LAPDm
Encyclopedia
LAPDm is a data link layer
protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface
between the Base Transceiver Station
and Mobile station
, 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 and specified in 3GPP
specifications TS 04.05 and 04.06. LAPDm is similar to the ISDN Layer 2, LAPD
, but with these simplifications:
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
RF interface is very similar to LAPDm.
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface
Um Interface
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station and the Base transceiver station . It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of...
between the Base Transceiver Station
Base Transceiver Station
A base transceiver station or cell site is a piece of equipment that facilitates wireless communication between user equipment and a network. UEs are devices like mobile phones , WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc...
and Mobile station
Mobile Station
The mobile station comprises all user equipment and software needed for communication with a mobile network.The mobile station refers to global system connected to the mobile network, i.e. mobile phone or mobile computer connected using a mobile broadband adapter. This is the terminology of 2G...
, 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 and specified in 3GPP
3GPP
The 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
specifications TS 04.05 and 04.06. LAPDm is similar to the ISDN Layer 2, LAPD
Link Access Procedures, D channel
Link Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
, 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 recevier-not-ready condition (GSM 04.06 Section 3.8.7).
- LAPDm supports only two SAPsService Access PointA Service Access Point is an identifying label for network endpoints used in Open Systems Interconnection networking.When using the OSI Network Layer , the base for constructing an address for a network element is an NSAP address, similar in concept to an IP address...
: SAP3 for SMSSMSSMS is a form of text messaging communication on phones and mobile phones. The terms SMS or sms may also refer to:- Computer hardware :...
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
Iden
For other uses of the word, see Iden The Iden was an English automobile manufactured from 1904 until 1907. Designed by George Iden, formerly of Daimler, they were four-cylinder 10/17 hp and 25/35 hp shaft-driven cars; each came with "Idens's frictionless radial gearbox"....
RF interface is very similar to LAPDm.
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