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

t is denoted by B(t) which is independent of t in the static channel allocation scheme.

The workload model used in the GPRS simulator is a Markov-modulated Poisson

Process (MMPP) [7]. It is used to generate the IP traffic for each individual user in

the system. The MMPP has been extensively used for modeling arrival processes,

because it qualitatively models the time-varying arrival rate and captures some of the

important correlations between the interarrival times. It is shown to be an accurate

model for Internet traffic which usually shows self-similarity among different time

scales. For our purpose the MMPP is parameterized by the two-state continuous-time

Markov chain with infinitesimal generator matrix Q and rate matrix Ë:

0

The two states represent bursty mode and non-bursty mode of the arrival process.

The process resides in bursty mode for a mean time of 1/á and in non-bursty mode for

a mean time of 1/â respectively. Such an MMPP is characterized by the average

arrival rate of packets, ëavg and the degree of burstiness, B. The former is given by:

1 2

The degree of burstiness is computed by the ratio between the bursty arrival rate and

the average arrival rate, i.e., B = ë1/ëavg.

4 Simulation Results

Table 1 summarizes the parameter settings underlying the performance experiments.

We group the parameters into three classes: network model, mobility model, and

traffic model. The overall number of physical channels in a cell is set to N = 20

among which at least one channel is reserved for GPRS. The overall number of GPRS

users that can be managed by a cell is set to M = 20. As base value, we assume that

5% of the arriving calls correspond to GPRS users and the remaining 95% are GSM

calls. GSM call duration is set to 120 seconds and call dwell time to 60 seconds, so

that users make 1-2 handovers on average. For GPRS sessions the average session

duration is set to 5 minutes and the dwell time is 120 seconds. Thus, we assume

longer “online times” and slower movement of GPRS users than for GSM users. The

average arrival rate of data is set to 6 Kbit/sec per GPRS user corresponding to 0.73