Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 97

One important aspect of a mobile communications network is the “signaling message” that is exchanged between its network elements. A signaling message is nothing but an exchange of a series of information between the network elements to establish, maintain, and release of resources allocated for communication services being provided to the service users. Information in a particular signaling message being transmitted is encoded (packed) and decoded (unpacked) differently across the mobile communications systems, i.e. from the GSM to 5G. Depending on the protocol stack and its layers supported by a network element, it may use different methods of encoding and decoding of signaling messages at each protocol layer. Also, a network element may decode the contents of a message that it receives or may not decode but transparently forward to the destination network element using another encoding method.

A protocol layer of a network element may send a signaling message to its peer layer like a long series of ordered bits where an octet alignment may or may not be required. Another protocol layer of the same network element may send a signaling message as a series of octets with octet alignments. To sum up, as far as the development of a protocol layer is concerned, two aspects of signaling messages defined in its technical specifications are required to be considered:

 The method used to describe a signaling message

 The method used for encoding and decoding of a signaling message to transfer/receive among network elements.

It may be noted that the method of description, i.e. tabular format, and encoding/decoding of signaling messages differs from layer to layer. In the subsequent sections that follow, the following methods of descriptions, encoding, and decoding of mobile communications networks signaling messages over their respective air interfaces are discussed.

1 Encoding and Decoding of Air Interface Layer 3 Messages

This method is used by the GSM air interface Radio Resource (RR) sublayer of Layer 3 protocol between an MS and the base station controller (BSC). This method is also used by the Call Control (CC) and Mobility Management (MM) sublayers of GSM; GPRS Mobility Management (GMM), Session Management (SM) layers of Universal Mobile Telecommunication System (UMTS); Evolved Packet System Session Management (ESM) and Evolved Packet System Mobility Management (EMM) layers of Long‐Term Evolution (LTE)/Evolved Packet System (EPS) system; and 5GMM and 5GSM layers of 5G system. These protocol layers work between an MS/UE and the CN.

1 Concrete Syntax Notation.1 (CSN.1) Encoding/Decoding

This method is used by the General Packet Radio Service (GPRS) air interface Layer 2 radio link control (RLC)/Medium Access Control (MAC) protocol between the MS and BSC.

1 Abstract Syntax Notation.1 (ASN.1) Encoding/Decoding Using Packed Encoding Rule (PER)

This method is used by the following protocol layers over their respective logical interfaces:

 UMTS Radio Resource Control (RRC) air interface Layer 3 between the UE and UMTS Terrestrial Radio Access Network (UTRAN)/Radio Network Controller (RNC),

 LTE RRC air interface Layer 3 between the UE and Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN)/eNodeB, and

 5G New Radio (NR) RRC air interface Layer 3 between the UE and Next Generation Radio Access Network (NG‐RAN)/5G Base Station (gNB).