4.4 physical and logical channels
TRANSCRIPT
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
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Frame k Frame k+1 Frame k+2 Frame k+3 Frame k+4
Pyhsical channel
Logical channel A
Logical channel B
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❑ 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
Mobile communications
❑ 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
Mobile communications
❑ 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
Mobile communications
Synchronization channel
Abbreviation SCH
Direction downlink
Use TDMA frame number/BSIC, time
synchronization
Allocation Permanent
Point to point/ point to multipoint Point to multipoint
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)
Allocation Permanent
Point to point/ point to multipoint Point to multipoint
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
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Mobile communications
Abbreviation SDCCH
Direction Uplink and downlink
Use authentication, call setup before
allocation of TCH, location updating,
SMS (MS in idle mode)
Allocation On demand
Point to point/ point to multipoint Point to point
Burst type Normal
❑ Dedicated control channels
- used by a single MS is dedicated mode
Standalone dedicated control channel
Mobile communications
Abbreviation SACCH
Direction Uplink and downlink
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
Allocation On demand
Point to point/ point to multipoint Point to point
Burst type Normal
Slow associated control channel
Mobile communications
Abbreviation FACCH
Direction Uplink and downlink
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)
Allocation On demand
Point to point/ point to multipoint Point to point
Burst type Normal/Access
Fast associated control channel
Mobile communications
Abbreviation RACH
Direction Uplink
Use Request allocation of a SDCCH as
a response to paging or location
updating
Allocation On demand
Point to point/ point to multipoint Point to multipoint
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
Mobile communications
Abbreviation PCH
Direction Downlink
Use Signaling of an incoming call
Allocation Permanent
Point to point/ point to multipoint Point to multipoint
Burst type normal
Paging channel
- used for signaling an incoming call ; carries an identity of the MS :
TMSI/IMSI
Mobile communications
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
Allocation Permanent
Point to point/ point to multipoint Point to multipoint
Burst type normal
Access grant channel
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❑ 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
Mobile communications
- 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)
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❑ 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
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Mobile communications
❑ 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
Mobile communications
❑ 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
Mobile communications
Mobile communications
❑ 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 …
Mobile communications
❑ 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
Mobile communications
Traffic multiframe Traffic multiframe
Signaling multiframe
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5. Signaling protocols and procedures in the GSM
system
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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
Mobile communications
-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
Mobile communications
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
Mobile communications
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
Mobile communications
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
Mobile communications
❑ 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
Mobile communications
❑ 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
Mobile communications
❑ 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
Mobile communications
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