GSM Protocol Architecture - Part 2 : User Plane:

As it mentioned in the previous blog post, User Plane handles the transportation of user data across the physical layer from a subscriber or a data channel. Thus it can be categorized under Layer 1 protocols. The communication services offered by this layer are defined under a set of connection types. In GSM connection is defined between reference points.

Three connection types that define GSM connection are;

1. Speech Transition

2. Transparent Data Transmission

3. Non-transparent Data Transmission

1. Speech transmission

In here, digital, source-coded speech signal of MS is transmitted across the physical layer in an error-protected and encrypted form. At the BTS (Base Transceiver Station), signal is de-ciphered and removes the error-protection prior to passing on.

2. Transparent Data Transmission (TDT)

When it comes to digital radio channels there would be severe quality variations and may generate burst errors. If the signal quality is too low, the resulting errors cannot be corrected.

Therefore in the TDT user data protected against transmission errors by Forward Error Correction (FEC – a mechanism for error controlling while data transmission, by the sender adding redundant data bit to its message. This enables receiver to correct errors if an erroneous message is received). But, in TDT further data transmission between the network and next MSC occurs unprotected.

3. Non-transparent Data Transmission

For the worst conditions even TDT quality is not enough and hence more redundancy bits will be added here to the data stream. Further an error-detection with automatic re-transmission of faulty data-blocks will be used. The protocol used in GSM for this purpose is RLP (Radio Link Protocol).