LAPD vs LAPDm(Technical)
Layer 2 Functionalities
Framing
Error detection & Correction
Multiplexing
Flow Controller
Framing Issues
LAPD uses HDLC frame format for framing.
Where as LAPDm uses synchronization scheme of radio interface to convey information on frame limits. LAPDm avoids using flags by ready-made blocks of the physical layer. LAPDm(LAPD modified) frame has a maximum length of 23 octets on all TCH channels and 21 octets on SACCH blocks(remaining 2 octets are for Timing Advance and Transmission Power Control). But the actual frame length is less than 23/21 octets, so Length Indicator(LI) is included in each frame to identify the length of the frame.
Segmentation & Reassembly
There is no need of segmentation & Reassembly in LAPD(the length of the Abis LAPD frame is 264 octets excluding flags, which corresponds to 260 octets of upper layer information).
Where as LAPD requires segmentation& reassembly because maximum frame length of 21 or 23 octets is not sufficient for most signalling needs.
Error Detection & Correction
LAPD and MTP2 uses 16 redundancy(FCS – frame check sequence) bits to each frame for errpr detection. For LAPDm, error detection is done by coding scheme of the physical layer.
LAPDm frame,in particular lacks TEI, FCS and flags at both ends.
The objective of LAPD is to provide a secure, error-free connection between two end-points so as to reliably transport Layer 3 messages. LAPD protocol provides framing, sequence control, error detection, and recovery of multiple logical data links on the same D channel. The Layer 2 operation starts with a Layer 3 request to establish a call, which prompts Layer 2 to demand Layer 1 to initiate the start-up procedure. Afterwards Layer 2 initializes itself through the SABME ( Set Asynchronous Balanced Mode Extended) procedure. The SABME is conducted between two peer processes by means of unnumbered frames. Initialization guarantees that the correct sequence numbers are used in processes on both ends. Layer 2 protocol maintain the correct sequence of information due to many different types of error. Firstly, if frames lost more than three consecutive times, the re-establishment of the LAP will be done under the assumption that the connection has failed. Secondly, a timer mechanism and preset values of time-out intervals detect lost frames in the transmit direction. Thirdly, frames received out of sequence denote a lost frame in the receive direction. Finally, frame Check Sequence(FCS) value control and discard frames corrupted by errors due to noise.
The opening flag of a frame is followed in octet 2 and 3 by an address field. In order to have each device support multiple logical data links, the data link address is divided into the Service Access Point Identifier (SAPI) and the Terminal End point Identifier (TEI). These two fields are together called Data Link Control Identifier (DLCI). This provides a form of multiplexing. The six bits allocated to the SAPI allow the specification of up to 64 distinct service access point. Table 1. is representing the meaning according to the different SAPI values. The SAPI identifies the network entity for which the information in the LAPD frame is intended. A LAPD entity may have more than a single Layer 3 entity above it and therefore will have more than one SAPI in use (Figure 5).
The TEI is associated with the user side of the user-to network interface. The TEI identifies the logical terminal or final destination for the Layer 3 information. One or more TEIs can be used for point to point data transfer. The TEI for broadcast connection occurs when a message is transmitted with the TEI set to 127. Broadcast TEI will be transmitted to all the logical terminals on the Layer 3 entity (Figure 6). The TEI is either assigned by the TEI management entity automatically or by the user. In either case no two TEIs can be the same. Table 2 is showing the TEI values.
The remainder of the address field consists of a single bit that identifies the frame as a Command or Response (C/R) frame. The TE will set the C/R bit to one for responses and reset it to zero for commands. The network side does the opposite, setting the bit for commands and responses. Bit 1 in octet 2 set to 0 (EA0) means that another address is to be transmitted, and bit 1 in octet 3 set to 1 (EA1) indicates that the associated address is the last one in the transmission.
Control Field
The control field determines the type of LAPD frame being transmitted as well as containing the sequence numbers for the acknowledged information transfer service. The structure of the control field is shown in Figure 7 and it occupies either octet 4 or octets 4 and 5 depending on the type of frame. There are three different formats for the control field, I-format, S-format, and U-format frame. The numbered information transfer format (I-format) frames are used to transfer information between network layer entities. I-frame frame has a send sequence number, N(S), a receive sequence number, N(R), and a poll bit. The supervisory format (S-format) frame has N(R) field in the control field but not N(S) field. The N(S) and N(R) counters occupy seven bits in the control field. It gives the range 0 to 127. The single bit identified as P or F in the control field format is refereed to as the poll bit in command frames and the final bit in response frames. P is set to 1 by a data link layer entity to elicit or poll a response frame from its peer entity on the other side of the connection. F must be set to 1 in the response frame resulting from the poll command frame. In the I- format frames, there is not F bit, so I-format frame never used as a response to command. The unnumbered format (U-format) frame provides additional functionality and is used for information transfer during unacknowledged information transfer. Modifier bits (M) in the U-format frame are used to define seven different frames. The purpose of U-format frames are establishing, maintaining, and disconnecting a data link connection between peer data link layer entities. Table 3 is listing the command, response, and different LAPD control field types.
LAPd Frame Types
I-Frame(Information): Used to send the LAPD field, which consists of layer 3 message or user data traffic.
Receive Ready(RR): Indicates station is ready to receive traffic and or acknowledge previously received frames by using the N(R) field.
Receive Not Ready(RNR): Indicates the transmitting station that the receiving station is unwilling to accept additional incoming data. The RNR frame may acknowledge the preiviously transmitted frames by using the N(R) field.
Reject(REJ): Requests retransmission of frames starting with frame numbered N(R).
Unnumbered Information(UI): Allows for transmission of user data in an unnumbered frame(i.e unsequenced).
Disconnect(DISC): Places the station in disconnected mode.(It terminates multiple frame operation)
Disconnect Mode(DM): Transmitted to indicate a station is in the disconnected mode(not operational)
Set Asynchronous Balanced Mode Extended(SABM): Sets SABM with two octets in the control field for extended sequencing.
Reset(RESET): Used for reinitialization. Previously unacknowledged frames remain unacknowledged.
Unnumbered Acknowledgement(UA): Acknowledges frames that don not contain an I field, for example SABME frame.
Frame Reject(FRMR): Rejects a frame that passed CRC check, but it is not coded correctly.
Exchange Identification(XID): Used to exchange an I-field without the use of sequence numbers.
Channels:
1.Control Channels(BCCH, CCCH, DCCH)
2.Traffic Channels(TCH/F, TCH/H)
BCCH(Broadcast Control Channel):
This downlink channel contains specific parameters needed by a mobile in order that it can identify the network and gain access to it. Typical information includes the LAC (Location Area Code) and RAC (Routing Area Code), the MNC (Mobile Network Code) and BA (BCCH Allocation) list.
The Location Area Code uniquely identifies a LA (Location Area) within a PLMN (Public Land Mobile Network). It may range from 0 to 65,535.
Routing Area Code is a fixed length of 1 octet and identifies a routing area within a location area. The RAC is part of the RAI (Routing Area Identity).
The Mobile Network Code is either a two or three digit number used to uniquely identify a given network from within a specified country (MCC (Mobile Country Code)
BA: This is a list of frequencies supported on the neighbouring cells. This parameter is broadcast on the BCCH (Broadcast Control Channel) giving mobiles the frequencies of the BCCH carriers on neighbouring cells. It is typically used by the MS (Mobile Station) in the cell selection and re-selection processes.
CCCH(Common Control Channel)
The Common Control Channel supports common procedures required to establish a dedicated link with the network. These Channels include the RACH (Random Access Channel), FACH (Forward Access Channel) and PCH (Paging Channel).
RACH : The channel used for initial access into a system. The RACH function is different depending on the technology of the system. The term RACH applies to cdmaOne™, GSM, UMTS, TETRA, CDMA2000® and other telecommunication systems.
Random Access Channel (RACH) is an uplink channel (mobile to base) used by a MS (Mobile Station) when it initially attempts to request access to the network. The MS uses the RACH to request a SDCCH (Stand-Alone Dedicated Control Channel), in response to being paged or trying to make a call.
FACH: A UMTS transport channel that forms the downlink half of a transport channel pair known as the RACH (Random Access Channel) / FACH (Forward Access Channel) combination. It is used for downlink signalling and small quantities of data.
PCH: The Paging Channel is used to alert a mobile that there is a call or text message waiting. The alert is broadcast from all cells within a given area.
AGCH(Access Grant Channel): The Access Grant Channel is used to assign resources to a user requesting access to the network. These resources will include the dedicated channel to be used along with timing advance information.
Access Grant Channel (AGCH) is a downlink channel (base to mobile) used by a BS (Base Station) to tell the MS (Mobile Station) which DCCH (Dedicated Control Channel) to use, after the MS has previously requested access to the network by sending a message over the RACH (Random Access Channel).
The AGCH is used to assign resources to the user who has requested access to the network, and the BS allocates a TCH (Traffic Channel) or SDCCH (Stand-Alone Dedicated Control Channel) to the MS, allowing it access to the network.
DCCH(Dedicated Control Channel)
These channels are used for signalling between the network and the mobile. They comprise of the SDCCH (Standalone Dedicated Control Channel), the SACCH (Slow Associated Control Channel) and the FACCH (Fast Associated Control Channel).
SDCCH (Standalone Dedicated Control Channel): This channel is used in the GSM system to provide a reliable connection for signalling and SMS (Short Message Service) messages. The SACCH (Slow Associated Control Channel) is used to support this channel.
SACCH (Slow Associated Control Channel): A GSM signalling channel that provides a relatively slow signalling connection. The SACCH is associated with either a traffic or dedicated channel. The SACCH can also be used to transfer SMS (Short Message Service) messages if associated with a traffic channel.
FACCH (Fast Associated Control Channel): The Fast Associated Control Channel appears in place of the traffic channel when lengthy signalling is required between a GSM mobile and the network while the mobile is in call. The channel is indicated by use of the stealing flags in the normal burst. Typical signalling where this may be employed is during cell handover.
Framing
Error detection & Correction
Multiplexing
Flow Controller
Framing Issues
LAPD uses HDLC frame format for framing.
Where as LAPDm uses synchronization scheme of radio interface to convey information on frame limits. LAPDm avoids using flags by ready-made blocks of the physical layer. LAPDm(LAPD modified) frame has a maximum length of 23 octets on all TCH channels and 21 octets on SACCH blocks(remaining 2 octets are for Timing Advance and Transmission Power Control). But the actual frame length is less than 23/21 octets, so Length Indicator(LI) is included in each frame to identify the length of the frame.
Segmentation & Reassembly
There is no need of segmentation & Reassembly in LAPD(the length of the Abis LAPD frame is 264 octets excluding flags, which corresponds to 260 octets of upper layer information).
Where as LAPD requires segmentation& reassembly because maximum frame length of 21 or 23 octets is not sufficient for most signalling needs.
Error Detection & Correction
LAPD and MTP2 uses 16 redundancy(FCS – frame check sequence) bits to each frame for errpr detection. For LAPDm, error detection is done by coding scheme of the physical layer.
LAPDm frame,in particular lacks TEI, FCS and flags at both ends.
The objective of LAPD is to provide a secure, error-free connection between two end-points so as to reliably transport Layer 3 messages. LAPD protocol provides framing, sequence control, error detection, and recovery of multiple logical data links on the same D channel. The Layer 2 operation starts with a Layer 3 request to establish a call, which prompts Layer 2 to demand Layer 1 to initiate the start-up procedure. Afterwards Layer 2 initializes itself through the SABME ( Set Asynchronous Balanced Mode Extended) procedure. The SABME is conducted between two peer processes by means of unnumbered frames. Initialization guarantees that the correct sequence numbers are used in processes on both ends. Layer 2 protocol maintain the correct sequence of information due to many different types of error. Firstly, if frames lost more than three consecutive times, the re-establishment of the LAP will be done under the assumption that the connection has failed. Secondly, a timer mechanism and preset values of time-out intervals detect lost frames in the transmit direction. Thirdly, frames received out of sequence denote a lost frame in the receive direction. Finally, frame Check Sequence(FCS) value control and discard frames corrupted by errors due to noise.
The opening flag of a frame is followed in octet 2 and 3 by an address field. In order to have each device support multiple logical data links, the data link address is divided into the Service Access Point Identifier (SAPI) and the Terminal End point Identifier (TEI). These two fields are together called Data Link Control Identifier (DLCI). This provides a form of multiplexing. The six bits allocated to the SAPI allow the specification of up to 64 distinct service access point. Table 1. is representing the meaning according to the different SAPI values. The SAPI identifies the network entity for which the information in the LAPD frame is intended. A LAPD entity may have more than a single Layer 3 entity above it and therefore will have more than one SAPI in use (Figure 5).
The TEI is associated with the user side of the user-to network interface. The TEI identifies the logical terminal or final destination for the Layer 3 information. One or more TEIs can be used for point to point data transfer. The TEI for broadcast connection occurs when a message is transmitted with the TEI set to 127. Broadcast TEI will be transmitted to all the logical terminals on the Layer 3 entity (Figure 6). The TEI is either assigned by the TEI management entity automatically or by the user. In either case no two TEIs can be the same. Table 2 is showing the TEI values.
The remainder of the address field consists of a single bit that identifies the frame as a Command or Response (C/R) frame. The TE will set the C/R bit to one for responses and reset it to zero for commands. The network side does the opposite, setting the bit for commands and responses. Bit 1 in octet 2 set to 0 (EA0) means that another address is to be transmitted, and bit 1 in octet 3 set to 1 (EA1) indicates that the associated address is the last one in the transmission.
Control Field
The control field determines the type of LAPD frame being transmitted as well as containing the sequence numbers for the acknowledged information transfer service. The structure of the control field is shown in Figure 7 and it occupies either octet 4 or octets 4 and 5 depending on the type of frame. There are three different formats for the control field, I-format, S-format, and U-format frame. The numbered information transfer format (I-format) frames are used to transfer information between network layer entities. I-frame frame has a send sequence number, N(S), a receive sequence number, N(R), and a poll bit. The supervisory format (S-format) frame has N(R) field in the control field but not N(S) field. The N(S) and N(R) counters occupy seven bits in the control field. It gives the range 0 to 127. The single bit identified as P or F in the control field format is refereed to as the poll bit in command frames and the final bit in response frames. P is set to 1 by a data link layer entity to elicit or poll a response frame from its peer entity on the other side of the connection. F must be set to 1 in the response frame resulting from the poll command frame. In the I- format frames, there is not F bit, so I-format frame never used as a response to command. The unnumbered format (U-format) frame provides additional functionality and is used for information transfer during unacknowledged information transfer. Modifier bits (M) in the U-format frame are used to define seven different frames. The purpose of U-format frames are establishing, maintaining, and disconnecting a data link connection between peer data link layer entities. Table 3 is listing the command, response, and different LAPD control field types.
LAPd Frame Types
I-Frame(Information): Used to send the LAPD field, which consists of layer 3 message or user data traffic.
Receive Ready(RR): Indicates station is ready to receive traffic and or acknowledge previously received frames by using the N(R) field.
Receive Not Ready(RNR): Indicates the transmitting station that the receiving station is unwilling to accept additional incoming data. The RNR frame may acknowledge the preiviously transmitted frames by using the N(R) field.
Reject(REJ): Requests retransmission of frames starting with frame numbered N(R).
Unnumbered Information(UI): Allows for transmission of user data in an unnumbered frame(i.e unsequenced).
Disconnect(DISC): Places the station in disconnected mode.(It terminates multiple frame operation)
Disconnect Mode(DM): Transmitted to indicate a station is in the disconnected mode(not operational)
Set Asynchronous Balanced Mode Extended(SABM): Sets SABM with two octets in the control field for extended sequencing.
Reset(RESET): Used for reinitialization. Previously unacknowledged frames remain unacknowledged.
Unnumbered Acknowledgement(UA): Acknowledges frames that don not contain an I field, for example SABME frame.
Frame Reject(FRMR): Rejects a frame that passed CRC check, but it is not coded correctly.
Exchange Identification(XID): Used to exchange an I-field without the use of sequence numbers.
Channels:
1.Control Channels(BCCH, CCCH, DCCH)
2.Traffic Channels(TCH/F, TCH/H)
BCCH(Broadcast Control Channel):
This downlink channel contains specific parameters needed by a mobile in order that it can identify the network and gain access to it. Typical information includes the LAC (Location Area Code) and RAC (Routing Area Code), the MNC (Mobile Network Code) and BA (BCCH Allocation) list.
The Location Area Code uniquely identifies a LA (Location Area) within a PLMN (Public Land Mobile Network). It may range from 0 to 65,535.
Routing Area Code is a fixed length of 1 octet and identifies a routing area within a location area. The RAC is part of the RAI (Routing Area Identity).
The Mobile Network Code is either a two or three digit number used to uniquely identify a given network from within a specified country (MCC (Mobile Country Code)
BA: This is a list of frequencies supported on the neighbouring cells. This parameter is broadcast on the BCCH (Broadcast Control Channel) giving mobiles the frequencies of the BCCH carriers on neighbouring cells. It is typically used by the MS (Mobile Station) in the cell selection and re-selection processes.
CCCH(Common Control Channel)
The Common Control Channel supports common procedures required to establish a dedicated link with the network. These Channels include the RACH (Random Access Channel), FACH (Forward Access Channel) and PCH (Paging Channel).
RACH : The channel used for initial access into a system. The RACH function is different depending on the technology of the system. The term RACH applies to cdmaOne™, GSM, UMTS, TETRA, CDMA2000® and other telecommunication systems.
Random Access Channel (RACH) is an uplink channel (mobile to base) used by a MS (Mobile Station) when it initially attempts to request access to the network. The MS uses the RACH to request a SDCCH (Stand-Alone Dedicated Control Channel), in response to being paged or trying to make a call.
FACH: A UMTS transport channel that forms the downlink half of a transport channel pair known as the RACH (Random Access Channel) / FACH (Forward Access Channel) combination. It is used for downlink signalling and small quantities of data.
PCH: The Paging Channel is used to alert a mobile that there is a call or text message waiting. The alert is broadcast from all cells within a given area.
AGCH(Access Grant Channel): The Access Grant Channel is used to assign resources to a user requesting access to the network. These resources will include the dedicated channel to be used along with timing advance information.
Access Grant Channel (AGCH) is a downlink channel (base to mobile) used by a BS (Base Station) to tell the MS (Mobile Station) which DCCH (Dedicated Control Channel) to use, after the MS has previously requested access to the network by sending a message over the RACH (Random Access Channel).
The AGCH is used to assign resources to the user who has requested access to the network, and the BS allocates a TCH (Traffic Channel) or SDCCH (Stand-Alone Dedicated Control Channel) to the MS, allowing it access to the network.
DCCH(Dedicated Control Channel)
These channels are used for signalling between the network and the mobile. They comprise of the SDCCH (Standalone Dedicated Control Channel), the SACCH (Slow Associated Control Channel) and the FACCH (Fast Associated Control Channel).
SDCCH (Standalone Dedicated Control Channel): This channel is used in the GSM system to provide a reliable connection for signalling and SMS (Short Message Service) messages. The SACCH (Slow Associated Control Channel) is used to support this channel.
SACCH (Slow Associated Control Channel): A GSM signalling channel that provides a relatively slow signalling connection. The SACCH is associated with either a traffic or dedicated channel. The SACCH can also be used to transfer SMS (Short Message Service) messages if associated with a traffic channel.
FACCH (Fast Associated Control Channel): The Fast Associated Control Channel appears in place of the traffic channel when lengthy signalling is required between a GSM mobile and the network while the mobile is in call. The channel is indicated by use of the stealing flags in the normal burst. Typical signalling where this may be employed is during cell handover.
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