LAPDm in

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

is a

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

protocol used in

GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...

cellular networks. LAPDm forms Layer 2 of the

Um interface The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station (MS) and the Base transceiver station (BTS). It is called Um because it is the mobile analog to the U interface of ISDN. ...

between the

Base Transceiver Station A base transceiver station (BTS) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phones (handsets), WLL phones, computers with wireless Internet connectivity, o ...

and

Mobile station A mobile station (MS) comprises all user equipment and computer software, software needed for communication with a Cellular network, mobile network. The term refers to the global system connected to the mobile network, i.e. a mobile phone or mob ...

, which is to say that it is used in the radio link between the cellular network and the subscriber handset. LAPDm is derived from a much older link layer protocol called

HDLC High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...

and specified in 3GPP specifications TS 04.05 and TS 04.06. LAPDm is similar to the

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wo ...

Layer 2, LAPD, but with these simplifications: * LAPDm frames are always 184 bits, with segmentation for larger messages. * LAPDm allows no more than one outstanding unacknowledged I-frame (GSM 04.06 Sections 5.8.4 and 6). * LAPDm does not support extended header formats (GSM 04.06 Section 3). * LAPDm supports only the SABM, DISC, DM, UI and UA U-Frames (GSM 04.06 Sections 3.4, 3.8.1). * LAPDm supports the RR and REJ S-Frames (GSM 04.06 3.4, 3.8.1), but not RNR (GSM 04.06 Sections 3.8.7 and 6). * LAPDm has just one internal timer, T200 (GSM 04.06 5.8). * LAPDm supports only one terminal endpoint, whose TEI is implied. * The BTS is always able to enter asynchronous balanced mode when requested. * LAPDm can never be in a receiver-not-ready condition (GSM 04.06 Section 3.8.7). * LAPDm supports only two SAPs: SAP3 for

SMS Short Message/Messaging Service, commonly abbreviated as SMS, is a text messaging service component of most telephone, Internet and mobile device systems. It uses standardized communication protocols that let mobile devices exchange short text ...

and SAP0 for everything else (GSM 04.06 Sections 3.3.3 and 6). * In SAP0, asynchronous balanced mode is always initiated by the MS (GSM 04.06 Sections 5.4.1.1 and 6). Another important difference between LAPDm and LAPD is the establishment contention resolution procedure of GSM 04.06 Section 5.4.1.4, wherein the MS sends an L3 message in the information field of the SABM frame which is then echoed back by the BTS in the corresponding UA frame. This procedure is required in LAPDm because of the possibility of a handset accidentally attempting to use the wrong channel. LAPD does not require contention resolution, since hard-wired ISDN devices cannot accidentally use a wrong channel. The Associated Control Procedure used in Layer 2 of the

iDEN Integrated Digital Enhanced Network (iDEN) is a mobile telecommunications technology, developed by Motorola, which provides its users the benefits of a trunked radio and a cellular telephone. It was called the first mobile social network by ma ...

RF interface is very similar to LAPDm.

References

External links

*http://www.3gpp.org/ftp/Specs/archive/04_series/04.06/0406-840.zip *http://www.3gpp.org/ftp/Specs/archive/04_series/04.05/0405-802.zip {{Portal bar, Electronics, Telecommunications, Technology, Science Link access protocols Integrated Services Digital Network