4.5 Data Services on SignalingBroadcast ChannelsSMS


4.5 Data Services on Signaling/Broadcast ChannelsSMS

The GSM technology is the first in providing data services on signaling and broadcast channels. These services are commonly known as SMS. There are two different types of SMS: SMS point-to-point and SMS broadcast. We will look into both of these services in this section. Data services like SMS are also provided by other radio technologies, such as IS-136 and IS-95.

SMS point-to-point is the data service that uses only the signaling channel (SDCCH) to transport the data over the air interface. In SMS, a short string of text (maximum 126 characters ) is carried from one subscriber to another. There is another type of SMS service called SMS broadcast, which is the only broadcast channel data service. As the name implies, the user terminal can only receive the data broadcasted from the network. This service transports data on a specially defined broadcast channel, CBCH, over the air interface. This service is also limited to a short text string based on the carrying capacity of the CBCH. This service was anticipated to be used by the broadcast data applications, such as traffic reports and weather alerts. This service didn't get much attention from service providers because it didn't provide a good revenue generation model.

4.5.1 SMS Point-to-Point

SMS point-to-point (SMS p-p) is a dedicated service between two users. GSM has defined two teleservices for SMS: SMS-MO (mobile origination), and SMS-MT (mobile termination). Using this service, a user sends a short string of alphanumeric characters to another user. SMS can also be used by the network operators to notify a user about certain status (e.g., number of messages waiting in voice mail). SMS is based on the concept that the signaling channel capacity can be utilized for carrying a few bytes of user data. SMS has also opened up GSM PLMN for support of telematics . For example, with a reduced GSM user equipment, a vending machine can become a short message sending entity to send a message to the vendor when a specific supply is needed.

SMS is defined with a store-and-forward mechanism. In this way the message is saved in case the addressee is not reachable and is later delivered based on availability. A subscriber must subscribe to this service, and it is provisioned in the subscriber's profile in HLR.

NETWORK DESCRIPTION

To understand how the SMS works, let's look into the network elements involved in SMS p-p (Figure 4-17).

Figure 4-17. Network architecture for SMS p-p.

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An SMS message sending/receiving entity is called short message entity (SME). The SME can be a GSM-MS or a computer. SMS has introduced a new NE short message service center (SM-SC). The SM-SC handles all the functions related to SMS p-p. It receives a message and forwards it to the addressee.

The SME is connected to the SM-SC via the short message transport protocol (SM-TP). The SM-SC allows connection to a computer, which makes SMS possible from e- mails and Web pages. When SME is a GSM-MS, it uses the short message relay protocol (SM-RP) and short message control protocol (SM-CP) to deliver short messages to the MSC/VLR.

The SMS-GMSC is the gateway to a GSM PLMN for SM delivery addressed in the PLMN. The SMS-GMSC interrogates the HLR to locate the subscriber. The interrogation protocol is a generic protocol used also by the GSM call control procedures for locating a called party. The HLR informs about the current location in terms of MSC/VLR, which is used by the SMS-GMSC for eventual delivery of the SM to the MS. In case the MS is not reachable, the HLR sends a response back to the SMS-GMSC.

The SMS-GMSC can be connected to the external servers or terminals to provide SMS application services, such as e-mail, stock quotes, and highway toll collection. These services can be managed with the help of a database management system. Many of the operators provide Web interface to send SMSs to the mobile subscribers.

The SMS-IWMSC is the MSC connected to the SM-SC. It directs the SM originated by an MS in a GSM PLMN to the SM-SC. The SMS-IWMSC functionality might exist together with the SMS-GMSC on one physical MSC.

PROCEDURE DESCRIPTION

The SMS transaction basically consists of two parts. An SME originates a SM, which is delivered to SC. The SC originates a session to deliver the SM to the addressee. In the following procedure description, we consider a generic case of an MS to MS short message delivery, encompassing both SMS-MO and SMS-MT parts :

  1. MS originates a short message, with the SM-SC directory number, and uses SM-CP and SM-RP protocols to transfer the message to the MSC/VLR.

  2. The MSC/VLR looks at the SM-SC directory number and forwards the SM to the SMS-IWMSC, which in turn contacts the SM-SC and delivers the SM.

  3. The SM-SC, based on the terminating party address, contacts the SMS-GMSC. It notifies the SMS-GMSC that there is a short message waiting for an MS in the addressee's network.

  4. The SMS-GMSC asks the HLR about the current location of the addressee in terms of visited MSC/VLR.

  5. If the addressed MS is reachable, the HLR sends the visited MSC/VLR information to the SMS-GMSC. The SMS-GMSC passes the SM on to the visited MSC.

  6. If the addressed MS is not available, the HLR sends that status back to the SM-SC. It also sets a flag to save the information that the attempted delivery was not successful. When the addressed MS becomes reachable, the HLR notifies the MS for the attempted delivery. The MS may contact the SM-SC to retrieve the waiting short messages.

  7. The MSC/VLR pages the MS through the BTSs in its registered location area. The currently visited BTS establishes a radio channel and delivers the message to the addressed mobile.

4.5.2 SMS Cell Broadcast

SMS cell broadcast provides a mechanism to broadcast short messages from a radio network to the MSs in a cell area (Figure 4-18). The sources of SMSCB can come from broadcast applications, such as traffic reports and weather reports. The message is limited by the capacity of the broadcast channel that carries it. A single CB message can carry up to 88 bytes. The service broadcasts on a cell level because the SMS-CB uses a specially defined channel called CBCH. The presence of a CBCH is indicated by the system information messages broadcasted for an individual cell on the BCCH. The system information tells the MSs camped on a cell on which frequency and channel CB message are sent.

Figure 4-18. Network architecture for SMS-CB.

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A CB message has a message header and a payload. The CB message header has an identifier, which identifies the source and subject of the SMSCB message. It also has a sequence number, which enables the MS to determine when a new message from a given source is available. SMS-CB messages are not acknowledged by the MS. Reception of SMS-CB messages by the MS is only possible in idle mode.

The SMS cell broadcast service is designed to minimize the battery usage requirements for an MS. An MS can read the first part of a CB message and then decide whether or not to read the rest of the message. In addition, the network may broadcast schedule messages, which provide information in advance about the CB messages that will be sent immediately afterward. The MS may use this scheduling information to restrict reception to those messages the customer is interested in receiving.

The CB short messages are generated in the cell broadcast entity (CBE). The functionality of a CBE is not specified in the GSM standards. The CBE can be understood as a source of SMS-CB, such as a weather information center. It includes all aspects of formatting the CB messages as well as splitting a message into various segments, which will eventually be transmitted on one channel.

The CBC actually handles all the GSM related functions of SMS-CB. It may be getting input from multiple CBEs and could be connected to one or multiple BSCs. The CBC coordinates the formatting and organization of the messages it receives from the CBE into GSM form. It performs functions such as determining the rate at which certain messages must be delivered; setting the language; and determining area where a certain message is to be sent.

The BSS takes care of the radio part of transmitting CB messages. The BSC performs functions such as storing the messages as long as they are to be transmitted; routing the messages to the appropriate BTSs; and scheduling of the CB messages according to the repetition rate. BTS puts the message on the CBCH at the time specified by the BSC.



IP in Wireless Networks
IP in Wireless Networks
ISBN: 0130666483
EAN: 2147483647
Year: 2003
Pages: 164

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