Basics of data transmission in a UMTS system

In a UMTS system, the maximum supported downlink data rate is 2.048 Mbps. With other technologies such as HSPA, LTE, higher data rates are possible, but we look at data rates for UMTS only systems.

Each application has a certain Quality of service (QOS) requirement. A user who wishes to run a application that demands high data rate informs the network of his throughput requirement in Quality Of Service (QOS) attributes during PDP context activation. It is from this message that the network becomes aware of the user’s data rate requirement.

Apart from data rate, other parameters like application’s error resilience, time sensitivity etc is negotiated.

If the network could fulfill the requirement, it informs the user by accepting and activating the PDP context and sets up a Radio Access Bearer (RAB). Network provides channel details that UE has to use in order to send and receive data in a RAB setup Message.

RAB message also provides parameters like transport block (TB) size and number of Transport blocks per Transmission Time Interval (TTI). Figure A shows the message exchange between Mobile and Network.

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Fig A. Message exchange between Network and Mobile during and after PDP context activation

The payload is segmented into several TB’s and are sent over the air interface.

As an example, let’s look at the configuration for a user downloading a file at the rate of 2.048 Mbps.

  • TB size = 1720 bits
  • Number of TBs per TTI = 12
  • TTI = 10 ms
  • RLC header (Doesn’t contain user data) = 16 bits
  • Payload size = 1704
  • 12 TBs of size 1704 bits are transmitted every 10 milliseconds.

So, the data rate is 1704 * 12 * 100 = 2048 kbps.

Fig. B shows how user data is contained in 10 ms TTI and transmitted.

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Fig B data blocks contained in 10ms TTI

However, Apart from user data, network is also required to send signaling data. Each TB also carries CRC bits and RLC header (discussed above) which add up further to the total data transmitted.

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