The Um interface interconnects the mobile network elements (i.e. MSs) with the fixed network
( in this case BTSs).
Air Interface Layer Functions
transmission structure is based on a 16kbps IDSN D-channel configuration with a composite
carrier data rate of 271kbps. The Access method uses Time Division Multiple Access
(TDMA).
Layer 1 functions include compression, channel coding for Forward Error Correction (FEC),
interleaving, encryption and modulation.
ISDN LAPD protocol used in fixed networks. LAPDm has been specifically adapted to the air
interface for ISDN D(mobile) channels rather than standard ISDN D channels.
In essence LAPDm is a protocol similar to the High-level Data Link Protocol (HDLC) which
offers a number of services on the various logical Dm channels of Layer 3.
The main task of Layer 2 is the transparent transport of messages between Layer 3 protocol
entities. Layer 2 functions also include call setup and clearing, protected data transfer (using
Backward Error Correction (BEC)) and operates in either acknowledged or unacknowledged
mode.
User speech and data is generated within the application layers (layers 4-7) and is passed
down through layer 3 into layer 2 where it is prepared for transportation over the air
interface. This ‘preparation’ (or coding) is covered in detail in Section 6 of these course notes.
· Call Control (CC) Management
· Mobility Management (MM)
· Radio Resource (RR) Management
The messages generated by each of these functions are combined at layer 2 and passed
transparently across layer 1 before being reconstituted into their respective functional groups
at the BTS Layer 2 before being passes back into their layer 3 functional areas.
Note that the RR Management functional relationship exists between the MS and the BSS
whereas the CM and MM functional relationship exists between the MS and the MSC. In all
cases, these messages are passed transparently over Layers 1 and 2 of the Um interface.
( in this case BTSs).
Air Interface Layer Functions
5.6.1 Um INTERFACE - LAYER 1
The physical medium at layer 1 is a GMSK modulated UHF radio path. The physicaltransmission structure is based on a 16kbps IDSN D-channel configuration with a composite
carrier data rate of 271kbps. The Access method uses Time Division Multiple Access
(TDMA).
Layer 1 functions include compression, channel coding for Forward Error Correction (FEC),
interleaving, encryption and modulation.
5.6.2 Um INTERFACE - LAYER 2
The Um interface layer 2 (data link layer) uses LAPDm which is a modified version of theISDN LAPD protocol used in fixed networks. LAPDm has been specifically adapted to the air
interface for ISDN D(mobile) channels rather than standard ISDN D channels.
In essence LAPDm is a protocol similar to the High-level Data Link Protocol (HDLC) which
offers a number of services on the various logical Dm channels of Layer 3.
The main task of Layer 2 is the transparent transport of messages between Layer 3 protocol
entities. Layer 2 functions also include call setup and clearing, protected data transfer (using
Backward Error Correction (BEC)) and operates in either acknowledged or unacknowledged
mode.
User speech and data is generated within the application layers (layers 4-7) and is passed
down through layer 3 into layer 2 where it is prepared for transportation over the air
interface. This ‘preparation’ (or coding) is covered in detail in Section 6 of these course notes.
5.6.3 Um INTERFACE - LAYER 3
Network signalling messages are generated at Layer 3 and relate to one of three functions:· Call Control (CC) Management
· Mobility Management (MM)
· Radio Resource (RR) Management
The messages generated by each of these functions are combined at layer 2 and passed
transparently across layer 1 before being reconstituted into their respective functional groups
at the BTS Layer 2 before being passes back into their layer 3 functional areas.
Note that the RR Management functional relationship exists between the MS and the BSS
whereas the CM and MM functional relationship exists between the MS and the MSC. In all
cases, these messages are passed transparently over Layers 1 and 2 of the Um interface.
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