4. GPRS Network Structure

Transcription

1 GPRS-network structure Page GPRS Network Structure The network structure known in GSM has to be enhanced to guarantee the data transfer of packet oriented data. In circuit-switched communication, a connection between two points is established at the beginning and maintained over the duration of the call. We talk about time oriented billing. Packet data does not appear continuously, but erratically and in different lengths. To convey them, it is necessary to pack them into a so called packet and combine every data packet with a so called packet header. This packet header contains the destination address of the target. Each packet has to be routed independently of other packets through the packet oriented network. Such networks are for example: TCP/IP-networks or ATMnetworks. Packet orientation requires an extension of GSM-network structure by the following elements: HLR GPRS Info circuitswitched G PSTN ISDN BSC PCU packetswitched PDN MS BSS NSS external Picture: New elements in the GSM-network 4.1. Serving GPRS Support Node The forms the switching node for packet oriented data. It possesses the same hierarchical level as the Mobile Switching Center,. Tasks of the are the mobility management of the subscribers location information, the control of network access, ciphering and authentication and the routing of the received data packets to the next node. The is connected to the BSS via the Gb-interface, a Frame Relay network and to the and other s via the Gn-interface. The Gninterface is based on the IP-Protocol. Features and tasks of the Hierarchically, the is on the same level as the Visitor, the in which area, the subscriber actually stays in dedicated or idle mode. Task of the V is to store or keep the location information of the subscriber, to establish a mobile terminated call if necessary. In the case of the mobile originated call, the determines the endpoint, this means, the which allows access to

2 GPRS-network structure Page 34 the desired external network. Concerning authentication, the has more or less the same tasks as the. In that case, that a subscriber puts into effect a PDP context activation process, for example, calling the address of a special website in the internet, the has to determine the correct. The is responsible for the mobility management, this means, it has to store the location information about the subscriber. This location information could be the routing area or the cell area where the subscriber MS stays. There is the possibility of MS GSM attached as well as GPRS attached. In such a case, the has to perform all paging calls. We remember the different location information. In GSM case this was the so called location area, in GPRS case this is the routing area. But the routing area is a subset of a location area, forms the more exact location information. This is the reason, why the has to carry out all the paging calls if the MS is GPRS attached. If the optional Gs-Interface is available, the performs the location update via this interface and informs the / about the new position of the ms: The performs the admission and network access control procedures. The authentication process known from GSM will be continued also in GPRS in the same way. One exception, the 2 points of ciphering will be the and the MS, comparing GSM, where ciphering happens between the MS and the. Billing and charging will be based on the data collected by the 4.2. Gateway GPRS Support Node The is used as inter-working node between the PLMN NSS and external packet-switched networks. Due to this, we can realise the direct access to external packet date networks, without using any more some inter-working networks. The Giinterface is to be adapted to the needs of the external network. The s and s are interconnected via an IP-based backbone. Concerning its functions, the can be compared with the Gateway, the G. Characteristics and tasks of the : Interface to external packet data networks Carrying out the routing functions, with the aid of routing tables. The routing table contains as information the visitor to enable the correct delivery of any incoming data unit. Due to a screening filter, the forms a firewall to protect the GPRS network against unwanted data Same as the, the provides data for the billing and charging Realisation of the can be in a combined mode, together with a.

3 GPRS-network structure Page Inter- and Intra-PLMN-netzworks. Interface to external networks: In the following picture, we can see once more the interfaces between networks, network internally and between GPRS PLMN and external packet data networks. There will be no longer the need for a interworking network, e.g. ISDN to provide the access to the packet data network. Between the different PLMN s and PLMN internally, we find the so called backbone network. There are 2 species: Intra-PLMN-Backbone Network, between the switching nodes and, network internally. Inter-PLMN-Backbone Network as a connection of 2 different networks via the Gpinterface. The only difference to the intra-plmn-backbone network will be a so called Border Gateway, as a security function to protect the own network against influences from outside. Access to PDN via intermediate network Intermediate network, e.g. ISDN Packet Data Network Inter-PLMN Backbone Gi Gp Gi Direct access to external PDN s BG G BG BG Intra-PLMN Backbone Intra-PLMN Backbone Intra-PLMN Backbone Security function GSM only PLMN GSM + GPRS PLMN A GSM + GPRS PLMN B Picture: Connection GPRS-PLMN PDN and Inter- + Intra-PLMN Network

4 GPRS-network structure Page HLR-Extension The subscriber specific data in the Home Location Register must be expanded by some new GPRS-Data. For that reason it is necessary to enhance the memory capabilities of the HLR. The information in the HLR can be divided into subscriber specific or location specific. Field IMSI MSISDN Number Address SMS Parameters MS Purged for GPRS MNRG Description IMSI is the main reference key. The basic MSISDN of the MS. The SS7 number of the currently serving this MS. The IP address of the currently serving this MS. SMS-related parameters, e.g., operator-determined barring. Indicates that the MM and PDP contexts of the MS are deleted from the. Indicates that the MS is not reachable through an, and that the MS is marked as not reachable for GPRS at the and possibly at the. -list The GSN number and optional IP address pair related to the that shall be contacted when activity from the MS is detected and MNRG is set. The GSN number shall be either the number of the or the protocol-converting GSN as described in the subclauses "MAP-based - HLR Signalling" and "GTP and MAP-based HLR Signalling". Picture: GPRS-data of the subscriber in the HLR There will be two more functional elements in the whole network structure: 4.6. Packet Control Unit PCU This unit is responsible for the radio channel management and the access to the GPRS channels, the PDCH s. There will be the possibility of sharing the physical resource, more subscribers are using the same physical channel. One subscriber can get bundled resources to enhance the data rate and there will be a co-existence between circuit switched and packet switched. The allocation of the channels will be done by this unit PCU. In the uplink path, the PCU separates between circuit switched data and packet switched data, it determines the next point, or. Tasks of the PCU are: Access request and channel access control functions Managing the radio channel, control of transmitting power and strategies to avoid congestion Structuring the packet data channel PDCH.

5 GPRS-network structure Page 37 In the specifications are defined 3 different positions where to implement the PCU: Physically related to the Integrated in the BSC Realised internally in the. Preferred is the variant B, integrated in the BSC. Um Abis Gb BSC site GSN site PCU Position A BSC site PCU GSN site Position B BSC site PCU GSN site Position C Key: Circuit-switching function (16 or 64 kbit/s) Gb Packet-switching function Picture: Possible positions of the PCU in the GPRS-network 4.7. Channel Codec Unit As mentioned elsewhere, GPRS uses different Coding schemes to enhance the net data rate and to adapt the composition between data and error protection dynamically. This coding procedure has to be performed in the by this unit. Functions of the are: Channel coding due to the Coding schemes CS-1 until CS-4, Forward Error Correction and Interleaving. Measuring of the radio parameters, signal level RX-LEV and quality RX_QUAL, Adaption of the Timing Advanced Parameter.

6 GPRS-network structure Page GPRS-Interfaces E SMS-G SMS-IW D C HLR Signalling Signalling + Data A Gs Gd Gr Gc MS BSC Um Abis Gb Gn Gi PDN Gn Gp Gf EIR other PLMN Picture: Interfaces in the GPRS-network The introduction of new network elements requires the definition of new interfaces between them. Each of the new GPRS-interface identifiers starts with the capital G. In the following description the distinction is drawn between interfaces, which carry both signalling and user data, and interfaces, which exclusively carry signalling information. Interfaces where both signalling and user data are transferred: Gi is a reference point between the GPRS PLMN and the external packet data network (PDN). It has to adapted to the requirements of the external network by the operator. Gn is the interface between an and or, between an and another within the same PLMN. The hereby used transmitting protocol is the internet protocol. Gb is the interface between the and the BSS. Gp is an interface to interconnect the s of different GPRS operators. The two operators have to reach an agreement about the way, this interface is realised. Gd is an interface between an SMS-G and an, and/or between an SMS-IW and an, to transmit the SMS-Messages via Packet data channels, PDCH.

7 GPRS-network structure Page 39 Interfaces sorely used to transmit signalling information: Gs is the interface between the and the /. Both elements have to carry out the mobility management for the subscriber. Via this interface, some location information can be exchanged between these two nodes and does not have to occupy the air interface. Gr is the interface between the and the HLR. Subscriber related network access information, the needs to perform security aspects, processes and access functions, can be conveyed via this interface Gc is an optional interface between the and the HLR. This enables the to access directly the HLR without occupying an inter-working. Gf is an interface between the and the Equipment identity register, EIR, if this is used in the network Preview at the UMTS-network structure The integration of GPRS in GSM-networks is also regarded as an intermediate step towards the 3. Generation, UMTS, Universal Mobile Telecommunication Services. This results in the fact that the 3. Generation in mobile communication also has to convey packet switched data. But this can be contrived to do by the integration of the GPRS-network elements. The general network structure of UMTS can be divided into the following hierarchical layers: UE (User Equipment), UTRAN (=UMTS Terrestrial Radio Access Network) and CN (=Core Network). Or otherwise the blocks: endpoint at the subscriber side, elements of the radio network and a backbone switching system. CN Iu UTRAN UE Uu Picture: General structure of UMTS-network The elements of the network structure are combined via the air interface, the Uuinterface, and the Iu-interface between the radio network and the backbone or core network.

8 GPRS-network structure Page 40 Due to the integration of GPRS, this backbone or core-network already exists and in the future the transition to the 3 rd generation of mobile network can be realised by substituting the radion network. On the network side, the operator will have to integrate the Radio Network Subsystem RNS and on the subscriber side, the subscriber has to get a new User Equipment, UE. In reality, there will be a co-existence of GSM/GPRS-networks and the 3 rd generation network, tantamount to the step-by-step-integration of new network elements into existing networks. The following picture shows such a UMTS-network structure and its co-existence with the 2 nd generation network structure. For the present, there will be no components replaced, but some new elements are added. We can see the continued use of the GPRS-network elements in the core-network of an UMTS-network. PSTN Gi Gp G PSTN PSTN C AUC H HLR Gc Gn B G E B D F EIR Gs Gf Gr UE / MS A IuCS IuPS IuCS IuPS CN Gb ME BSS BSC RNS RNC Iur RNC RNS BSC BSS SIM-ME i/f or Cu Abis Iubis Iubis Node B Node B Node B Node B Abis SIM USIM Um Uu Um MS = GSM Network components = GPRS Network components = UMTS/UTRAN Network components Picture: Combined network structure for UMTS and GSM/GPRS