Modern telecommunication standards like GSM and LTE are some of the most complex engineering works in the world. They are very intricate and depend on a multitude of protocols. A simple way to understand telecom protocol stacks is to divide them in three different layers: the physical layer, the data link layer, and the network layer.

Figure 1: Protocol stack layers

These layers are implemented differently in each wireless protocol. In this blog post, I describe GSM and LTE in order to illustrate the differences and similarities between protocol stack implementations.

The physical layer

The physical layer is the protocol’s interface to the real world. It consists, in electrical engineering terms, of the transmission and reception hardware as well the software controlling it. Its purpose is to handle the transmission and reception of data through the protocol’s defined physical channels.

  • GSM: Uses GMSK modulation, encoding 1 bit per symbol. Its physical channels consist of 156,25 bit durations of GMSK symbols per burst.
  • GSM/EDGE: Uses PSK8 modulation, 3 bits per symbol.
  • LTE: Uses OFDM or OFDMA modulation and includes potential multiple-input multiple-output (MIMO) features.

The data link layer

The data link layer enables the different network entities to transfer data between themselves through the physical layer.

Figure 2: Data link layer (layer 2) comparision

GSM uses the LAPDm protocol to establish communication between the base transceiver station and the mobile unit.

For GSM/EDGE and LTE, the data link layer is divided into three sub-layers, the Media Access Controller (MAC), the Radio Link Control (RLC) and the Packet Data Convergence Protocol (PDCP, for LTE) or the Sub Network Dependent Convergence protocol (SNDCP for GSM/EDGE).

The MAC sub-layer handles the mapping between the logical and transport channels, linking the RLC to the physical layer. Logical channels define what type of data is transported whereas transport channels define how data is transported. The MAC sub-layer also manages the priority between the logical channels of a single user equipment (UE) unit or between multiple UEs.

Figure 3: LTE data link layer flow

The RLC sub-layer handles the transfer of user data through the logical channel connection with the MAC.

The PDCP and SNDCP sublayers handle the link to the network protocols, for example performing the compression and decompression of IP packets.

The network layer

The network layer’s function is the interconnection of nodes within a network. In wireless telecommunications protocols, this enables UE mobility.

Figure 4: Network layer (layer 3) comparision

The GSM network layer is divided in three sublayers:

  • Radio resources management (RRM): Manages the radio parameters to maximize spectrum utilization and radio network resources. Besides point-to-point concerns, it handles multi-user and multi-cell concerns like bandwidth allocation and channel scheduling.
  • Mobility management (MM): Enables the GSM network to track mobile units as they move through the network cells. It handles user authentication and cell-to-cell handover.
  • Connection management (CM): Handles the connection of phone calls.

The following diagram shows the layer 2 and 3 interconnections in a GSM system.

Figure 5: GSM layer 2 and 3 interconnections

The LTE network layer is divided in two sublayers:

  • Radio resource control (RRC): Handles the point-to-point connections to the user equipment.
  • Non-access stratum (NAS): Handles the communications sessions within the network. It’s in charge of mobility management and user identification.

Nutaq’s PicoSDR and wireless protocol stack development

The PicoSDR 2X2E’s architecture makes it the perfect development platform for the development and testing of telecommunication protocol stacks. The combination of a Radio420M FMC, a Virtex-6 FPGA, and an i7 CPU card enables the distribution of MIMO physical layer resources. The CPU’s Linux environment can be used to develop layer 2 and parts of layer 3 (i.e. radio resources control). The backplane Ethernet port enables the interconnection of multiple units to a desktop PC where the rest of layer 3 can be developed.