This page contains the answers to the questionnaire in chapter 2 (GPRS)
All answers have been held as short as possible and require an
understanding and study of the corresponding chapter of the book.
Chapter 2, GPRS:
When data is transferred over a circuit switched channel, a dedicated connection is established between two parties. Data is sent without any overhead like lower level addressing. Bandwidth and delay are constant. In a packet switched network on the other hand, there is no direct connection between the endpoints of a session. Resources in the network are only used for the connection when data is sent. Data is sent in packets which have to contain a source and destination address in order to be transported through the network. This also enables N:N connections in the network, i.e. a subscriber can communicate with any subscriber without establishing a physical connection first. Depending on the load of the network, bandwidth and delay for a connection can vary. This is a clear disadvantage compared to a circuit switched channel. Due to the bursty nature of many information exchanges the advantage of the packet switched approach on the other hand is to use more bandwidth during the burst which decreases transmission time.
As GPRS is a packet switched network, resources or the air interface are only assigned to a user when data is actually sent. This tremendously increases the capacity of the network especially for applications such as web surfing which only send and receive data at irregular intervals. Several timeslots can be assigned to a subscriber simultaneously to increase throughput. If the physical connection to the network is lost (e.g. due to bad reception quality) the logical connection persists. As soon as the physical connection has been reestablished, data transfer on higher layer resumes without the user having to reestablish another channel manually as would be the case for a circuit switched connection.
Dynamic coding schemes allow to adapt the ratio of error correction and detection bits to user data bits. For good transmission conditions the redundancy information in a block can be reduced which in turn increases the overall transmission speed of the user data. During times of bad reception, more error detection and correction bits are inserted which ensures that the link remains stable.
While in GPRS ready state the SGSN can send data to the mobile terminal without delay. In this state, the SGSN is aware of the cell which the subscriber uses to communicate and thus can forward incoming packets directly to the PCU responsible for this cell. The PCU does not need to page the subscriber and can immediately assign resources on the air interface. When changing the cell in ready state the mobile station has to send a cell update message to the SGSN. Once the mobile station is in GPRS standby state, the SGSN is only aware of the location are of the subscriber, as the mobile station only has to report cell changes when a location area boundary is crossed. This reduces the mobiles energy consumption. In order to send data frames to a mobile in standby state, the SGSN has to page the subscriber first. The mobile station responds with an empty frame and thus implicitly changes into the ready state again. Once the SGSN receives the empty frame it is aware again of the cell the mobile station uses and can then forward the frame.
In practice, no handovers are performed for GPRS today (Network Control Order = 0). The mobile station has to perform cell changes on its own. In case a cell change has to be performed during an ongoing data transfer due to deteriorating reception conditions it is necessary to interrupt the transmission and perform the cell change. Afterwards the mobile station reports to the SGSN from the new cell by continuing to send data. The SGSN detects the cell change as the cell global ID is part of every incoming frame and can thus change its routing of incoming Internet packets to the new cell.
GPRS requires the following network nodes: A) The serving GPRS support node (SGSN) which is responsible for mobility management and session management (GMM/SM). B) The gateway GPRS support node (GGSN) which is the interface between the GPRS network and the Internet. The GGSN is responsible for the assignment of IP addresses to the mobile subscribers and hides subscriber mobility from the Internet. C) The packet control unit (PCU) is the interface between the GPRS core network and the radio network. The PCU is responsible for packet scheduling, assignment of timeslots to the subscribers and terminates the RLC/MAC protocol.
GPRS assigns resources (timeslots) to a subscriber only for the time required to send the data. Furthermore, timeslots are not exclusively assigned to a single subscriber but only in blocks of four bursts. This way, timeslots can be used to transfer data to several subscribers at the same time. The temporary block flow with the temporary block identifier describes which data blocks are addressed to which device currently listening on a timeslot.
An Inter-SGSN routing area update is performed if the mobile device roams into a cell which is connected to a new SGSN. As the new cell belongs to a new routing area, the mobile device attempts a routing area update. The new SGSN then detects that the mobile device is currently registered with a different SGSN and thus sends a message to the previous SGSN to retrieve authentication information. After authenticating the mobile station, the HLR is informed that the subscriber has changed its location to the new SGSN. Furthermore, the GGSN is informed of the position change so it can forward incoming packets to the new SGSN in the future. Once all of these actions are performed, the routing area update in the core network is complete and the subscriber gets a confirmation from the SGSN the operation was performed successfully.
The GPRS core network between the SGSN and GGSN use the IP protocol for routing the IP data frames of subscribers. These are not transferred directly, however, but are encapsulated into GPRS tunneling protocol (GTP) frames. Part of the encapsulated frame is the IP address of the mobile device and the source/destination of the frame. Thus, a GTP frame contains two source and two destination IP addresses. This mechanism has the advantage that no routing table updates are required in routers between these two network components if the user is roaming into the area of another SGSN. In addition, the GPRS core network is decoupled from the Internet and the GPRS user as it is not possible to directly access these components from outside the local GPRS core network.
The user does not have to change any settings on his/her device for international roaming. All packets that are sent and received are always routed through the GGSN in the subscriber’s home network. This is possible as the access point name (APN) is a qualified domain name and the SGSN inserts the mobile country code (MCC) and the mobile network code (MNC) as well as a top level domain (‘.gprs’) to the APN string received by the subscriber during the connection establishment. This domain name is then sent to a DNS server which resolves the domain name into the IP address of the GGSN in the subscriber’s home network.
During a GPRS attach, the mobile device registers with the network. Afterwards, the network is aware that the device has been switched on and in which routing area it is located. Up to this point no IP address has been assigned to the mobile device and no data can be transmitted. The IP address is only assigned to a mobile device during the PDP context activation procedure. Billing is also only invoked during the activation of a PDP context.
In order to transfer data via GPRS to and from the Internet, a PDP context has to be established between the mobile device and the GPRS network. During the establishment of a PDP context the mobile device sends the access point name which identifies the GGSN and profile to be used to establish a connection to the internet. (Also see answer 10)
MMS messages are always exchanged between a mobile device and the MMS gateway which is located behind the GGSN. For sending an MMS the mobile device first establishes a GPRS connection and uses the APN which the network operator has foreseen for the MMS service. Usually it is only possible to reach the MMS gateway via this APN. Afterwards the MMS, which has many similarities to an eMail, is sent by using an HTTP-PUT push command. This command is also used by web browsers to send the input the user has made on a web page in text fields, etc. to the web server. Once the MMS is received by the MMS gateway it is stored and attempt is made to deliver the message to the destination. If the destination is a mobile subscriber, an SMS is sent to inform the mobile device of the waiting MMS message. Depending on the configuration of the mobile device it either establishes a GPRS connection immediately or queries the user first before doing so. To receive the MMS message the mobile device uses the HTTP-GET command. This command is also used by web browsers to request web pages from a web server.
An MMS message has many similarities to an eMail. The header for example is structured in a similar way as an eMail header and just contains additional X-MMS-tags which contain MMS specific information. Text and pictures are sent in the “body” of the MMS and are separated by Multipurpose Internet Mail Extension (MIME) separators. The first part of an MMS body is the description of the general layout of the message. SMIL, an XML language, is used for this purpose. Further MIME parts of the MMS then contain the text, pictures, videos, etc.