulator

To model the different quality of service profiles GPRS provides, the simulator

implemented a Weighted Fair Queueing (WFQ) strategy. The WFQ scheduling

algorithm can easily be adopted to provide multiple data service classes by assigning

each traffic source a weight determined by its class. The weight controls the amount

of traffic a source may deliver relative to other active sources during some period of

time. From the scheduling algorithm's point of view, a source is considered to be

active if it has data queued at the BSC. For an active packet transfer with weight wi

the portion of the bandwidth Âi(t) allocated at time t to this transfer should be

( ) ( ) = ⋅ ∑