GPRS

GPRS, stands for General Packet Radio Service, is used to give higher data speed over GSM. It is not the replacement of GSM. It is just a extension to the older GSM technology to gain faster speed.

Multimedia Messaging System or MMS is the feature of GPRS. It allowed subscribers to send videos, pictures, or sound clips to each other just like text messages. GPRS also provided mobile handset the ability to surf the Internet at dial-up speeds through WAP enabled sites.

GPRS offered higer bit rate ( Up to 171kb/s) by usage of A packet-linked technology over GSM.

Architecture of GPRS

The most important network nodes added to the existing GSM networks are:

SGSN (Serving GPRS Support Node). GGSN (Gateway GPRS Support Node).

The serving GPRS support node (SGSN) is responsible for routing the packet switched data to and from the mobile stations (MS) within its area of responsibility. The main functions of SGSN are packet routing and transfer, mobile attach and detach procedure (Mobility Management (MM)), location management, assigning channels and time slots (Logical Link Management (LLM)), authentication and charging for calls. It stores the location information of the user (like the current location, current VLR) and user profile (like IMSI addresses used in packet data networks) of registered users in its location register.

The gateway GPRS support node (GGSN) acts as interface between the GPRS backbone and the external packet data network (PDN). It converts the GPRS packet coming from the SGSN into proper packet data protocol (PDP) format (i.e. X.25 or IP) before sending to the outside data network. Similarly it converts the external PDP addresses to the GSM address of the destination user. It sends these packets to proper SGSN. For this purpose the GGSN stores the current SGSN address of the user and his profile in its location register.The GGSN also performs the authentication and charging functions. In general there may be a many to many relationship between the SGSN and GGSN. However a service provider may have only one GGSN and few SGSNs due to cost constraints. A GGSN proved the interface to several SGSNs to the external PDN.

With GPRS providing a move from circuit switched technology to packet switched technology, it was necessary to upgrade the network architecture to accommodate this. To accommodate this the GPRS network architecture added new elements including the GGSN and SGSN to the existing GSM network to be able to accommodate this.

However it was still necessary for the GPRS network elements and those from the existing GSM elements to work along side one another. Accordingly the introduction of GPRS technology saw the addition of some new entities within the over network architecture.

GPRS network architecture upgrades

With GPRS providing additional connectivity in terms of packet data, there are naturally a number of upgrades needed to the network architecture required. A number of new elements are needed for the network, but these can operate alongside the existing elements meaning that the GPRS capability is an upgrade to the network and not a completely new network structure.

The main new network architecture entities that are needed are:

SGSN:   GPRS Support Node - this forms a gateway to the services within the network. GGSN:   Gateway GPRS Support Node which forms the gateway to the outside world. PCU:   Packet Control Unit which differentiates whether data is to be routed to the packet

switched or circuit switched networks.

A simplified view of the GPRS network architecture can be seen in the diagram below. From this it can be seen that it is very similar to the more basic GSM network architecture, but with additional elements.

GPRS network architecture

SGSN

The SGSN or Serving GPRS Support Node element of the GPRS network provides a number of takes focussed on the IP elements of the overall system. It provides a variety of services to the mobiles:

Packet routing and transfer Mobility management Attach/detach Logical link management Authentication Charging data

There is a location register within the SGSN and this stores location information (e.g., current cell, current VLR). It also stores the user profiles (e.g., IMSI, packet addresses used) for all the GPRS users registered with the particular SGSN.

GGSN

The GGSN, Gateway GPRS Support Node is one of the most important entities within the GPRS network architecture.

The GGSN organises the interworking between the GPRS network and external packet switched networks to which the mobiles may be connected. These may include both Internet and X.25 networks.

The GGSN can be considered to be a combination of a gateway, router and firewall as it hides the internal network to the outside. In operation, when the GGSN receives data addressed to a specific user, it checks if the user is active, then forwarding the data. In the opposite direction, packet data from the mobile is routed to the right destination network by the GGSN.

PCU

The PCU or Packet Control Unit is a hardware router that is added to the BSC. It differentiates data destined for the standard GSM network (circuit switched data) and data destined for the GPRS network (Packet Switched Data). The PCU itself may be a separate physical entity, or more often these days it is incorporated into the base station controller, BSC, thereby saving additional hardware costs.

GPRS network upgrading

One of the key elements for any network operator is the cost of capital expenditure (capex) to buy and establish a network. Capex costs are normally very high for a new network, and operators endeavour to avoid this and use any existing networks they may have to make the optimum use of any capital. In addition to the capex, there are the operational costs, (opex). These costs are for general maintenance and other operational costs that may be incurred. Increasing efficiency and reliability will reduce the opex costs.

Any upgrade such as that from GSM to GPRS will require new investment and operators are keen to keep this to the minimum. The upgrades for the GPRS network are not as large as starting from scratch and rolling out a new network.

The GPRS network adds to the existing GSM network. The main new entities required within the network are the SGSN and GGSN, and these are required as the starting point.

The base station subsystems require some updates. The main one is the addition of the PCU described above. Some modifications may be required to the BTS, but often only a software upgrade is required, and this may often be achieved remotely. In this way costs are kept to a minimum.

The GPRS network architecture can be viewed as an evolution of the GSM network carrying both circuit switched and packet data. The GPRS network architecture was also used as the basis for the 3G UMTS network. In this way network operators could evolve their networks through GPRS and possibly EDGE to the full 3G networks without having to replace and install more new equipment than was absolutely necessary.

3G- WCDMA

The introduction of 3G changed a lot of the accepted standards in the mobile phone industry. It allows the use of a greater bandwidth that allows more features to be implemented on it.

3G gives many features like video calls and TV applications because of the speed of 3G which began at 384kbps; well within DSL speeds. Further development on 3G technologies have also created even faster data rate reaching 3.6 and even 7.2Mbps.

Existing GSM networks are not compatible with the 3G networks. To keep it, requires a new infrastructure.

According to popularity and demand Telecom Operators place 3G towers in those areas.They have to operate 2 radios in particular areas; one for GSM and one for 3G.

Mobile phone Users are also required to switch mobile phones in order to take advantage of the new features of 3G.

The WCDMA system is part of the UMTS. It is developed by the 3G Partnership Program, which is composed of evolved core cellular networks that belong to the Global System for Mobile (GSM) communications networks worldwide.

WCDMA features two modes:

Frequency Division Duplex (FDD): Separates users by employing both codes as well as frequencies. One frequency is used for the uplink, while another is used for the downlink.

Time Division Duplex (TDD): Separates users by employing codes, frequencies and time, wherein the same frequency is used for both uplink and downlink.

Although WCDMA is designed to operate on evolved GSM core networks, it uses code division multiple access (CDMA) for its air interface. In fact, the majority of the 3G systems in operation employ CDMA, while the rest use time division multiple access (TDMA). The TDD mode of WCDMA actually employs a combination of TDMA and CDMA.

CDMA allows multiple users to share a channel at the same time, while TDMA allows users to share the same channel by chopping it into different time slots. CDMA offers the benefits of multipath diversity and soft handoffs.

As an air interface technology, WCDMA is able to artificially increase a signal's bandwidth. It does so by modulating each baseband symbol with a binary or quaternary signature with a much higher rate than that of the original data symbol.

3G Networks are based on WCDMA i.e. Wideband Code Division Multiple Access, a mobile technology that improves upon the capabilities of current GSM networks.W-CDMA (Wideband Code-Division Multiple Access), an ITU standard derived from Code-Division Multiple Access (CDMA), is officially known as IMT-2000 direct spread. W-CDMA is a third-generation

(3G) mobile wireless technology that promises much higher data speeds to mobile and portable wireless devices than commonly offered in today's market.

Parameters of WCDMA

Channel Bandwidth: 5 MHzDuplex Mode: FDD and TDDSpread Spectrum Technique: Direct SpreadChip Rate: 3.84 MHzFrame Length: 10 ms (38400 chips/sce)Slot Length: 15 Slots per Frame (2560 chips/slot)Spreading Modulation: Balanced QPSK (downlink) and Dual-Channel QPSK (uplink) with complex spreading circuit.Data Modulation: QPSK (downlink) and BPSK (uplink)Channel Coding: Convolution code, Turbo code, and no coding.Spreading Factors: 4-256 (uplink) and 4-512 (downlink).Modulation symbol rates vary from 960 k symbols/s to 15 k symbols/s (7.5 k symbols/s) for FDD uplink (downlink).Spreading (downlink): OVSF sequences for channel separation. Gold sequences 218-1 for cell and user separation (truncated cycle: 10 ms).Spreading (uplink): OVSF sequences for channel separation. Gold sequences 225-1 for user separation (truncated cycle: 10ms)

Architecture of WCDMA

USER EQUIPMENT

HIPERLAN

The standard covers the Physical layer and the Media Access Control part of the Data link layer like 802.11. There is a new sublayer called Channel Access and Control sublayer (CAC). This sublayer deals with the access requests to the channels. The accomplishing of the request is dependent on the usage of the channel and the priority of the request.

CAC layer provides hierarchical independence with Elimination-Yield Non-Preemptive Multiple Access mechanism (EY-NPMA). EY-NPMA codes priority choices and other functions into one variable length radio pulse preceding the packet data. EY-NPMA enables the network to function with few collisions even though there would be a large number of users. Multimedia applications work in HiperLAN because of EY-NPMA priority mechanism. MAC layer defines protocols for routing, security and power saving and provides naturally data transfer to the upper layers.

On the physical layer FSK and GMSK modulations are used in HiperLAN/1.

HiperLAN features:

range 50 m slow mobility (1.4 m/s) supports asynchronous and synchronous traffic Bit rate - 23.2 Mbit/s Description- Wireless Ethernet Frequency range- 5 GHz

HiperLAN does not conflict with microwave and other kitchen appliances, which are on 2.4 GHz. An innovative feature of HIPERLAN 1, which may other wireless networks do not offer, is its ability to forward data packets using several relays. Relays can extend the communication on the MAC layer beyond the radio range. For power conservation, a node may

set up a specific wake up pattern. This pattern determines at what time the node is ready to receive, so that at other times, the node can turn off its receiver and save energy. These nodes are called p-savers and need so called p-supporters that contain information about wake up patterns of all the p-savers they are responsible for. A p-supporter only forwards data to a p-saver at the moment p-saver is awake. This action also requires buffering mechanisms for packets on p-supporting forwaders.

HIPERLAN 1 – Characteristics

 

Data transmission point-to-point, point-to-multipoint, connectionless 23.5 Mbit/s, 1 W power, 2383 byte max. packet size Services asynchronous and time-bounded services with hierarchical priorities compatible with ISO

MACTopology infrastructure or ad-hoc networks transmission range can be larger then coverage of a single node  („forwarding“ integrated

in mobile terminals)

Further mechanisms

power saving, encryption, checksums

Services and protocols

CAC service

  definition of communication services over a shared medium   specification of access priorities   abstraction of media characteristics MAC protocol MAC service, compatible with ISO MAC and ISO MAC bridges uses HIPERLAN CAC

CAC protocol

provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or several channels

Physical protocol

send and receive mechanisms, synchronization, FEC, modulation, signal strength

HIPERLAN 1 – Physical layer

Scope

  modulation, demodulation, bit and frame synchronization   forward error correction mechanisms measurements of signal strength   channel sensing

Channels

  3 mandatory and 2 optional channels (with their carrier frequencies) mandatory channel 0: 5.1764680 GHz   channel 1: 5.1999974 GHz   channel 2: 5.2235268 GHz   optional (not allowed in all countries)   channel 3: 5.2470562 GHz   channel 4: 5.2705856 GHz

HiperLan is the total opposite of 802.11. This standard has been designed by a committee of researcher within the ETSI, without strong vendors influence, and is quite different from existing products. The standard is quite simple, uses some advanced features, and has already been ratified a while ago (summer 96 - we are now only waiting for the products).

The first main advantage of Hiperlan is that it works in a dedicated bandwidth (5.1 to 5.3 GHz, allocated only in Europe), and so doesn't have to include spread spectrum. The signalling rate is 23.5 Mb/s, and 5 fixed channels are defined. The protocol uses a variant of CSMA/CA based on packet time to live and priority, and MAC level retransmissions. The protocol includes optional encryption (no algorythm mandated) and power saving.

The nicest feature of Hiperlan (apart from the high speed) is the ad-hoc routing : if your destination is out of reach, intermediate nodes will automatically forward it through the optimal route within the Hiperlan network (the routes are regularly automatically recalculated). Hiperlan is also totally ad-hoc, requiring no configuration and no central controller. HiperLan II

HiperLan II is the total opposite of HiperLan (see above ;-). The first HiperLan was designed to build ad-hoc networks, the second HiperLan was designed for managed infrastructure and wireless distribution systems. The only similarities is the HiperLan II is being specified by the ETSI (Broadband Radio Access Network group), operate at 5 GHz (5.4 to 5.7 GHz) and the band is dedicated in europe.

HiperLan II was the first standard to be based on OFDM modulation (see chapter   4.7.4 ). Each sub-carrier may be modulated by different modulations (and use different convolutional code, a sort of FEC), which allow to offer multiple bit-rates (6, 9, 12, 18, 27 and 36 Mb/s, with optional 54 Mb/s), with likely performance around 25 Mb/s bit-rate. The channel width is 20 MHz and includes 48 OFDM carriers used to carry data and 4 additional are used as references (pilot carriers - total is 52 carriers, 312.5 kHz spacing).

HiperLan II is a Wireless ATM system (see chapter   5.1.4 ), and the MAC protocol is a TDMA scheme centrally coordinated with reservation slots. Each slot has a 54 B payload, and the MAC provide SAR (segmentation and reassembly - fragment large packets into 54 B cells, see chapter   5.2.2 ) and ARQ (Automatic Request - MAC retransmissions, see chapter   5.2.1 ). The scheduler (in the central coordinator) is flexible and adaptive, with a call admission control, and the content of the TDMA frame change on a frame basis to accommodate traffic needs. HiperLan II also defines power saving and security features.

HiperLan II is designed to carry ATM cells, but also IP packets, Firewire packets (IEEE 1394) and digital voice (from cellular phones). The main advantage of HiperLan II is that it can offer better quality of service (low latency) and differentiated quality of service (guarantee of bandwidth), which is what people deploying wireless distribution system want. On the other hand, I'm worried about the protocol overhead, especially for IP traffic

The main deficiency of Hiperlan standard is that it doesn't provide real isochronous services (but comes quite close with time to live and priority), doesn't fully specify the access point mechanisms and hasn't really been proved to work on a large scale in the real world. Overhead tends also to be quite large (really big packet headers).

HiperLan suffers from the same disease as 802.11 : the requirements are tight and the protocol complex, making it very expensive.

Architecture of Hiperlan