Circuit switched network

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1 GPRS-Services Page GPRS-Services GPRS integrates a vast sum of additional services in a GSM-network. For this it will be necessary to define a subscriber profile that corresponds with services the subscriber has negotiated with its operator. This profile comprises the specified data transfer services, their quality of service parameters and their guarantee of service. Furthermore it has to be discussed how security aspects are taken into account and how the billing and charging is done. This chapter discusses the question: What can a subscriber expect from the offer of a GPRS-Service in his GSM network. It examines the network from the point of view of a subscriber Bearer Services of GPRS: Packet oriented data transfer: GPRS uses the principle of packet oriented data transfer. In comparison to circuit switched data transfer, there is no continuous connection established. The data packets are transferred sporadically and each packet is independent from the others. The maximum size of a packet data unit, that the GPRS-network has to transfer, is 1500 octets. Packet switched network Circuit switched network Picture: Circuit switched and packet switched data transfer

2 GPRS-Services Page 13 Point-to-point connection: GPRS establishes a point-to-point connection between transmitter and receiver. We distinguish between the Service requester, the one who requests a service and the Service receiver, the one who gets the service. Offered bearer services are: Retrieval Services: This service allows access to databases and the download of data out of a database. One example for this service is the download of any websites out of the internet, the classical surfen in the World Wide Web WWW. Messaging Services: Transmitting any kind of messages between 2 subscribers, for example transmitting and receiving of s. Or the still known Short Message Service, now transmitted via packet oriented channels. Conversational Services: Establishes a bidirectional communication in realtime. Allows the establishment of a communication. After a further development step this conversational service shall enable voice-over-ip. This means transfer of voice data over packet oriented channels will be possible. GPRS simplified reference model: Mobile Station GPRS-Network PDN TE TE Air- Interface Gi - Interface Picture: GPRS simplified reference model PDN = external Packet Data Network Gi = Interface between GGSN and PDN TE = Terminal Equipment GPRS provides data transfer capabilities between a sending entity and one or more receiving entities. These entities may be an MS or Terminal Equipment, the latter being attached either to a GPRS network or to an external data network. The base station provides radio channel access for MSs to the GPRS network.

3 GPRS-Services Page 14 Point-to-Multipoint data transfer: In further development steps, GPRS enables the Point-to-Multipoint data transfer, e.g. conference calls or group calls. The first introduction of GPRS comprises only point-to-point. For this reason, the following chapters are restricted in describing this kind of service. Within a point-to-point connection it is distinguished between 2 transportation services: Point-to-point Connectionless Network Service PtP-CLNS: Point-To-Point Connectionless Network Service (PTP-CLNS) is a service in which one (or more) single packet(s) is (are) sent from a single service subscriber "A" to a single destination user "B". Each packet is independent of the preceeding and succeeding packet. For that reason, different transportation routes and different delay times over multiple packets are possible. Applications of this service are the so-called bursty applications. The GPRS network has to support the Internet Protocol IP. Connectionless signifies, that there is no peer-to-peer connection between both sides established. All information, necessary for the transport, has to be handed out together with the data to the layer which is carrying out the data transport. One example for the transport service Connectionless is the sending of a letter via the normal post way. Between both partners, there is no connection established, the necessary transport information, i.e. the destination address has to be handed out together with the packet content. Also the sender side does not get an acknowledgement of the delivery of the packet data unit. Point-to-point Connection Orientated Network Service PtP-CONS: Point-To-Point Connection Orientated Network Service (PTP-CONS) is a service in which multiple packets are sent between a single service subscriber "A" and a single destination user "B". This service provides a logical relation between the users, a so called connection orientated virtual circuit which may last between seconds to hours. The service is intended to support bursty transactive or interactive applications. All packet data units are conveyed via the same route. Interactive applications use this transportation service, but also bursty applications. GPRS has to support every application which is based on the OSI Connection Orientated Network Protocol CONP, e.g. the X.25 Protocol. Connection Oriented or Connection-mode transfer requires a peer-to-peerconnection established between both partners. All lower protocol layers have to support the maintenance of this connection and the data transport. Every data transfer runs in 3 phases: 1. Establishment of the link between the 2 sides 2. Data transfer 3. Release of the link between both communication partners Connection mode data transfer implies the 3 characteristics of the transfer service: Acknowledged data transfer, In-order-delivery and No-duplicated data. The air interface supports acknowledged transfer mode for reliable delivery and unacknowledged transfer mode in PtP-CLNS-Mode and PTP-CONS-Mode.

4 GPRS-Services Page 15 Transfer characteristics: The following table out of the Specifications gives an overview of the established transfer characteristics within GPRS: Attributes Supported capabilities Information Transfer Capability Unrestricted digital information Information Connection mode Connection orientated, connectionless Transfer Traffic type (a) Variable bit rate and variable delay Mode Timing end-to-end (b) Asynchronous Information Transfer Rate (c) Maximum bit rate. Values from one TCH inclusive. Information Structure Communication Configuration - Service data unit integrity - Data sequence integrity - Point to point Establishment of Communication (d) Negotiation Symmetry Radio Channel assignment Demand mobile originated or mobile terminated - Out of band - In band - Bi-directional - Unidirectional Shared, multi-user Table: Transfer characteristics of GPRS. From GSM Spec Subscriber Profile: The subscriber profile comprises all the service profile parameters a subscriber has negotiated with its network operator. One parameter could be that he uses Connectionless or Connection Oriented data transfer. It is possible to combine some of these parameters depending on the application. The subscriber profile contains information about the arranged quality of the bearer services, the so called Quality of Service Profile. Before every service request the network checks if the subscriber is authorized for the demanded service. Only with a positive result will the service be provided to the subscriber Quality of Service, QoS: Quality of Service covers all the bearer service parameters that guarantee a satisfactory performance of customer duties. These parameters shall be measured at the access points for the requested service. The assumption is that there is enough radio wave coverage at the considered point. The following scenarios shall be discussed in detail: - Speed, Data rate - Reliability and accuracy - Dependability

5 GPRS-Services Page 16 The QoS within GSM/GPRS refers to normal network operating conditions1. In the GSM.Rec the QoS profile for PtP services consists of following parameters: - service precedence (priority), - reliability, - delay, - user data throughput, and - scheduled repeated transmission - Those parameters can either be negotiated between the GPRS subscriber and the GPRS network operator or set to default values. In detail, the meanings of these parameters are the following: Service precedence The service precedence indicates the relative priority of maintaining the service. The following precedence levels are defined: - High precedence: Service commitments will be maintained ahead of all other precedence levels. - Normal precedence: Service commitments will be maintained ahead of low priority users. - Low precedence: Service commitments will be maintained after the high and normal priority commitments have been fulfilled. What is the meaning of relative priority of maintaining the service? Data packets are conveyed through the network, from one node to the next node. In each node, these packets are buffered or stored in a memory for the time the node determines the next hop. But some congestion can appear. In that case, the data will still be stored in the memory, but new packet data units are arriving at this node. This implies a stack overflow and some packets have to be discarded. The parameter service precedence describes the order, in which, data packets are discarded. The classification high represents a packet class that is discarded at last. 1 exception: the service precedence parameter.

6 GPRS-Services Page 17 Reliability: This parameter represents the reliability profile of the data transfer. The reliability class defines the probability of loss of, duplication of, mis-sequencing of or corruption of Service Data Units. The following table from GSM Spec shows all specified reliability classes. Reliability class Lost SDU probability (a) Duplicate SDU probability Out of Sequence SDU probability Corrupt SDU probability (b) Example of application characteristics Error sensitive, no error correction capability, limited error tolerance capability Error sensitive, limited error correction capability, good error tolerance capability Not error sensitive, error correction capability and/or very good error tolerance capability. Table: Reliability classes from GSM Spec Delay: Each communication requires a time > 0. Within a circuit switched network, the delay time is fixed. Once established, the data transport always takes the same time. But in a packet oriented network, different delay times are possible. Depending on the application, delay times can be tolerated more or less. The delay parameter defines the end-to-end transfer delay incurred in the transmission of SDUs through the GPRS network(s). Measurement points are the access point air-interface at the MS and the Gi-interface at the GGSN, the access point for Packet Data Networks. The time is measured over all switching nodes of the GPRS network, but possible intermediate network are excluded. Resulting time values are the 2 magnitudes: Average delay and 95% delay, i.e. the 95% average of all the delays happened. In GPRS- Specifications, the following delay classes are defined: Delay Class Delay (maximum values) SDU size: 128 octets SDU size: 1024 octets Mean 95 percentile Mean 95 percentile Transfer Delay (sec) Transfer Delay (sec) Delay (sec) Delay (sec) 1. (Predictive) < 0.5 < 1.5 < 2 < 7 2. (Predictive) < 5 < 25 < 15 < (Predictive) < 50 < 250 < 75 < (Best Effort) Unspecified Table: Delay classes of GSM Spec

7 GPRS-Services Page 18 Throughput This parameter presents the guaranted amount of data which can be transfered during a time interval. It is distinguished between 2 parameters: Peak Throughput: It specifies the maximum data rate for an individual PDP context (=application). Is measured between the reference points air interface and Gi-interface and is the amount of data transfered within 1 second. There is no guarantee, how long, such a throughput stays stable and established. In total, there exist 9 different classes. Peak Throughput Class Peak Throughput in octets per second 1 Up to (8 kbit/s). 2 Up to (16 kbit/s). 3 Up to (32 kbit/s). 4 Up to (64 kbit/s). 5 Up to (128 kbit/s). 6 Up to (256 kbit/s). 7 Up to (512 kbit/s). 8 Up to (1 024 kbit/s). 9 Up to (2 048 kbit/s). Table: Peak throughput Spec Mean Throughput: It specifies the average data transmission rate, which a subscriber may expect during the activation of a PDP context. It is measured at the reference points air interface and Gi-interface and contains the magnitude: mean throughput of data in 1 hour. 19 different classes exist. The average is also calculated for these times, where no data transfer happened. We find different levels of throughput, presented in the following table: Mean Throughput Class Mean Throughput in octets per hour (~0.22 bit/s) (~0.44 bit/s) (~1.11 bit/s) (~2.2 bit/s) (~4.4 bit/s) (~11.1 bit/s) (~22 bit/s) (~44 bit/s) (~111 bit/s) (~0.22 kbit/s) (~0.44 kbit/s) (~1.11 kbit/s) (~2.2 kbit/s) (~4.4 kbit/s) (~11.1 kbit/s) (~22 kbit/s) (~44 kbit/s) (~111 kbit/s). 31 Best effort. Table: Mean throughput per hour from Spec

8 GPRS-Services Page Security aspects: As we know from GSM-Networks, also in GPRS-networks, security aspects are of overriding importance. The following aspects are considered: MS Authentication: The confirmation of each MS by the land-based part of the system that the subscriber identity, transferred by the MS within the identification procedure on the radio path, is the one claimed. The purpose of this authentication is to protect the network against unauthorized use. It also enables the protection of GPRS subscribers by denying intruders the ability to impersonate authorized users Access Control: The network can support restrictions on access by or to different GPRS subscribers, such as restrictions by location, screening lists, and so on User identity confidentiality: The property that the user identity on the radio link is not made available or disclosed to unauthorized individuals, entities or processes. The purpose is to provide privacy of identities of the subscribers who are using GPRS radio resources. It allows for the improvement of other security features, e.g., user information confidentiality, and also provides for the protection against tracing the location of a mobile subscriber by listening to the signalling exchanges on the radio path. This is realized by the allocation of a temporary valid identity to the subscriber. The Temporary Mobile Subscriber Identity TMSI becomes a derivative in GPRS, the Packet Temporary Mobile Subscriber Identity, the so called P-TMSI. User information confidentiality: The property that the user information is not made available or disclosed to unauthorized individuals, entities or processes. The process of ciphering stays the same in comparison to GSM, only the point of ciphering at the network side has changed. In GPRS-network, the responsable part for ciphering is the SGSN.

9 GPRS-Services Page Addressing of subscriber Every subscriber has to be identified by an international valid identifier, the well known IMSI. Furthermore a subscriber can have a temporary or permanent address, valid in external packet data network to guarantee his availability in these networks. This could be for example a X.121 address or X.25 CONS adress or IP-adress Inter-Network-Operations GPRS shall provide means to interwork with external data networks. The GPRS operator may provide an appropriate address to the external data network for the subscriber as part of the GPRS subscription. That address can be either dynamic (e.g. the user's IP address is allocated from a pool of unused IP addresses every time the subscriber activates the access to an IP network) or static (e.g. a certain IP address is permanently allocated to a particular subscriber). When connected with some external data networks, the routing protocols of these networks may limit the data network addresses that can be allocated. For example, when interworking with IP networks, the IP address for the GPRS subscriber shall belong to that GPRS operator's IP subnetwork that allocates the address. In the case of a simple point to point connection, a GPRS subscriber doesn t need an associated network address Roaming: The worldwide distribution of GSM-networks and the provided worldwide utilisation of GSM-Services will be continued within GPRS-networks. Between GPRS-network operators, contracts or enhanced existing contracts of roaming will be concluded to guarantee the subscriber the services also in Visitor-PLMN s.