The Data Link Control layer

In the Data Link Control layer of the DECT standard (EN 300 175-4) the physical world is left behind and the data communications realm is entered. This is also the layer in which the difference between control data (from the Network layer) and the user data (from the U-plane), e.g. speech data, becomes apparent. Of course a physical link has to be open between the base system and the portable for these communications to occur. Therefor the services provided by the Medium Access Control layer are used.

Servicing the Network layer

The data link services provided to the Network layer, and therby the C-plane, are fairly simple. They use 'Link Access Protocol C (LAPC)', a variant of the widely known High Level Datalink Control protocol (HDLC) of which the point-to-point 'Asynchronous Balanced Mode (ABM)' is used.

This is a clip of a LAPC trace of a call setup. Arrows from the left to right are from the portable to the base system, and vice versa. The data link is opened by the exchange of an empty 'Information' frame (I-frame) with the 'New Link Flag' (NLF) set and a 'Receiver Ready' frame (RR-frame), also with its New Link Flag set. Now the data link is established and NWK layer messages are exchanged contained in I-frames.

This is the LAPC link setup, initiated by the portable

          PP          FP
           |          |
           | -------> |    Addr: 0x91 Contr: 0x0
           |          |    NLF: 1 LLN: 1 SAPI: 0 CR: 0 RES: 1
           |          |    I-frame  N(R): 0  N(S): 0 
           |          |    Contents: empty
           |          | 
           | <------- |    Addr: 0x91 Contr: 0x21
           |          |    NLF: 1 LLN: 1 SAPI: 0 CR: 0 RES: 1
           |          |    RR-frame N(R): 1

Now the first NWK layer messages can be relayed

 
           |          |
           | -------> |    Addr: 0x11 Contr: 0x2
           |          |    NLF: 0 LLN: 1 SAPI: 0 CR: 0 RES: 1
           |          |    I-frame  N(R): 0  N(S): 1 
           |          |    Contents:
           |          |    0x03 ---> CC-Message
           |          |    0x05 --> {CC-SETUP}
           |          |    0x05 -> <>
           |          |    0x07 0x80 0xa8 0x20 0x00 0x92 0x61 0x50
           |          |    0x06 -> <>
           |          |    0x06 0xa0 0xa0 0x20 0x00 0x90 0x10
           |          |    0xe0 -> <>
           |          |    0x80
           |          |    0x6c -> <>
           |          |    0x02 0x41 0x81
           |          |    
           | <------- |    Addr: 0x11 Contr: 0x1
           |          |    NLF: 0 LLN: 1 SAPI: 0 CR: 0 RES: 1
           |          |    RR-frame N(R): 0
           |          |

Here the encryption kicks in

           |          |
           | <------- |    Addr: 0x13 Contr: 0x0
           |          |    NLF: 0 LLN: 1 SAPI: 0 CR: 1 RES: 1
           |          |    I-frame  N(R): 0  N(S): 0
           |          |    Contents:
           |          |    0x05 ---> MM-Message
           |          |    0x4c --> {CIPHER-REQUEST}
           |          |    0x19 -> <>
           |          |    0x02 0x81 0x98
           |          |   
           | -------> |    Addr: 0x13 Contr: 0x21
           |          |    NLF: 0 LLN: 1 SAPI: 0 CR: 1 RES: 1
           |          |    RR-frame N(R): 1 
           |          |

The call setup is accepted

           |          |
           | <------- |    Addr: 0x13 Contr: 0x2
           |          |    NLF: 0 LLN: 1 SAPI: 0 CR: 1 RES: 1
           |          |    I-frame  N(R): 0  N(S): 1
           |          |    Contents:
           |          |    0x83 ---> CC-Message
           |          |    0x0d --> {CC-SETUP-ACK}
           |          |    0x1e -> <>
           |          |    0x02 0x80 0x88
           .          .    

     etc. etc. etc.

Servicing the U-plane

The Data Link Control layer can provide for a wide variety of data links. Currently the following are described in the DECT standard:

LU1	Transparent unprotected service
LU2	Frame replay service
LU3	Frame switching service
LU4	Forward error correction service
LU5	Basic rate adaption service
LU6	Secondary rate adaption service
LU7	64 kbit/s data bearer service w/ ARQ
LU8	64 kbit/s data bearer service without ARQ
LU9	Unprotected Rate Adaption for V series Equipment service
LU10	Enhanced Data service

For voice communication the LU1 service is used. This service provides a minimum delay transparent unprotected data link. This means that the voice data is transported without delay (eg. no buffering), without alteration and without error detection or correction. This may not be acceptable for pure data transport. Therefor other services are defined. The LU10 service is supposed to be the main data transport serivce.

Written by my own two hands and an ASCII editor. Problems? write me Last updated January 27, 2000

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