4.4 physical and logical channels

4.4 Physical and logical channels

❑ Physical channel – a recurrence of a particular TS during subsequent

TDMA frames on a carrier frequency

- transports a single or multiple logical channels

❑ Logical channel – a term used in GSM to identify the type of information

exchanged :

- traffic channels (data at various rates, voice)

- control channels (only signaling information)

- a specific logical channel carries information of a similar type

(Ex: PCH- paging channel)

- logical channels are mapped onto physical channel according to

a specific pattern

Mobile communications

Frame k Frame k+1 Frame k+2 Frame k+3 Frame k+4

Pyhsical channel

Logical channel A

Logical channel B


❑ Logical channel organisation

Logical channels

Common

channels

Dedicated

channels

Broadcast

channels

Common control

channels

Dedicated control

channels

Traffic

channels

FCCH

SCH

BCCH

PCH

RACH

AGCH

TCH/F TCH/H

FACH

SDCCH

SACCH


❑ Control channels

- common control channels: channels used for managing access

to a GSM PLMN

- dedicated control channels: channels used for exchanging signaling

information coming/going to a single MS

- broadcast channels: mapped onto the beacon frequency; information

flows only in the downlink direction, allocation is carried out on a cell per

basis

❑ Traffic channels

Abbreviation TCH/F or TCH/H (ex. TCH/FS –voice,

TCH/F9.6 –data)

Direction Uplink and downlink

Use Transport of user data

Point to point/ point to multipoint Point to point

Allocation On demand from a MS

Burst type normal


❑ Broadcast channels

-cell specific information , point to multipoint channels

-placed on a beacon frequency emitted continuously

Abbreviation BCCH

Direction downlink

Use cell specific information: CI, LAI, beacon

frequencies of the neighboring cell(s),

control channels configuration, MNC,

MCC etc.

Allocation Permanent

Point to point/ point to multipoint Point to multipoint

Burst type normal

Broadcast control channel


Synchronization channel

Abbreviation SCH

Direction downlink

Use TDMA frame number/BSIC, time

synchronization



Burst type synchronization

- SCH carries: BSIC and the TDMA frame number- time for

synchronization to a hyper frame structure (to be discussed later)

Frequency correction channel

Abbreviation FCCH

Direction downlink

Use Frequency synchronization (the MS fine

tunes to the beacon frequency)



Burst type Frequency correction

-at power on time:- all channels that a PLMN operator uses are scanned/

only those carrying FCCH are corresponding to beacon frequencies

-once registered to the network – the MS is informed on BCCH which

beacon frequencies has to monitor for an eventual cell reselection

procedure and possibly a new location updating procedure



Abbreviation SDCCH


Use authentication, call setup before

allocation of TCH, location updating,

SMS (MS in idle mode)

Allocation On demand


Burst type Normal

❑ Dedicated control channels

- used by a single MS is dedicated mode

Standalone dedicated control channel


Abbreviation SACCH


Use Always associated with a TCH or a

SDCCH

Measurement reports (uplink), TA

(downlink), power control of MS

(downlink), SMS (MS in dedicated

mode), list of frequencies to be

monitored in dedicated mode



Burst type Normal

Slow associated control channel


Abbreviation FACCH


Use Used for exchanging time critical

information (handovers); works in

stealing mode – replaces a speech

segment in the TCH channel(odd

or/and even bits)



Burst type Normal/Access

Fast associated control channel


Abbreviation RACH

Direction Uplink

Use Request allocation of a SDCCH as

a response to paging or location

updating



Burst type Access burst

❑ Common control channels

- dedicated to several MSs in order to allow them to pass from idle

to dedicated mode

Random access control channel


Abbreviation PCH

Direction Downlink

Use Signaling of an incoming call



Burst type normal

Paging channel

- used for signaling an incoming call ; carries an identity of the MS :

TMSI/IMSI


Abbreviation AGCH

Direction Downlink

Use Indication that a channel was

allocated by the network (usually a

SDCCH) as a response to paging

or following a request of the MS



Burst type normal

Access grant channel


❑ TCHs and associated SACCHs – must be mapped on the same

physical channel

❑ No unique way of mapping for the other logical channels

❑ Typical mapping

- FCH+SCH BCCH, RACH, PCH, and AGCH – beacon frequency

on TS0

- SDCCH and the associated SACCHs – on any TS on the beacon

frequency ; typically they are mapped on TS1

4.5 Mapping of logical channels onto physical channels

❑ TCH, SACCH

- a SACCH is always allocated to a TCH

- mapping is defined based on a recurrent pattern of 26 TDMA frames

(traffic multiframe)

-4x26 TDMA frames are necessary to transmit a FEC encoded SACCH

information (120x4=480 ms)

-on the above example a MS has as allocated TS for TCH and SACCH TS2


- instead all TSs are used by the MS in order to measure/monitor

beacon frequencies from neighboring cells, indicated by the network

- for the previous example, during TS2 in the idle frame the MS is

not emitting or receiving

- Why all 8TS are idle? - to ensure that information sent on TS0

(FCCH, SCH, BCCH) on beacon frequencies from neighboring cells

will be read25th TDMA frame Idle frame

TS0 on neighboring beacon ?

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

-the MS must tune to and perform measurements on beacon frequencies

between reception and emission (a FCCH burst must be detected for

handovers)-reports must be indexed by BSIC (a SCH burst on the same beacon

carrier must be read)


❑ FCCH, SCH, BCCH, PCH, AGCH, RACH

- mapped according to a 51 TDMA frames periodic pattern (control

signaling/ multi frame) on the beacon frequency on TS0

Downlink

Uplink

R-RACH, F-FCH, S-SCH, B-BCCH, P-AGCH + PCH

I- idle frame



❑ Typical scenarios

Power on

- the MS searches for beacon frequencies, it finds one and

synchronizes to it by reading FCCH

… F S B B B B …

- it synchronizes in time

… F S B B B B …

- reads system information

… F S B B B B …

- it must register to the network by demanding a dedicated control

channel

TS0 downlink

TS0 downlink

TS0 downlink

… R R R R R R …TS0 uplink

- the network allocates a SDCCH using the AGCH

… P P P P P P …

- registration is carried out using SDCCH

Paging

- the system signals an incoming call using PCH

TS0 downlink

… P P P P P P …TS0 downlink

- the mobile responds by placing an access grant on RACH

… R R R R R R …TS0 uplink


❑ SDCCH, SACCH

- usually mapped according to a 102 TDMA frames periodic pattern

on the beacon frequency on TS1- 8 SDCCH sharing the same

physical channel

- a supplementary TS can be dedicated but it will not be allowed to

hold TCH

Downlink

- Dx- SDCCH allocated to MSx, Ax- SACCH allocated to SDCCH

- SDCCH/SACCH information issued from channel coding is mapped onto

4 consecutive TDMA frames

- on the uplink direction – similar structure time shifted with 3 TS



❑ Typical scenario – access for a call

- the mobile requests a channel RACH

… R R R R R R …

- the mobile allocates a SDCCH using AGCH

… P P P P P P …

TS0 uplink

TS0 downlink

- set-up (authentication, IMEI check, ciphering mode etc) takes

place on the SDCCH; power control and measurement reports are

sent using SACCH

… D0 D0 D0 D0 … A0 A0 A0 A0 …

- call is started on a TCH

TS1 downlink

and uplink

… T T T A T T T …


❑ Frame hierarchy

- TDMA frame number – important for synchronization

- numbering according to a periodic pattern of 3h28 min – hyper

frame

- TDMA frame number modulo 51 – position inside a signaling multiframe

- TDMA frame number modulo 26 – position inside a traffic multiframe

Cadru TDMA

TDMA superframe (26x51 frames)

Signaling multiframe

Hyperframe

Traffic multiframe

❑ Why 26 and 51?

- deliberate choice since 26 and 51 do not have common divisors –sliding

of the beginning of multi frames occurs

- in the above example the first Idle frame doesn’t allow detection of the

FCCH channel whilst the second one does

- after decoding FCCH the mobile will decode the SCH and will found the

BSIC (essential for handovers) the TDMA frame number (for ciphering) on

neighboring cells


Traffic multiframe Traffic multiframe

Signaling multiframe


5. Signaling protocols and procedures in the GSM

system


5.1 Introduction

-user data is not the single type of information transported in a GSM

network

5 Signaling in the GSM system

Signaling information – denotes information (other than user data)

exchanged between several pieces of equipment for ensuring the

network’s functionality

Example – handovers : before a MS can switch to another TCH, the new

channel must be previously activated on another BTS by a BSC (possibly a

different one); a MSC can also be involved - > several entities need to

communicate through signaling messages

-exchange of signaling messages is triggered by events (for example the

signal level falls below a threshold) and can initiate some actions on a

distant machine

-for dealing with signaling info packet switching is employed in GSM


-when transporting packet data reliability is an important issue –>

GSM must include data link layer capabilities on the signaling path

- distant entities must be able to communicate also via intermediate

equipments (ex: in location updating MS and VLR are actively

involved and BSS plays no role) -> relaying capabilities must be

supported by GSM equipments i.e. the signaling information is to be

transported without interpretation

- a signaling message might arrive to an intermediate machine that

needs to add some extra data before sending data to its final

destination; this is achieved by protocol interworking

- in order to be capable to communicate with different distant pieces

of equipment routing and addressing capabilities must be

implemented


5.2 The GSM’s stack of signaling protocols

❑layered approach - each layer passes information to other independent

layers in standard format

❑different set of protocols for

each interface

Main reason: each interface has its own needs/ standardization carried out

by different workgroups

m


5.3 Signaling protocols on the radio interface

5.3.1 Physical layer

-communication takes place in logical control channels mapped onto

physical channels using modulation, channel coding, burst formatting,

ciphering etc.

5.3.2 Data link layer-reliability for point-to-point communications (SDCCH for example)

-both link layer protocols for the radio LAPDm) and the A (MT2) and Abis

(LAPD) interfaces are based on the classical HDLC (High-level Data Link

Control) protocol

Functions implemented by HDLC:

-segmentation/reassembly

-frame structuring and synchronization using start/stop patterns

-addressing - allowing addressing of the right function and right function at

the receiving end

-error detection using frame check sequences (FCS) and ARQ

retransmissions


LAPDm – is the GSM’s data link layer protocol for the radio interface

– (Link Access Protocol for D channel); m- stands for mobile

❑ frame structuring – not needed since is provided by the physical layer

(burst formatting)

❑ segmentation and reassembly – using a “more” bit that allows

distinction between the last frame(0) and other frames(1)

❑ error detection and correction – no FCS; relies on the Fire code

technique used to encode signaling information at the physical layer level

❑addressing – SAPI (Service Acces Point Identifier – SAPI=0 signaling

RR, MM or CM – SAPI=3 SMS or SS)

Structure

of a LAPDm frame:

23 bytes=184 bits


❑ 3 sub layers with different functionality

RR (Radio Resource Management)

MM (Mobility Management)

CM (Connection Management)

❑ RR –Radio Resources Management

-includes all procedures related to the establishment and maintenance

of physical connections (RR connections) in order to allow point-to-point

dialogue between MS and the network on dedicated control channels

- implements procedures for the reception and decoding of the BCCH,

SCH, FCCH channels

- handles transmissions on AGCH/PCH /RACH

- RR terminates at BSS

5.3.3 Layer 3


❑ MM –Mobility Management

- implements mobility / confidentiality related procedures such as:

- location updating

- IMSI attach/detach

- authentication, ciphering

- some MM messages are triggered on request from the upper layer

(CM) -> MM connection related procedures (ex. authentication)

- exchange of MM information can be done not necessarily in connection

with a call -> MM specific procedures (ex: location updating)

- all MM procedures need prior RR connections

- exchange of MM messages takes place between MS and MSC/VLR (in

some cases HLR) with BSS acting as a transparent relay


❑ CM – Connection Management

- layer regrouping 3 different protocols

SMSCC SSCM <=>

- CC – Call Control - call establishment/release, routing information for

MT calls

- SMS – Short Message Service – sending/receiving of short messages

- SS– Supplementary Services management – enabling/disabling/querying

status of supplementary services

-exchange of CM messages takes place between MS and HLR via MSC

or MS and the SMS SC via dedicated MSCs

❑ The structure of a layer 3

message


Protocol discriminator (PD) – 4 bit identifier inserted by the originator

and used by the receiver in order to distribute the message to the right

software module ( to the corresponding layer 3 protocol) for processing

- acts also like a networking address

What if a user is involved in several communications (transactions)?

- a supplementary field – Transaction Identifier - is used

Message type –mandatory field ; together with PD uniquely defines the

function and format of a layer 3 message pertaining to a given protocol;

it can trigger some events at destination

4.4 physical and logical channels

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