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Evolution from 2G to LTE (Part 1 – GSM)

Category: 2G GSM, Telecom, Wireless
Author: gokhankosem, on 15 Jan 12 - 2 Comments
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Evolution from 2G to LTE (Part 1 – GSM)

The evolution of the 2G technology to LTE technology is in three types:

  • Technical evolution : The new network elements are developed with new technology
  • Network Evolution : General functionality of network will change
  • Service Evolution : Demand generated by the end user will increase

To understand deeply, we will summarize the mobile technologies in order beginning with 2G.

GSM (Global System for Mobile Communication)

GSM is an ETSI standard to describe the second generation(2G) digital cellular networks. The original name was “Groupe Spécial Mobile”. Uses circuit switched network. With its evolution and adding packet switched networks, new generation technologies GPRS, EDGE, UMTS and at last LTE is introduced.

The basic GSM architecture is below. Here, as you can see GSM architecture is divided 4 subsystems:

  • Mobile Station (MS)
  • Base station Subsystem (BSS) : Radio path control
  • Network Subsystem (NSS) : Call control
  • Network Management Subsystem (NMS) : Operation and maintenance


mobile equipment + subscriber data (Service Identity Module-SIM)


Base Station Controller (BSC): Maintains radio connections towards MS and terrestrial connection towards the NSS.

Base Transceiver Station (BTS): Air interface signalling, ciphering and speech processing.


Mobile Service Switching Centre (MSC): Call control, switching for CS, mobility management.

Serving MSC: BSS connections, mobility management.

Gateway MSC: Interface to external networks, access HLR, locate mobile device, route the call, serve several MSCs, first access point from the outside.

Major databases in NSS

Home Location Register (HLR) : Information about each subscriber, what services they are subscribed,their identifiers like IMSI,MSISDN, their status, how can they be reached, which is the current VLR, etc…

Visitor Location Register (VLR): Typically locaten within the MSC,nlocal store for all the variables and functions needed to handle calls in the area related to VLR.

Authentication Center(AuC): Often located within HLR, stores a unique authentication key for eaxch subscriber,supports cpihering for voice and data.

Equipment Identification Register(EIR): Databases of IMEIs, database of stolen or defective phones.

Here, the interface between MS and BTS is “Um interface”. The interface between BTS and BSC is “Abis interface”. And lastly the interface between BSS and MSC is “A interface”. In the next lessons we will see that with addition of packet swiched network, additional interfaces will be added to the network architecture.

Additional network parts

VAS (Value Added Services) : This is a simple platform for supporting certain type of services in GSM for example short message service centre and video mail system.

IN (Intelligent Networks) : It is a platform for creating and providing additional services.

GSM Protocol Stack

Some Important Points for GSM

Data Rate : 9.6 kbps
Channel coding : Cyclic Redundancy encoder for error detection
Convolutional encoder of rate 1/2
Interleaving codes for error bursts
Modulation : Gaussian Minimum Shift Keying GMSK (BT = 0.3)
Multiple Access : TDMA/FDMA
Transmit Frequency bands : Mobile Station Uplink Reverse ch. 890 – 915 MHz
Base Station Downlink Forward ch. 935 – 960 MHz
Duplex seperation : 45 MHz
RF carrier spacing : 200 kHz
Number of RF Duplex channels : 124
Traffic channels per carrier : 8
Cell radius : upto 35 km
International roaming capability : more than 16 countries
Frame Interval : 4.615 ms (8 users)
Time slot for each user : 0.577 ms
Frequency Hopping : 217 hops/s (slow)
Equalizer : 16 µs time dispersion

You can continue with the other Evolution from 2G to LTE articles…

Evolution from 2G to LTE (Part 1 – GSM)
Evolution from 2G to LTE (Part 2 – GPRS)
Evolution from 2G to LTE (Part 3 – EDGE)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

2 comments for “Evolution from 2G to LTE (Part 1 – GSM)”


Thank you so much for this piece, can u pls try to include VAS Application to the architecture.

February 27th, 2012 at 14:52

Good One, Please also include how can we integrate the devices such as MSS & MGW in NSS domain with the IP Transport devices such as routers and switches and what devices are involved at each step of call setup in NSS Domain.
The role of Signaling Transfer Protocol and its integration with IP devices as well.

Thanks & Regards

September 30th, 2013 at 22:42

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