Lecture overview Modifications and derivatives of GSM Data transmission in GSM: HSCSD GPRS part one EDGE
Modifications and derivatives of GSM Introduction of half-rate speech coding (BR 6.5 kb/s) Two users can utilize the same TS increased capacity Diminished signal quality especially for GSM-GSM connection New, high quality speech coder Enhanced Full Rate Encoder Handsets often have coders fulfilling all 3 standards Modifications lead to enhancement in transmission capacities and possibility of providing new services
Digital Cellular System (DCS) In 1990 the work started on adaptation of GSM standard to 1800 MHz band (1900 MHz in USA) Personal Communications Networks (PCN) category Primary for urban & suburban areas with very high traffic density Lower power for BS and MS Up to 374 frequency channels Much higher traffic capacity (lower power, smaller cells) More prone to Doppler shift max speed 130 km/h Possibility of roaming within the country using other operator network when no coverage from user s mother network impossible in GSM 900 In dual band phones and operators exploiting both bands switching between GSM and DCS automatic
Data Transmission in GSM Data transmission possible but procedures optimised for voice transmission Max. BR 9.6 kb/s and some special conditions 14.4 kb/s Error correction to adjust the BR to 22.8 kb/s Data transmission blocks parallel chain to voice transmission Transmission asynchronous and synchronous (BR 0.3/1.2/2.4/4.8/9.6 kb/s) Interworking Function (IWF) interface modifying data stream from GSM to properties of other networks Rate adaptation ARQ procedures Acoustic modem functions
GSM offers 2 types of services: Teleservices full service within the network, no need for external device (speech, SMS) Bearer services GSM implements lower layers of services (transport mechanism between access points). Terminal Equipment required e.g. laptop, palmtop or computer server on both ends Data transmission is a bearer service Circuit-switched transmission link established between communicating terminals
Short Message Service (SMS) teleservice Store-and-forward mechanism if recipient not available message is stored and delivered when user reappears Can be delivered parallel to speech transmission (using DCCH) Max. 160 signs mapped into 140 bytes using special alphabet
High-Speed Circuit Switched Data Service (HSCSD) Answer to slow data rates in GSM & high cost (duration) Increase data rates with min. system modifications: Assigning multiple TS to one connection (HSCSD link) Keeping circuit-switched communications Max 8 TS with full-rate channel 8*9.6 kb/s=76.8 kb/s In practice max. 57.6 kb/s when 4 TS with 14.4 kb/s used All assigned channels follow common frequency hopping procedure, same training sequences in midamble, channel coding, interleaving and rate adaptation Each TS has its own SACCH, but common FACCH All traffic treated as one radio link e.g. during handover
During connection set-up MS indicates: The max. - Desired Number of Channels - DNC and min. - Required NC Acceptable channel coding (transparent with error correction, non-transparent no correction) Allowable type of remote modem Value of the fixed network user data rate System tries to fulfil requirements depending on resources Connection can be: Symmetric the same no. of TS in both directions Asymmetric higher no. of TS for downlink HSCSD has higher probability of blocking multiple TS assigned to one user Price depends on duration of connection and no. of TS used
General Packet Radio Service (GPRS) Circuit-switched system not suitable for data transmission Burst & asymmetric nature inefficient usage of resources Packet transmission with statistical multiplexing The same TS can be used by multiple users Cost based on no. of packet sent not duration GPRS uses network parallel to GSM system, but uses many of GSM blocks Network elements connected using separate backbone network based on IP Main novel element: GPRS Support Node (GSN): Deliver data packets Determine packet routes between MS and external networks
SGSN Serving GPRS Support Node GPRS system Architecture
Serving GSN: Delivering and reception of packets from and to MS within its service area (similar to MSC) User authentication Storage of information of all GPRS subscribers in this SGSN Gateway GSN interface between GPRS and external data packet networks
GPRS Physical Layer Multislot operation up to 8 slots assigned to one user Asymmetric traffic means that there is separate resources assignment for up- and downlink Channel allocation only for duration of packet Other MS can use TS during intervals between packets Capacity on demand no. of assigned TS function of traffic load, service priority etc. Channels can be shared between GSM and GPRS Physical channel called Packet Data Channel (PDCH)
Several logical channels: Packet Data Traffic Channel (PDTCH) user data (1 or more) Packet Broadcast Control Channel (PBCCH) information on organisation of GSM and GPRS system downlink Packet Common Control Channel (PCCCH): Packet Random Access Channel (PRACH) MS requests PDTCH Packet Access Grant Channel (PAGCH) confirmation of allocation of PDTCH to MS Packet Paging Channel (PPCH) Packet Notification Channel (PNCH) inform MS about message in multicast of group calls Packet Associated Control Channel Packet Associated Control Channel (PACCH) signalling information associated with 1 or more PDTCH, bidirectional
Packet Timing advance Control Channel (PTCCH/U or /D) uplink of downlink adjust timing advance on frame clock To connect to GPRS network MS looks for PBCCH and then PCCCH If not found it looks for BCCH and CCCH of GSM Logical channels mapped on to physical channels in a similar way as in GSM but: 4 frames create a block unit for applied channel coding Block has 4*114 bits=456 bits 12 block constitute a multiframe (52 frames not 26 as in GSM) Two-level channel coding is applied: block and convolution code with puncturing
There are 4 coding schemes depending on channel conditions and service requirements CS-1 strongest poor channel conditions 9.05 kb/s resulting BR CS-4 for very good conditions only block coding 21.4 kb/s per slot i.e. 171.2 kb/s for 8 slots There is a set of different services in GPRS due to different channel coding characterized by Quality of Service (QoS) QoS defined using parameters: Priority Reliability specific values of parameters Delay Throughput max. and mean BR QoS is negotiable by MS depending on service and resources
Enhanced Data rate for Global Evolution (EDGE) Response to demand for higher BR In Europe extension of GSM and GPRS (Enhanced GPRS, Enhance Circuit Switched Data) In USA extension of TDMA personal mobile communication system IS-136 Main improvement 8-PSK modulation (phase shift keying) Retains 270.833kb/s at 200 khz but send 3 times more data Except for information controlled phase shift there is additional shift by 3π/8 at each symbol period avoid low levels of signal envelop, reduces the peak-to-average power ratio (3.2 db) Lower BR GMSK still used
Slow frequency hopping: Fight against channel fading Lower co-channel interference level Increases overall system capacity Each block sent on 4 different frequency carriers Link quality control MS informs BS about the quality of the channel As a result best combination of modulation and coding used 2 modulations and 9 coding rates each combination results in a different throughput
EDGE burst has the same structure as GSM If 8-PSK used it transmits 348 bits/burst (not 114) Multiframe = 25 frames = 12 blocks, 13 th frame without data (timing advance correction or measurements) Max. data rate per carrier with 8 slots 556.8 kb/s for 8-PSK and 185.6 kb/s for GMSK User data rate lower due to applied coding
Summary Modifications and derivatives of GSM DCS Half-rate speech coding Data transmission in GSM Standard GSM max. 14.4 kb/s HSCSD circuit switched, Multiple TS allocation Priced by duration and no. of TS GPRS Packet switched with statistical multiplexing Architecture, Logical channels EDGE