Kelvin Hilton 2nd Generation Mobile Telecommunications Part 2

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.2 TDMA vs CDMA Recall two main competitors in the 2G space were TDMA & CDMA The major difference between the new 2G technologies was the access scheme 1G had used FDMA Initially all 2G technologies were TDMA (before arrival of CDMA)

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.3 Review on sharing a medium Time division multiplexing brought digital technology to mobile communications Recall, “multiplexing describes how several users can share the same medium with minimum or no interference” [Schiller 2003] In mobile communications multiplexing can be applied in 4 dimensions Space Frequency Time Code

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.4 Space Division Multiplexing (SDM) Premise that if we have entities wishing to communicate using a single channel, then as long as we space them far enough apart interference will not occur To reduce further, the risk of interference place guard space between the frequency spaces

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.5 Frequency Division Multiplexing (FDM) Divides the available frequency into non- overlapping bands with guard spaces between to avoid overlapping (adjacent channel interference) Receiver only has to know the frequency to tune in to Used in analogue systems

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.6 Time Division Multiplexing (TDM) Allows access to entire frequency bandwidth but for a limited amount of time All senders use same frequency in at different time If two transmissions overlap known as co- channel interference Precise clock synchronisation required

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.7 Combining FDM/TDM By allowing a channel to use a certain frequency for a certain period of time more efficient use of resource is achieved More robust against interference and tapping This is the scheme used by GSM between the handset and base station

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.8 Combining FDM/TDM Requires coordination between the different senders Two senders will interfere if they select the same frequency To avoid this the senders hop between frequencies, if the hop is fast enough the period of interference may be so small that if the coding of the data signal is sufficient to allow the receiver to recover the data the interference is deemed acceptable

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.9 Code Division Multiplexing (CDM) All channels use the same frequency, however, each channel is given its own unique code Each code must be sufficiently orthogonal to allow appropriate guard spaces Large range of codes provides significant expansion, security, etc

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.10 Code Division Multiplexing (CDM) Highly complex scheme Receiver has to know the code & be able to separate out other traffic on different codes which appear as background noise Receiver & transmitter must be synchronised to provide correct decoding All signals must reach the receiver with relatively equal strength or the receiver will not be able to distinguish between them

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.11 Medium Access Control (MAC) Whilst SDM, FDM, TDM, CDM describe how the medium is accessed at the physical layer, how the selected multiplexing scheme is “regulated” is called the Medium Access scheme (equivalent to OSI Layer 2 the Data Link Layer) In mobile this layer is divided between the Logical Link Control (2b) and the MAC (2a)

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.12 Medium Access Control (MAC) Why can we not simply use proven data MAC’s such ac CSMA/CD used on ethernet? On a fixed wire, the propagation etc, is a known factor, the sender is responsible for detecting collisions, etc. If collision occurs everyone using the medium will be aware In wireless networks attenuation, etc means signal decreases as it propagates out from the transmitter, therefore a collision may occur but will not be detected by the sender

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.13 Problem of Hidden Terminals Consider three adjacent mobile phones Signal from A reaches B but not C Signal from C reaches B but not A B reaches both A and C Both A and C want to transmit to B

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.14 Problem of Hidden Terminals A senses the medium, it’s clear so it starts transmission C senses the medium and as it cannot see A as far as C is concerned the medium is available Result is collision at B Neither A or C will detect the collision because b is hidden from C and vice versa

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.15 Problem of Exposed Terminals As before, consider three adjacent mobile phones Signal from A reaches B but not C Signal from C reaches B but not A B reaches both A and C Suppose B wants to transmit to A and C wants to transmit to anywhere else

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.16 Problem of Exposed Terminals C will sense the medium is busy and so will postpone transmission However, because anything C does with any other mobile has no impact A delay is not necessary Even if there are collisions at B due to C sending it will not matter because B is sending not receiving and thus A is not effected Therefore we can say that C is exposed to B

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.17 Problem of Near/Far Terminals Proximity to a transmitter can also cause problems Because of reduction in signal strength over distance, even though C should be able to receive A’s transmissions, B’s signal is much stronger and will drown out A’s This is called the near/far effect and is particularly a problem on CDM

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.18 The role of the Access Scheme Clearly conventional digital access schemes cannot be transferred to mobile Each access scheme has its own solution Mobile networks use a combination of the schemes to overcome the problems

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.19 Space Division Multiple Access (SDMA) Allocates separate space to users of the network For example allocating the optimal base station to the mobile phone The mobile phone may receive signals from several base stations with its range The SDMA MAC algorithm decides which is best based on the FDM, TDM or CDM available Basis for SDMA is formed by the cells and antennas of the appropriate infrastructure

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.20 Frequency Division Multiple Access (FDMA) Allocation may be fixed or dynamic Can allocate the same frequencies or select a pattern from those available (eg where FDMA is combined with TDMA) using a hopping pattern FDM is often used for full-duplex communication between a base station and a mobile phone The two channels of the duplex are separated by frequency (called frequency division duplex)

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.21 Frequency Division Multiple Access (FDMA) The channel from the mobile to the base station is called the uplink channel From the base station to the mobile is called the downlink For example in GSM 900 the basic frequency allocation is All uplinks use between and 915 MHz All downlinks use between 935 and 960 MHz Base station selects the uplink/downlink based on f u = n MHz f d = f u n MHz Each channel’s bandwidth is 0.2 MHz making 124 channels per direction

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.22 Time Division Multiple Access (TDMA) The 124 channels of FDMA is clearly not sufficient TDMA is more flexible Receiver can stay on same frequency just has to listen on a different time slot Again fixed and dynamic

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.23 Fixed TDMA Allocates time slots in a fixed pattern Each mobile knows when its turn is Fits well with fixed bandwidth systems Allows for guarantee fixed delay as the maximum will only ever be n-1 slots (where n is the total number of slots in the scheme) Duplexing is achieved by allocating the same slot number in the uplink/downlink (called Time Division Duplex (TDD)) Used in GSM

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.24 Fixed TDMA

GSM Network Features

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.26 Roaming Major feature of GSM is automatic world wide location of users using the same phone number HLR always contains data about MS location As soon as user moves location, HLR transmits data to appropriate VLR Changing VLR’s without interruption in service is called roaming

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.27 Roaming To locate MS requires Mobile Station ISDN number (MSISDN) Consists of country code (CC) National Destination Code (NDC) usually the number of the network provider Subscriber number (SN) the phone number allocated to the SIM International Mobile Subscriber Identity (IMSI) Consists of Mobile country code (MCC) Mobile network code (MNC) Mobile Subscriber Identification Number (MSIN)

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.28 Roaming To locate MS requires Temporary Mobile Subscriber Identity (TMSI) Used to hide the IMSI over the air interface to protect their identity Mobile Station Roaming Number (MSRN) Temporary address generated by the VLR containing the Visitor country code (VCC) and Visitor National Destination Code (VNDC) Access to this internal network operator data is restricted Only operators with roaming agreements provide access to the data

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.29 Handover Crossing from one cell to another requires that the network update user location data, etc. Process is called handover GSM aims at maximum handover duration of 60ms Two primary reasons for handover Network cannot guarantee QOS due to distance from current BTS Loading on one BTS may necessitate transfer to another, load balancing

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.30 Handover BTS and MS perform periodic tests on the quality of uplink & downlink (approx every 0.5s) The values are compared to a handover margin (HO_MARGIN) Dependent upon difference between the current value and the HO_MARGIN handover decision is made by the BSC

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.31 Handover Dependent upon difference between the current value and the HO_MARGIN handover decision is made by the BSC MSC is notified and it manages the connection to the new BSC/BTS MS has to drop existing connection once new one is established This is an example of a hard handover If terminal makes connection QoS maintained If not connection is lost

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.32 GSM Security Original specification identified three security algorithms A3 – used for authentication A5 – used for encryption A8 – used to generate cipher key Only A5 was published by the ETSI In 1998 A3/A8 leaked on the Internet, transpired that the claimed 64bit key used for cipher frequently only used 54bits Network providers may add additional layers of security Only BTS to MS is encrypted

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.33 Call setup – MS terminated (MTC) PST N GMSC HLR VLR MSC 1User dials mobile number 2Fixed network identifies target as mobile & contacts the network via the gateway 3GMSC identifies the targets HLR & signals call setup 4After HLR checks subscriber data, it contacts VLR for current MSRN 5HLR passes the MS’s current MSC to the GMSC 6GMSC forwards call setup to MSC 7MSC requests current MS status from VLR 8Initiates paging in all cells in its LA 9BTS’s transmit paging call to MS 10If MS available, MSC requests VLR to set security. VLR returns all clear for connection to be established BSS 8 MS 9 10

2nd Generation Mobile Telecommunications kch/soc/mccs/04 page15/07/2003Copyright: All rights reserved. Not to be reproduced without consent.34 Call setup – MS Originated (MOC) Network VLR MSC 1MS transmits request for connection 2Request forwarded to MSC 3MSC checks subscriber services with VLR 4MSC checks available resources throughout network & if all are available sets up connection. Target may be: -Serviced by same MSC -Serviced by MSC on same network -Another network (mobile or fixed) 3 BSS 2 MS 1 4

Questions ?