Introduction to Throughput

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1 Throughput
Throughput measures the number of bits transferred over the communication link over a definite amount of time. In the case of cellular network the data throughput may not be same at each interface; also it is a well known fact that the radio interface is the bottleneck. To get a more accurate value of the user experience in terms of throughput, it is highly desired to measure the value from the UE. But practically this may not be feasible as it requires sophisticated applications in the mobile handset. An alternative is to measure bearer path throughput from the RNC end. Though this appears simple, practically it is complex and resource hungry operation. Hence it is done on an experimental basis and not exactly in a production network. On the core network, it is more feasible to measure the throughput as the protocol complexity is not as much as on the RAN side. On the core side, throughput is normally measured from the Gn or the Gi interfaces. This is more logical since the TCP/IP session tunneled over GTP gives a more practical view of the bearer throughput, taking into account retransmissions and IP packet losses. Following sections give techniques for measuring throughput from various parts of the network. M KN

1.1 RLC Throughput measurement from Iub interface
This measures the throughput of all the RLC transport blocks per cell, i.e. this includes both user plane and control plane data. This quantity does not differentiate retransmissions from the actual data flow. One of the main point to be noted while calculating throughput using the RLC block flow rate is that not all of the RLC traffic in a cell are related to the actual call. This means there will be a portion of the RLC traffic which the mobile user does not observe. Another issue is that not all of the RLC traffic can be measured from the Iub interface. [Refer UMTS Performance Measurement by Ralf Kreher]. RLC throughput measurement is error prone due to variability and dynamism in the radio interface. None of the existing call trace applications can correctly capture RLC blocks on multiple channels especially – BCH and PCH. PCH need not always carry paging information, it can also carry other channel allocation details and information like when and how to read from BCH. BCH which carry SIBs like RRC signaling information is frequently sent to the downlink direction and hence is stored in the Node B. These SIBs are difficult to capture from Iub as sophisticated correlation functionality is required to map broadcasted SIBs with cells and corresponding Uu messages actually sent. RLC throughput is a valid measurable entity in GPRS networks as well since the RAN side layer 2 protocol stacks is same as in UMTS, except for the HSDPA related modifications that are incorporated in the UMTS. In the case of GPRS network, RLC throughput can be measured from the BSC over Abis interface. Similar to UMTS case, RLC user throughput tend to vary compared to the throughput value measured from other interfaces , mainly due to the control plane traffic and other overhead involved in throughput measurement. In GPRS network, each RLC data block occupies four time slots, irrespective of the type of channel coding used. In the case of coding scheme CS-1, each RLC block consists of 181 information bits, 40 block-check-sequence (BCS) bits, and seven tail/control bits. With single-slot operation [i.e., only one slot per GSM time-division multiple-access (TDMA) frame is allotted to a user], the information rate of coding scheme CS-1 is 9.05 kb/s (i.e., 181 b in four TDMA frames, where each TDMA frame occupies 4.615 ms). Similarly, the

maximum information rates possible using other coding schemes are 13.4 kb/s for CS-2, 15.6 kb/s for CS-3, and 21.4 kb/s for CS-4 (see Fig. 2). With multi slot operation (i.e., allocation of up to eight slots in a TDMA frame to a user), these maximum possible information rates are increased eight-fold. Following table gives theoretically...
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