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GSM Protocol Suite - Lecture 2 - Mobile Computing, Lecture notes of Mobile Computing

Indian Institute of Information Technology (IIIT)Mobile Computing

Detail Summery about Protocols involved in the radio interface, GSM Channel Multiplexing, LAPD message structure, LAPDm on radio interface,Protocols involved in the A-bis interface.

Typology: Lecture notes

2010/2011

Uploaded on 09/07/2011

manish
manish 🇮🇳

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Protocols involved in the radio interface
•Level 1 - Physical
–TDMA frame
–Logical channels multiplexing
•Level 2 - LAPDm(modified from LAPD)
–No flag
–No error retransmission mechanism due to real time constraints
•Level 3 -Radio Interface Layer (RIL3) involves three sub layers
–RR: paging, power control, ciphering execution, handover
–MM: security, location IMSI attach/detach
–CM: Call Control(CC), Supplementary Services(SS), Short Message
Services(SMS),
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Protocols involved in the radio interface

  • Level 1 - Physical
    • (^) TDMA frame
    • (^) Logical channels multiplexing
  • (^) Level 2 - LAPDm(modified from LAPD)
    • (^) No flag
    • No error retransmission mechanism due to real time constraints
  • Level 3 -Radio Interface Layer (RIL3) involves three sub layers
    • (^) RR: paging, power control, ciphering execution, handover
    • (^) MM: security, location IMSI attach/detach
    • (^) CM: Call Control(CC), Supplementary Services(SS), Short Message

Services(SMS),

GSM Channel Multiplexing

LAPDm on radio interface

  • (^) In LAPDm the use of flags is avoided.
  • (^) LAPDm maximum length is 21 octets of information. It makes use of “more” bit to distinguish last frame of a message.
  • (^) No frame check sequence (CRC) for LAPDm, it uses the error detecting performance of the transmission coding scheme offered by the physical layer
ADDRESS CONTROL INFORMATION 0-21 OCTETS
SAPI
N(S) N(R)

LAPDm Message structure

Protocols involved in the A-bis interface

  • (^) Level 1- PCM transmission (E1 or T1)
    • (^) Speech encoded at 16kbit/s and sub multiplexed in 64kbit/s time slots.
    • (^) Data which rate is adapted and synchronized.
  • (^) Level 2 - LAPD protocol, standard HDLC
    • (^) Radio Signaling Link (RSL)
    • (^) Operation and Maintenance Link (OML).
  • (^) Level 3 - Application Protocol
    • (^) Radio Subsystem Management (RSM)
    • Operation and Maintenance procedure (OAM)

Presentation of A-bis Interface

  • (^) Messages exchanges between the BTS and BSC.
    • Traffic exchanges
    • (^) Signaling exchanges
  • (^) Physical access between BTS and BSC is PCM digital links of E1(32) or T1(24) TS at 64kbit/s.
  • (^) Speech:
    • Conveyed in timeslots at 4X16 kbit/s
  • (^) Data:
    • Conveyed in timeslots of 4X16 kbit/s. The initial user rate, which may be

300, 1200, … is adjusted to 16 kbit/s

FLAG ADRESS CONTROL INFORMATION 0 – 260 OCT FCS FLAG
N(S) N(R)

LAPD message structure SAPI TEI

SAPI (Service access point identifier), 6-bits C/R (Command/Response) bit indicates if the frame is a command or a response EA0 (Address Extension) bit indicates whether this is the final octet of the address or not TEI (Terminal Endpoint Identifier) 7-bit device identifier EA1 (Address Extension) bit, same as EA Control (2 octets) - The frame level control field indicates the frame type (Information, Supervisory, or Unnumbered) and sequence numbers (N(r) and N(s)) as required. Information – Layer 3 protocol information and User data CRC (2 octets) - Cyclic Redundancy Check is a low-level test for bit errors on the user data. Flag (1 octet) - This is always 7E 16 (0111 1110 2 )

LAPD message structure

Radio PCM

O
A
M

R R L A P D

L

A

P

Dm

BTS
PCM PCM
MTP
SCCP
MTP
L
A
P
D
O
A
M
R
R

D T A P B S S M A P

BSSAP
BSC
PCMT1E1 MTP
MTP
MTP
SCCP
MTP
MTP

SCCP

I S U P / T U P
BSSAP
DTAP/
BSSMAP
T
C
A
P
MM
CM M
A
P
NSS

Radio

L

A

P

Dm

R
R
MM
CM
MS

Um Interface A bis Interface

A

Interface GSM protocol layers for signaling