GSM Frame Structure

- GSM frame structure uses slots, frames, multiframes, superframes and hyperframes to give the required structure and timing to the data transmitted.

GSM TUTORIAL INCLUDES

• GSM tutorial / introduction
• Network architecture
• Interfaces
• Radio access network
• Frames
• Frequency bands and allocations
• Power class, control & amplifiers
• Physical & logical channels
• Codecs / vocoders
• Handover / handoff

The GSM system has a defined GSM frame structure to enable the orderly passage of information. The GSM frame structure establishes schedules for the predetermined use of timeslots.

By establishing these schedules by the use of a frame structure, both the mobile and the base station are able to communicate not only the voice data, but also signalling information without the various types of data becoming intermixed and both ends of the transmission knowing exactly what types of information are being transmitted.

The GSM frame structure provides the basis for the various physical channels used within GSM, and accordingly it is at the heart of the overall system.

Basic GSM frame structure

The basic element in the GSM frame structure is the frame itself. This comprises the eight slots, each used for different users within the TDMA system. As mentioned in another page of the tutorial, the slots for transmission and reception for a given mobile are offset in time so that the mobile does not transmit and receive at the same time.

Eight slot GSM frame structure

The basic GSM frame defines the structure upon which all the timing and structure of the GSM messaging and signalling is based. are groupeThe fundamental unit of time is called a burst period and it lasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods d into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

In simplified terms the base station transmits two types of channel, namely traffic and control. Accordingly the channel structure is organised into two different types of frame, one for the traffic on the main traffic carrier frequency, and the other for the control on the beacon frequency.

GSM multiframe

The GSM frames are grouped together to form multiframes and in this way it is possible to establish a time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

• Traffic multiframe:   The Traffic Channel frames are organised into multiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These are numbered 0 to 11 and 13 to 24. One of the remaining bursts is then used to accommodate the SACCH, the remaining frame remaining free. The actual position used alternates between position 12 and 25.
• Control multiframe:   the Control Channel multiframe that comprises 51 bursts and occupies 235.4 ms. This always occurs on the beacon frequency in time slot zero and it may also occur within slots 2, 4 and 6 of the beacon frequency as well. This multiframe is subdivided into logical channels which are time-scheduled. These logical channels and functions include the following:
  • Frequency correction burst  
  • Synchronisation burst  
  • Broadcast channel (BCH)  
  • Paging and Access Grant Channel (PACCH)  
  • Stand Alone Dedicated Control Channel (SDCCH)  

GSM Superframe

Multiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 traffic multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and the control multiframes are 51 bursts long, the different number of traffic and control multiframes within the superframe, brings them back into line again taking exactly the same interval.

GSM Hyperframe

Above this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe which repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSM frame structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequential number comprising the frame number and time slot number. This is used to maintain synchronisation of the different scheduled operations with the GSM frame structure. These include functions such as:

• Frequency hopping:   Frequency hopping is a feature that is optional within the GSM system. It can help reduce interference and fading issues, but for it to work, the transmitter and receiver must be synchronised so they hop to the same frequencies at the same time.
• Encryption:   The encryption process is synchronised over the GSM hyperframe period where a counter is used and the encryption process will repeat with each hyperframe. However, it is unlikely that the cellphone conversation will be over 3 hours and accordingly it is unlikely that security will be compromised as a result.

GSM Frame Structure

4G Architecture

4 G Network  Architecture

Best LTE Network Architecture Diagrams

Wireless operators are rapidly expanding their LTE networks to take advantage of additional efficiency, lower latency and the ability to handle ever-increasing data traffic. 

 LTE network architecture diagrams.

This diagram from standards body 3GPP shows network evolution from GSM to LTE. The core technologies have moved from circuit-switching to the all-IP evolved packet core (moving left to right). Meanwhile, access has evolved from TDMA (Time Division Multiple Access) to OFDMA (Orthogonal Frequency Division Multiple Access) as the need for higher data speeds and volumes as increased.

According to Chris Pearson, president of 4G Americas, there are currently 101 commercial LTE deployments around the world. “That’s one of the fastest mobile broadband technology deployments ever,” Pearson said.

Ceragon’s diagram of basic LTE architecture demonstrates how LTE flattens network architecture.  The previous Gateway GPRS support node (GGSN) for connection between the GPRS network and the Internet, and the Serving GPRS support node (SGSN) for delivery of data packets from nodes within its reach, are replaced by an all-IP structure.

Previous generations of technology relied on a “hub and spoke” design where traffic from all base stations (NodeB) was sent to the Radio Network Controller (RNC). The diagram illustrates that in LTE, the enhanced nodes (eNodeB) have direct connectivity with each other (known as x2 traffic) that enable peer-to-peer applications without reaching deep into the network.

This LTE network architecture diagram (larger version) comes from testing company Breaking Point Systems. Again, it illustrates the connection points from the end user on the left, to the evolved base stations (eNodeB) and then traffic to and between the MME and SGW (Serving Gateway), to the HSS (Home Subscriber Servier) and ultimately to the Packet Data Network (PDN) on the right.

This diagram from Juniper Networks shows the relationship of the LTE radio access network (RAN) to the LTE Evolved Packet Core/System Architecture Evolution, with the PGW (packet data network gateway) connecting the EPC to the Internet in the user/data plane, which carries users’ data traffic. Dotted lines represent network connections within the control plane.

“Once past the cell site, it’s all IP,” said Kittur Nagesh, senior director of service provider solutions for Juniper Networks. “The migration to all-IP architecture mean better spectral efficiency, and a seamless migration for both CDMA and GSM is possible with LTE. There’s also a roadmap to higher performance.”

Interphase Networks’ diagram shows the complex reality in which LTE exists, where it must interact with pre-existing network elements, including 3G networks (and small cell deployments) and Internet Multimedia Subsystems (IMS). Real-world implementations are where the complexity and differences of LTE deployments show up. The MME (Mobility Management Entity, located in the blue 4G area) authenticates wireless devices and is involved in hand-offs between LTE and previous generations of technology.

LTE-Advanced HetNet, or heterogeneous network. Image courtesy of 4G Americas.

This image from 4G Americas illustrates the future of LTE deployments, as the technology moves into later releases. A heterogeneous network, or HetNet, is a multi-layer, multi-mode, multi-band network architecture. HetNets involves the use of standard base stations (macro sites) to cover wide areas; microcells to cover individual buildings; picocells to offer wireless on the scale of separate floors of a building; and femtocells to cover small areas such as individual apartments/homes, home offices or home businesses. The goal of HetNets is to optimize spectrum use, increase network capacity and coverage, reduce capital and operating costs, and provide a consistent user experience. Future challenges for LTE HetNets include backhaul for the small cells and effective use of interference cancellation so that the various overlapping cells do not interfere with one another.