SMS:PDU Format,DCS,UDH,UDHI

The SMS message, as specified by the Etsi organization (documents GSM 03.40 and GSM 03.38), can be up to 160 characters long, where each character is 7 bits according to the 7-bit default alphabet. Eight-bit messages (max 140 characters) are usually not viewable by the phones as text messages; instead they are used for data in e.g. smart messaging (images and ringing tones) and OTA provisioning of WAP settings. 16-bit messages (max 70 characters) are used for Unicode (UCS2) text messages, viewable by most phones. A 16-bit text message of class 0 will on some phones appear as a Flash SMS (aka blinking SMS or alert SMS).

User Data header was added to the SMS format specification to add new features. Originally SMS was made to send single small binary files or text messages, with a maximum of 140 bytes or 160 7-bit characters. But now mobile devices need to distinguish between different files, for example ringtones, operator logos and wap-push messages. Also users want to send larger messages which was impossible with the original SMS format specification.

Each short message has a flag to indicate if the message part includes a User Data Header or not. If this flag is set to 1 (or true), then the first few bytes of the message are the User Data Header, followed by the message text or data. A very general use of UDH is for plain concatinated messages, where UDHI is set.

What is SS7?

Common Channel Signalling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). Signalling System 7 was introduced by AT andT in 1975 and approved by worldwide standards bodies in 1980. The standards define the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signalling network to effect wireless (cellular) and wireline call setup, routing and control as well as network management and maintainence. SS7 uses out-of-band signalling i.e. it uses a separate digital channel for the exchange of signalling information

The SS7 network and protocol can be used for:

>Basic call setup, management, and tear down
>Wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
>Local number portability (LNP)
>Toll-free (800/888) and toll (900) wireline services
>Enhanced call features such as call forwarding, calling party name/number display, and three-way calling
>Efficient and secure worldwide telecommunications

SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bi-directional channels called signalling links. Signalling occurs out-of-band on dedicated channels rather than in-band on voice channels.

The networks consist of three types of network elements:

SSP: Signal Switching Points are telephone exchanges equipped with SS7 capable software. They are responsible for originating, terminating or switching calls. SSP can be an origination or destination point for signalling messages. Exp. MSC etc.

STP: Signal Transfer Points are packet switches that receive and route signalling messages to their proper destination. Exp. STP, SSCP etc.

SCP: Signalling Control Point is a database that provides information necessary for advanced call-processing capabilities. Exp. IN, HLR

Layers of GSM

In a GSM network, there are three functional areas:
Ø The Mobile Station such as a mobile phone which is carried by a subscriber,
Ø the Base Station Subsystem which controls the radio link with the Mobile Station,
Ø and the Network Subsystem which mainly performs the switching of calls between the mobile users, and between mobile and fixed network users.
The signalling protocol in GSM is structured into three general layers as shown in the following figure.
v Layer 1 (the Physical Layer) uses the data channel structures over the air interface.
v Layer 2 (the Data Link Layer) is a modified version of the LAPD protocol used in ISDN, called LAPDm. Across the A interface(bw BSS and MSC), the Message Transfer Part layer 2 (MTP2) of SS7 is used.
v Layer 3 is the GSM Signalling Protocol, which is divided into 3 sub-layers:
ü Radio Resources Management to control the setup, maintenance, and termination of radio and fixed channels, including handovers;
ü Mobility Management to manage the location updating and registration procedures, as well as security and authentication; and
ü Connection Management to handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.

Signalling between the different entities in the fixed part of the network, such as between the HLR and VLR, uses the Mobile Application Part (MAP), one of protocols in the SS7/C7 stack.

What is GSM?

The Global System for Mobile communications is a digital cellular communications system. GSM standards are used to provide the interface between the various entities in GSM network.

GSM networks presently operate in three different frequency ranges,
GSM 900 (operates in the 900 MHz frequency range and is the most common in Europe and rest of the world), GSM 1800 or DCS 1800 (operates in the 1800 MHz frequency range and is found in a rapidly-increasing number of countries including India, France, Germany, Switzerland, the UK, and Russia) PCS 1900 or DCS 1900 - the only frequency used in the United States and Canada for GSM).

Between these only differences, so far seem to be the power levels, frequency & some small changes in signaling.

My Photo