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Networks: L17
GPRS : General Packet Radio System
• A packet data overlay onto existing GSM networks - 2½G
– packet transmission at up to 115kbs claimed
Home
Location
Register
(HLR)
Radio Network
Base Station
Subsystem (BSS)
Packet
Control
Unit
IP Network/
Internet
Serving GPRS
Support Node
(SGSN)
Leased
Line
or
VPN
Business
network
Gateway GPRS
Support Node
(GGSN)
X25 Network
Upgraded GSM equipment
New GPRS equipment
– new mobile devices required – cellphones, PDAs etc.
» some support simultaneous packet-switched and circuit-switched, some not
– device will scan for a local GPRS channel when switched on
» and attempt to attach to the network
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• Applications that might benefit from higher bandwidth :
– chat : mobile users can chat with Internet chat groups
– images : photographs, greetings cards, presentations, static web pages etc.
» not just David Beckham!
– moving images : video messages, movie previews, video conferencing etc.
– document sharing : collaborative working
– audio : broadcast quality
– corporate email for road warriors e.g. reps
– internet email : mailbox services
– vehicle positioning : handsets can incorporate GPS e.g. Motorola A920
– remote LAN access
– file transfer
– home automation : remote control of lights, kitchen equipment
– security : burglar alarms, entry cameras etc.
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– upgrade to Base Stations required :
» software upgrade to Base Transceiver Stations and Controllers
» additional hardware module – a Packet Control Unit (PCU)
- to manage the packet transfer between mobile devices and GPRS network
- also supports data frame retransmission and other GPRS protocol functions
– new network components :
» Serving GPRS Support Node (SGSN)
- interacts with Home Location Register (HLR) to get subscriber profile information
- directly connected to the Base Station
- controls access, tracks user mobility and implements some security functions
» Gateway GPRS Support Node (GGSN)
- gateway to external data networks
¤ e.g. Internet, private intranets, X25 networks etc.
- provides services for authenticating external network access, Quality of Service
- provides tunnelling for access to Virtual Private Networks
- supports roaming by routing incoming traffic to the appropriate SGSN where the
user is located
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• Protocol stack :
– GTP : GPRS Tunnelling Protocol – tunnels user data and signalling between
GPRS support nodes in the GPRS backbone network
– TCP : carries GTP PDUs in the GPRS backbone network for protocols that
need a reliable data link; UDP for protocols not needing a reliable link
– IP : the GPRS backbone network protocol for user data and control signalling
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– SNDCP : Subnetwork Dependent Convergence Protocol – maps networklevel characteristics onto the characteristics of the underlying network
– LLC : Logical Link Control – provides a reliable ciphered logical link
– Relay : relays PDUs between the Um/Gb and Gb/Gn interfaces
– BSSGP : Base Station System GPRS Protocol – conveys routing and QoS
information between BSS and SGSN
– Network Service (NS) : transports BSSGP PDUs - based on Frame Relay (a
high-performance WAN protocol originally developed for ISDN)
» may traverse a multi-hop network of Frame Relay switching nodes
– RLC : Radio Link Control provides a radio-technique-dependent reliable link
– MAC : Medium Access Control controls access signalling (request and grant)
procedures for the radio channel
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• Mapping of Functions to Logical Architecture
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• Time Slot Aggregation
– GPRS allows one user to have more than one slot in a TDMA frame
GSM : 1 time slot per user :
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GPRS : more than 1 time slot per user :
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2 slots
3 slots
– effective bandwidth thereby increased
» nominally 13.4kbps per slot
– system notified at time of transmission how many slots needed
» for sending and receiving separately
– supports both low speed and higher speed in a single network
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• Coding Schemes :
– four coding schemes defined with varying degrees of error correction
» options : Forward Error Correction, CRCs, convolutional codes etc.
– transmission speed varied based on the coding scheme in use
» CS-1 : 9.05kbs - highest level of error correction
» CS-2 : 13.4kbs (equivalent to the GSM CSD bearer service)
» CS-3 : 15.6kbs
» CS-4 : 21.4bps - least error correction
– speeds  number of slots allocated
» 9.05kbps for 1 slot using CS-1
» 172.40kbps for 8 slots using CS-4
– the better the radio link, the less error correction needed
– all transparent to the user
– only CS-1 and CS-2 in use yet
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– theoretical 172kbs available but normal allocation includes :
» 1 slot needed for a control packet
» at least 2 slots reserved for voice traffic
» remaining slots possibly for data packets
- 29 combinations of downlink and uplink slots defined
» maximum data rate one user can usually expect : type 4+1
- 4 slots downlink = 413.4kbs = 53.6kbs, 1 slot uplink = 13.4kbs
– mobiles classified as to their ability to support simultaneous voice and data
» class A : simultaneous voice and data
» class B : automatic switching between voice and data
- only one active at a time
- user can put data on hold while they receive phone calls and vice versa
» class C : hand-operated switching between voice and data
- voice calls must be cleared before GPRS can be used
» classes B and C developed first for faster time-to-market, class A later
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EDGE : Enhanced Data for GSM Evolution
• a method of increasing the data rates on the radio link for GSM
– 3-4 times increase in data speeds over GPRS claimed
• different protocols and behaviour in the Base Station System
– same packet handling protocols in the core network
• introduces a new modulation technique and new channel coding schemes
GPRS
EDGE
Modulation
GMSK
8-PSK/GMSK
Symbol rate
270ksym/s
270ksym/s
Modulation bit rate
270kbs
810kbs
Radio data rate per time slot
22.8kbs
69.2kbs
User data rate per time slot
20kbs (CS4)
59.2kbs (MCS9)
User data rate (8 time slots)
160kbs
473.6kbs
– same symbol rate but different modulation rate
» three times as many bits as GPRS during the same period of time
» users data rates take header sizes into account, radio data rates do not
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– GMSK : one bit per symbol
» equivalent to 2-phase modulation
– 8-Phase Shift Keying : coded using eight phases – gives three bits per symbol
(0,1,0)
(0,0,0)
(0,1,1)
(0,0,1)
(1,1,1)
(1,0,1)
(1,1,0)
(1,0,0)
» three consecutive bits mapped into one symbol
» more susceptible to misinterpretation – Shannon’s theorem etc.
» under good reception conditions, better throughput
» under poor reception conditions, the extra bits can be used to add more errorcorrecting coding, and the correct information recovered
» in the worst reception conditions, GMSK better
- EDGE therefore defines a mixture of GMSK and 8-PSK modulation
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• Coding schemes
– nine more schemes introduced for EDGE – MCS1 to MCS9
» MCS1 to MCS4 for GMSK, MCS5 to MCS9 for 8PSK
– GPRS tops out at 20kbs, EDGE goes up to 59,2kbs
• Retransmission
– a packet sent using a higher coding scheme that is not properly received can be
retransmitted with a lower coding scheme
» requires changes in payload sizes to take account of extra error correction bits
– GPRS required same coding scheme to be used again in retransmissions
» even if radio environment had changed
– EDGE has a faster scheme for analysing the radio link quality
» allows faster adaptation to the current radio environment
• Interleaving
– entire block retransmitted in GPRS if any of the 4 bursts not received correctly
– EDGE interleaves over two bursts instead of four – better likelihood of success
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UMTS : Universal Mobile Telecommunication System
• Third Generation mobile telecoms – 3G
• to cope with increasing volumes of traffic
– and new capacity-demanding apps such as video streaming & music download
• developed as a global standard to assure international roaming
– though now unlikely to be universal e.g. USA, China may use different standards
• a development from GSM and CDMA – also known as W-CDMA
• auction of licenses for spectrum
– Vodafone paid £5.96billion, BT (now Mmo2), One2One and Orange paid £4bn
» for 2 or 3 paired 5MHz channels
» all now regretting the amounts paid!
– auctions also in other countries but beauty contests in some
– extremely expensive to set up the new infrastructure needed
• new handsets required to deal with new CDMA technology and applications
– dual purpose handsets – have to be back-operable with GSM
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– Paired frequencies in the ranges
» 1.92GHz – 1.98GHz uplink,
» 2.11GHz – 2.17GHz downlink
– B : Vodafone; C : BT (Mmo2); D : One2One; E : Orange
– A : reserved for a late entrant – taken by Hutchison 3G for £4.4bn
» but still the first in service!
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– similarities between GSM and WCDMA :
» Radio Access Network v. Base Station System (can be co-sited)
» Base Station Controller v. Radio Network Controller (RNCs cross-connected)
» Base Transceiver Station v. Radio Base Station (multiple cells per RBS)
– both connect to and share the same Core Network
» essential to minimise costs
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• WCDMA
– all users use the same frequency band and transmit at the same time
» orthogonal Walsh codes for spreading
» Gold codes used for synchronisation and scrambling
– should provide :
» higher bit rates – up to 2Mbs
» higher spectrum efficiency
» better QoS
– developments in DSP and fabrication costs make CDMA feasible for handsets
– chip rate of 3.84Mcps
– spreading factors variable from 4 to 512
– competing CDMA2000 in USA
» same frequency bands not available in USA
» uses 1.25MHz bands and chip rate of 1.22Mcps
» developed by QualComm – the main CDMA patent-holders
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• Fast Power Control
– regulates the transmit power of mobile and of base station
– the lower the power the less the interference
» less interference means more capacity on the same CDMA carrier
– aim for the base station to receive the same power level from all handsets in
the cell, regardless of distance from the base station
» if power level from one handset higher than needed, everyone else will suffer
» if power too low, quality will suffer
– WCDMA updates power levels 1500 times per second
» rapid changes in the radio environment can be handled
High Load
– implemented in both uplink and downlink
– cell breathing occurs :
» size of a cell varies depending on the traffic load
» when load is low, good quality achieved over long distance
» when load is high, interference is higher and users need to
be closer to the base station to get the same quality
Low Load
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• Soft and Softer Handover
– with soft(er) handover, a handset can communicate simultaneously with two
or more cells in two or more base stations
– continuity and quality of connection maintained while moving from one cell to
another
» important to operators – a very competitive marketplace
– during handover, handset will adjust its power to the base station that
requires the smallest amount of transmit power
» the preferred cell may change rapidly
– soft handover : handset connected to multiple cells at different base stations
– softer handover : connected to multiple cells at the same base station
– requires additional hardware resources on the network side
» since the handset has multiples simultaneous connections to cells
– 30-40% of users will be in soft or softer handover in a well-designed network
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• Handover to GSM
– limited initial 3G coverage requires interoperability with GSM
– need to swap to GSM when moving out of WCDMA cell range
» though same quality of service not possible e.g. fast data rates
– a positive effect on capacity through load sharing when possible
– handset has to identify which is the best GSM cell to swap to
• Inter-frequency handover
– in high capacity areas where multiple 5MHz carriers are deployed
– some users may need to be swapped to a lower-loaded band
• Channel type switching
– common channels and dedicated channels
– depending on the amount of data the user needs to transmit
– a dedicated channel when large amount of data e.g. voice, web page
– common channel for control information
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• Admission Control
– to avoid system overload and to maintain quality of service
– when a new user seeks to access the network, admission control estimates the
network load
– if the new load is sustainable, the new user is admitted
– if not, the user is blocked out
• Congestion Control
– overload can still occur e.g. when a user moves to a new area
– possible actions :
» reduce the bit-rate of non real-time applications
» if not sufficient, invoke an inter-frequency handover
» hand some users over to GSM
» discontinue the connection
• Synchronisation of base stations
– maintained by handsets measuring offsets in different cells and reporting back
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