Once GPRS data has been collected, various parameters can be used to analyze the performance of the network. Each parameter described here can be viewed using the Actix Analyzer platform, provided the parameter has been collected by the collection equipment.
• Throughput – GPRS logfiles contain throughput values collected
within the GPRS network and at the application level. RLC
Throughput and Application Throughput are the most commonly
used for analysis of GPRS networks.
o RLC Throughput (Radio Link Control) provides data rate for the logical link between the mobile and the PCU and can be found in the GSM → GPRS Data → Vendor Specific group.
RLC throughput provides the best indication of data transfer over the air interface.
o LLC Throughput (Logical Link Control) provides the data rate between the mobile and the SGSN and can be found in the
GSM → GPRS Data → Vendor Specific group.
o TCP Throughput provides the data rate on the logical link maintaining flow and congestion control between the two
communicating computers. TCP throughput can be found in the Data Testing → Protocol Measurements group.
o Application Throughput most closely approximates the end user experience and can be found in the Data Testing → Application Measurements group. Target application throughput for GPRS is 11-13 kbps per timeslot for Coding Scheme 2 (CS 2).
Figure 76 - Uplink RLC and Application Throughput
• Timeslot Usage – The more timeslots allocated to a GPRS data
session, the higher the resulting throughput. Timeslots may be
allocated based on data rate requested and network congestion. Check timeslot usage with the parameters in the GSM → GPRS Data → GPRS RMAC → Dedicated Radio Link group.
• Coding Scheme Usage – GPRS relies on four different coding
schemes to provide an appropriate balance between error correction and maximum data rate. CS 1 provides substantial error correction, resulting in a lower maximum data rate. CS 4 provides virtually no error correction and the highest maximum data rate, but should only be used in areas where RF conditions are excellent to avoid losing
• GPRS Radio Conditions – The best indicators of GPRS radio
conditions are RLC_DL_BLER and RLC_UL_Retransmissions, both of which can be found in the GSM → GPRS Data → Vendor Specific
group. RLC_DL_BLER indicates the percentage of corrupt blocks received that cannot be decoded by the mobile.
RLC_UL_Retransmissions indicates unacknowledged blocks sent by
the mobile that must be resent. High RCL_DL_BLER or
RLC_UL_Retransmission values typically result in lower throughput.
Figure 77 - RLC DL Throughput and BLER
• Cell Updates – In GPRS networks, cell updates are controlled by the
handset rather than by the network. The handset measures and updates the serving cell based on local radio conditions. Because the connection with the abandoned cell is broken before the connection is resumed with the target cell, there is a 3-5 second break in data
transfer during a cell update. Use the ServCI parameter in the GSM → Serving Cell Parameters group to determine whether multiple cell
updates are affecting GPRS throughput.
• TFI and TBF – Timeslots are shared between GPRS users. Each GPRS
user is granted a Temporary Flow Identity (TFI), Temporary Block Flow (TBF) on the uplink and on the downlink. Each data transmission in a timeslot includes a TFI indicating to which mobile the data
corresponds. During a constant stream of data (i.e. FTP download), the mobile will keep the same TFI. During sporadic transmit or receive activity, the mobile will constantly release and be allocated new TFIs. During sporadic activity, TFIs will be assigned in consecutive numerical order if no other users are on the cell. TFI numbers will jump if there
Diminished throughput and high BLER
are multiple users on the cell. GPRS performance will suffer the more users share the same resources.
Figure 78 - File Download Throughput and TFI
• GPRS Signaling – All GPRS signaling can be accessed via the
Analyzer message browser or Protocol Stack Browser. Knowledge of protocol signaling can help to identify and pinpoint network-based problems. Protocol analyzer traces on the Gb link may be used to see whether a problem is more widespread than just the test mobile.
o RLC-based Protocols provide communication between the mobile and the PCU. The RLC layer ensures that the mobile has sufficient resources to communicate on the radio link. RLC messages and procedures include Channel Request, Immediate Assignment, Acks & Nacks, and presence messaging.
o LLC-based Protocols provide communication between the mobile and the SGSN. LLC procedures include mobile attach messaging, PDP context activation, and acknowledgement of the mobile on the network.
Bursty TFI during bursty download activity
looks good, but application throughput is poor. Some potential TCP problems are:
o Missing Packets - TCP behavior causes the receiver to acknowledge packets with the number of the next expected packet. If the receiver misses a packet it will acknowledge the number of the missing packet, even as other packets are sent and received, until the missing packet is finally received.
o Receive Buffer Size Window – The receive buffer size window, or RWIN setting, defines how many packets can be sent out by the sender without receiving an acknowledgement from the
receiver. If the RWIN setting is too small, the sender may hold data and reduce throughput while waiting for earlier packets to be acknowledged.