2. Introduction
Logical channels:
•RACH(uplink): This channel is used by a MS to request an initial dedicated channel from the
BTS. This would be the first transmission made by a MS to access the network and request radio
resources.
•AGCH(downlink): This channel is used by a BTS to notify the MS of the assignment of an
initial SDCCH for initial signaling.
•PCH(downlink): This channel is used to inform the MS that it has incoming traffic. The traffic
could be a voice call, SMS, or some other form of traffic.
•DCCH(uplink/downlink): This channel is used for signaling and call setup between the MS
and the BTS.
•FACCH(uplink/downlink): This channel is used for control requirements such as handoffs.
There is no TS and frame allocation dedicated to a FAACH.
The FAACH is a burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH).
•SACCH(uplink/downlink): This channel is a continuous stream channel that is used for
control and supervisory signals associated with the traffic channels.
•TCH(uplink/downlink): Traffic Channels are used to carry two types of information to and
from the user(Encoded Speech and Data)
3. GSM nodes:
• MS: It is consisted of ME and SIM card
• BTS: Mobile Station's access point to the network.
It is responsible for carrying out radio communications between the
network and the MS but it doesn’t give any orders to the MS.
• BSC: The BSC controls multiple BTSs. It is responsible for taking all the
actions on the MS such as Handover, power and allocation of radio
channels, etc.
• MSC: The MSC is the heart of the GSM network. It handles call routing,
call setup, and basic switching functions.
4. Immediate Assignment Procedure
MS BTS BSC
Channel Request
RACH Channel Required
Channel Activation
Channel Activation Ack
Immediate Assignment Command
AGCH
Start T3101
SABM
DCCH
UA
DCCH
Establish Indication
Stop T3101
CA300J:Channel Requests
(Circuit Service)
CR330B:CHAN ACTIV
NACK Messages Sent by
BTS in Immediate
Assignment Procedure
K3101:Immediate
Assignment Commands
CA303J:Call Setup
Indications (Circuit
Service)
Counters
CR330A:Channel
Activation Attempts in
Immediate Assignment
Procedure
the BSC assigns a signaling channel and sends the BTS a Channel Activation
message to activate a radio channel. The Channel Activation message carries the
activation type and TA, and the BTS turns on the power amplifier on the specified
channel and sends a Channel Activation Acknowledge message to the BSC
The BTS sends the BSC a Channel Required message, Request
Reference and Access Delay parameters. Access Delay is the delay of
the access burst.
RACH can be used at random, without any coordination between the
mobiles. If two mobiles transmit on the channel at the same time their
messages will be lost in a collision. The mobiles will detect the collision
via timer T3120 and retransmit the message.
The MS sends the BTS a Channel Request message on the RACH. The
Channel Request message contains the Establish Cause. At this point
the mobile is listening to the AGCH for a reply.
Start T3120
Stop T3120
The BSC sends the MS an Immediate Assignment Command message on the AGCH,
instructing the MS to access the specified channel. The specified channel can be
either an SDCCH or a TCH. If the BSC has no available channels, it sends the MS an
Immediate Assignment Reject message. The BSC starts timer T3101
The MS tunes to the specified radio channel and sends an SABM (Set
Asynchronous Balanced Mode) frame on the main DCCH, the SABM is a frame in
the LAPDm protocol at the data link layer. The SABM frame conveys a message
which informs the BSC that the MS has changed to a new channel.
The BTS responds with a UA frame on the main DCCH. The UA frame is an
unnumbered acknowledge frame of the LAPDm protocol at the data link
layer. It confirms the SABM frame and at the same time the BTS sends the
BSC an Establish Indication message. This Establish Indication message
indicates the establishment of a radio connection, and the BSC stops T3101.
When T3101 expires, the system releases corresponding
radio resources.
5. TCH Assignment
MS BTS BSC MSC
Assignment Request
Channel Activation
Channel Activation Ack
Assignment Command
DCCH
Start T3107
SABM
FACCH Establishment Indication
UA
FACCH
Assignment Complete
FACCH
Assignment Complete
Stop T3107
Start T10
Stop T10
K3010A:TCH Seizure
Requests (Traffic
Channel)
Counters
CR331A:Channel
Activation Attempts in
Assignment Procedure
CA311:Assignment
Command
K3013A:Successful
TCH Seizures
(Traffic Channel)
CR331B:CHAN ACTIV
NACK Messages Sent by
BTS in Assignment
Procedure
If Timer T3107 expires, the BSC releases all radio resources.
The BSC sends an Assignment Complete message to the MSC. Upon receiving
the message, the MSC stops timer T10, the message carries the Channel type,
and Speech version.
Upon receiving the UA frame, the MS sends an Assignment Complete message
on the FACCH to notify the BSC of the channel assignment completion. Upon
receiving the message, the BSC stops timer T3107.
the BTS responds to the MS with a UA frame on the FACCH and at the
same time sends the BSC an Establishment Indication message to message
indicate the establishment of a radio connection on the new channel
the MS tunes to the TCH and sends the SABM frame on the FACCH.
The BSC sends the MS an Assignment Command message on the signaling
channel and starts timer T3107. The Assignment Command message
carries the Channel Description, Power Command, and Channel Mode.
The Channel Activation and Activation Acknowledgement is
similar to the procedure in the immediate assignment process
the MSC requests radio resources from the BSS, sends an Assignment
Request message to the BSS, and starts timer T10. The Assignment Request
message carries the Channel Type, Priority and QoS parameters, and
Downlink DTX Flag parameters.
6. Call Queuing
MS BTS BSC MSC
Immediate Assignment Procedure
Setup
DCCH Call Proceeding
DCCH Assignment Request
Queuing Indication
Start T11
TCH Assignment Procedure
Stop T11
Assignment Complete
Channel Request
RACH
R3135:Queuing
Requests
Start T10
Stop T10
The MSC sends the BSC an Assignment Request message and finds that no
speech channel is available. If queuing is allowed, the BSC sends Queuing
Indication message to the MSC and starts timer T11.
The assignment request is placed in a queue. The network can determine the
maximum length of the Queue.
If a speech channel becomes available, the BSC will undergo normal TCH
Assignment Procedure.
When a speech channel is assigned, the queuing procedure is complete, and the
BSC stops timer T11. The BSC sends Assignment Complete message to the MSC.
7. Call Release
MS BTS BSC MSC
FACCH
Disconnect
Start T305
Start T308
FACCH
Release Complete
Release
FACCH
Stop T305
Stop T308
Clear Command
Channel Release
FACCH
Start T3109
A03704:Number of
Speech
Disconnections
Counters
A03701:Normal
Speech
Disconnections
The BSC sends a Channel Release message and starts timer T3109. The
Channel Release message notifies the MS and the BTS of releasing the logic
channels on the Um interface.
the MSC stops timer T308, releases the MM connection, and sends the BSC a
Clear Command message. The Clear Command message notifies the BSC of
releasing the resources on the A interface and the Um interface.
The calling MS stops timer T305, releases the MM connection, and sends the
MSC a Release Complete message.
The MSC sends the calling MS a Release message and at the same time sends
the called MS a Disconnect message. The network starts timer T308.
The calling MS sends a Disconnect message to the MSC and starts timer T305.
The Disconnect message carries the cause for disconnection.
8. Call Release – Cont.
MS BTS BSC MSC
Channel Release
FACCH
Start T3109
Deactivate SACCH
DISC
FACCH
UA
FACCH
Release Indication
Stop T3109
Start T3111
Stop T3111
RF Channel Release
RF Channel Release Ack
Clear Complete
The BSC responds to the MSC with a Clear Complete message and the release
procedure is finished.
The BTS sends the BSC an RF Channel Release Acknowledge message,
indicating that the RF channels on the radio interface are released and can be
used for re-assignment.
Upon timer T3111 expiry, the BSC sends the BTS an RF Channel Release
message.
The BTS sends the MS a UA frame. Upon receiving the UA frame, the MS
returns to the CCCH and enters the idle state.
Upon receiving the DISC frame, the BTS sends the BSC a Release Indication
message, and the BSC stops timer T3109, and starts timer T3111. The Release
Indication message indicates that the MS has released the logic channels on
the Um interface.
Upon receiving the Channel Release message, the MS disconnects the uplink
signaling link and sends the BTS a DISC frame, indicating that the logic
channels are released.
The BSC sends the BTS a Deactivate SACCH message, deactivates the associated
channels, and releases the downlink channel associated signaling links.
The BSC sends a Channel Release message and starts timer T3109. The
Channel Release message notifies the MS and the BTS of releasing the logic
channels on the Um interface.
9. Call Release – Cont.
MS BTS BSC MSC
FACCH
Disconnect
Start T305
Start T308
FACCH
Release Complete
Release
FACCH
Stop T305
Stop T308
Clear Command
Channel Release
FACCH
Start T3109
In case of network originating release procedure, the Disconnect message is
sent by the MSC instead of the MS while the Release message is sent by MS,
also timer T305 is started from network side while T308 is started from MS
side, and the rest of the procedure is the same.
…….
10. Mobile Originated Call Establishment
MS BTS BSC MSC
Immediate Assignment Procedure
Establishment Indication
(CM Service Request)
CM Service Request
CC
CM Service Accepted
DCCH
Setup
DCCH
Call Proceeding
DCCH
TCH Assignment Procedure
Channel Request
RACH
A300A:Channel
Requests (MOC)
Counters
A3030A:Call
Setup Indications
(MOC Non SMS)
The BTS sends the BSC an Establishment Indication
message. The Establishment Indication message
accurately indicates the reason for the access of an MS,
mobile originated or mobile terminating
The BSC sends a CM Service Request message to the MSC over the A
interface.
The MSC sends the BSC a Connect Confirm message
The MSC sends the MS a CM Service Accepted message on the
DCCH on the Um interface.
The MS sends a Setup message on the DCCH. The Setup message
carries all the information required on the network side for
processing the call, including the number of the called MS.
The MSC sends the calling MS a Call Proceeding message on the
DCCH on the Um interface. The Call Proceeding message indicates
that a call is being processed and the mobile originated call
processing procedure is initiated.
11. Mobile Originated Call Establishment- Cont.
MS BTS BSC MSC
TCH Assignment Procedure
Alerting
FACCH
Connect
FACCH
Connect Acknowledge
FACCH
Talking
Release Procedure
A3201:Number of
Put-Throughs
Counters
A3200:Number of
Call Alertings
The MSC sends the MS an Alerting message on the FACCH on the
Um interface, then the calling MS hears a ringing tone.
The MSC sends the calling MS a Connect message on the FACCH on
the Um interface. The Connect message is used to notify the MS
that the connection is established.
The calling MS responds to the MSC with a Connect Acknowledge
message on the FACCH
The calling and the called MSs enter the session state.
12. Mobile Terminating Call Establishment
MS BTS BSC MSC
Paging
Paging Command
Paging Request
PCH
Channel Request
RACH
Mobile Originated Call Establishment Procedure
A330:Delivered
Paging Messages
for CS Service
Counters
A300C:Channel
Requests (MTC)
the MSC sends the BSC a Paging message. The Paging message
contains the TMSI and IMSI of the called number.
BSC will then send Paging Command
to the BTS, serving either all the cells in the BSC or a specific
Location Area.
the BTS sends the paged MS a Paging Request message on the
paging sub-channel. The Paging Request message carries the IMSI or
TMSI of the paged MS.
After decoding the Paging message, if the MS finds itself the paged
MS, it sends the BTS a Channel Request message to trigger an Initial
Channel Assignment procedure.
The rest is identical to Mobile Originated Call Establishment
Procedure
13. Intra-BSC Handover Procedure
MS BTS2 BSC MSC
BTS1 MS
Measurement Report
SACCH
The MS sends BTS1 a measurement report on an SACCH on the Um
interface and BTS1 forwards the measurement report to the BSC
Measurement Result
Channel Activation
Based on the measurement report, the BSC determines whether to
hand over the MS to another cell. If a handover is required, the BSC
sends BTS2 a Channel Activation message.
Channel Activation Ack
BTS2 turns on the power amplifier on the specified channel, and
sends a Channel Activation Acknowledge message to the BSC.
Handover Command
FACCH
Start T3103
The BSC sends a Handover Command message to BTS1 and BTS1
forwards the message to the MS on the FACCH. The BSC starts
timer T3103. The Handover Command message has all the
information to be transferred on the new channel.
Handover Access
FACCH
Upon receiving the Handover Command message, the MS attempts
the access and sends BTS2 a Handover Access message on the FACCH
and starts timer T3124.
Handover Detect
Start T3124
PHY INFO
FACCH
BTS2 sends the BSC a Handover Detect message and at the same time
to the MS a PHY INFO message on FACCH. The PHY INFO message
carries the new TA calculated by BTS2. Upon receiving the PHY INFO
message, the MS stops timer T3124.
Stop T3124
When sending PHY INFO, the network starts T3105. If the timer expires
before receiving any correct frames from MS, the network resends PHY
INFO and restarts the T3105. The maximum repeated times is NY1.
Start T3105
CR334A:Channel
Activation Attempts in
Incoming Internal Inter-
Cell Handover Procedure
CR334B:CHAN ACTIV
NACK Messages Sent by
BTS in Incoming Internal
Inter-Cell Handover
Procedure
CH310:Number of
Outgoing Internal
Inter-Cell Handover
Requests
CH311:Number of
Outgoing Internal
Inter-Cell Handover
Commands
H3247:Number of Incoming
Internal Inter-Cell Handover
Detection Messages Received
by BSC (TCHF) + H3248:Number
of Incoming Internal Inter-Cell
Handover Detection Messages
Received by BSC (TCHH)
Counters
14. Intra-BSC Handover Procedure – Cont.
MS BTS2 BSC MSC
BTS1 MS
FACCH
SABM
Upon receiving the PHY INFO message, The MS sends BTS2
an SABM frame on an FACCH
Establish Indication
BTS2 sends an Establishment Indication message to notify the BSC of
radio link establishment and sends a UA frame on an FACCH to notify
the MS of radio link establishment.
FACCH
UA
FACCH
Handover Complete
The MS sends BTS 2 a Handover Complete message on the FACCH.
BTS 2 forwards the message to notify the BSC of the handover
success.
If the MS does not receive the PHY INFO message or the UA frame
from the target cell, it sends a Handover Failure message on the old
channel.
Stop T3103
Stop T3105
Handover Performed
Upon receiving the Handover Complete message, the BSC stops
timer T3103 and sends a Handover Performed message to notify
the MSC of the handover completion. The Handover Performed
message carries the handover type. At the same time, the BSC
sends an RF Channel Release message to notify BTS1 of channel
release.
CH313:Number of Successful Outgoing
Internal Inter-Cell Handovers
15. Inter-BSC Handover Procedure
MS BTS1 BSC1 BTS2
MSC BSC2 MS
Measurement Report
SACCH
Handover Required
Start T7
Handover Request
Channel Activation
Channel Activation Ack
Handover Request Ack
BSC2 sends a Handover Request Ack message to notify the MSC that
the channel is now available. The Handover Request Ack message
carries the information on the allocated handover resources and the
handover command indicating that the BSC is ready. Upon receiving the
message, the MSC stops timer T101.
Handover Command
Stop T7
The MSC sends BSC a Handover Command message and starts timer
T102 to supervise the Handover process, then BSC1 stops timer T7.
Handover Command
FACCH
BSC 1 sends the MS a Handover Command message and starts timer T8,
requesting the MS to access the new channel. The Handover Command
message carries the CI and channel type.
Start T8
Intra-BSC Handover Procedure
Handover Complete
FACCH
Handover Complete
Clear Command
The MSC sends BSC1 a Clear Command message and BSC1 stops timer T8.
Stop T8
Clear Complete
BSC releases the old channel, and sends the MSC a Clear Complete
message, indicating that the clearing is complete.
CH330:Outgoing
External Inter-Cell
Handover Requests
CH331:Outgoing
External Inter-Cell
Handover
Commands
CH333:Successful
Outgoing External
Inter-Cell
Handovers
CR335A:Channel Activation
Attempts in Incoming
External Inter-Cell
Handover Procedure
CR335B:CHAN ACTIV NACK
Messages Sent by BTS in
Incoming External Inter-Cell
Handover Procedure
BTS2 undergoes channel activation procedure similar to intra-BSC
Handover
if the MSC finds that the location area of the target cell is under the
MSC, it sends BSC 2 a Handover Request message. The Handover
Request message carries the information, transmission mode, MS
classmark, and required channel classmark of the serving and the
target cells. The MSC starts timer T101.
Start T101
Stop T101
Start T102
Stop T102
Handover procedure identical to intra-BCS Handover procedure.
Upon receiving Handover Complete message from BSC2, the MSC stops
timer T102.
BSC 1 sends the MSC a Handover Required message and starts timer
T7. This message carries the Cell Identity of the target cell group and
the serving cell, and the handover cause.
16. Intra-BSC directed retry procedure
MS BTS1 BSC MS
MSC BTS2
Immediate Assignment Procedure
Channel Request
RACH
Establishment Indication
Signaling Phase
Assignment Request
Channel Activation Ack
Channel Request
Handover Command
SDCCH
Start T3103
Intra-BCS Handover procedure
Handover Complete
Stop T3103
Assignment Complete
MS undergo normal Immediate Assignment Procedure
After acquiring a signaling channel, the MS and the MSC
exchange signaling data (CM Service Accepted, Setup, and Call
Proceeding messages)
After the Signaling Phase Is finished, the MSC sends the BSC an
Assignment Request message requesting for TCH. If no qualified
TCH is available in the serving cell, the BSC attempts to handover
the MS to a neighbor cell
When the BSC receives Handover Complete message, it sends an
Assignment Complete message to the MSC.
17. Inter-BSC directed retry procedure
MS BTS1 BSC1 MS
MSC BTS2
BSC2
Immediate Assignment Procedure
Channel Request
RACH
Establishment Indication
Signaling Phase
Assignment Request
Handover Required
MS undergo normal Immediate Assignment Procedure
After acquiring a signaling channel, the MS and the MSC
exchange signaling data (CM Service Accepted, Setup, and Call
Proceeding messages)
Handover Request
The MSC sends BSC2 a Handover Request message.
Channel Activation Ack
Channel Request
Handover Request Ack
BSC2 activates a new channel and sends a Handover Request Acknowledge
message to notify the MSC that the channel is now available.
Start T7
Handover Command
Stop T7
Handover Command
SDCCH
the MSC sends BSC1 a Handover Command message. BSC1 forwards the
Handover Command message over SDCCH to the MS.
Start T8
After the Signaling Phase Is finished, the MSC sends BSC1 an
Assignment Request message requesting for TCH. If no qualified TCH is
available in the serving cell, BSC1 sends the MSC a Handover Required
message requesting the MS to initiate an outgoing inter-BSC handover.
18. Inter-BSC directed retry procedure – Cont.
MS BTS1 BSC1 MS
MSC BTS2
BSC2
Handover Command
SDCCH
Start T8
Intra-BCS Handover procedure
FACCH
Handover Complete
Handover Complete
Upon receiving the Handover Complete message from BSC2, the MSC
sends BSC1 a Clear Command message. BSC1 initiates a local release
procedure and sends the MSC a Clear Complete message.
Clear Command
Clear Complete
Stop T8
19. Location Update Procedure
MS BTS BSC MSC
Immediate Assignment Procedure
The Immediate Assignment Procedure is initiated. The BTS sends
the BSC an Establish Indication message, containing the Location
Updating Request message
Channel Request
RACH
Establish Indication
(Location Update Request) CR (Complete Layer3
Information
The BSC establishes a connection on the A interface and sends the
MSC a Complete Layer3 Information message, containing the CGI of
the current cell
CC
Location Updating Accepted
SDCCH
The MSC responds to the MS with a Location Updating Accepted
message, indicating that the location update is successful.
If the network reject this location update, it sends a Location
Updating Rejected message to the MS.
Clear Command
Release Procedure
The MSC sends the BSC a Clear Command message to start the
resource release procedure on the network side.
Clear Complete
Counters
A300F:Channel
Requests (Location
Updating)
A3030F:Call Setup
Indications
(Location Updating)
The MSC responds to the BSC with a Connection Confirm message.
20. Timers
Timers are used to supervise messages sent
across the network
Usually the entity starting the timer is
responsible for stopping the timer
Timers can be on MS side or on network (BSS
or Core) side
Timers on core side are generally of longer
duration than corresponding MS and BSS
timers
21. Timers List
Timer Category
Network
Element
Description Start Event Stop Event Expiry Event
T3120
Call setup
procedure
MS
This timer is included in the call
request procedure which
starts with RACH request
the MS sends a
Channel Request
message to the
network
Immediate
assignment
command message is
received
Channel request is re-
transmitted on RACH for a
number of times equal to
parameter of Max
retransmissions
T3101
Call setup
procedure
BSC
Timer for the BSC waiting for
an Establish Indication
message after sending an
Immediate Assignment
message
Immediate
assignment
command message
is sent to the MS
Establish indication
message is received
by the BSC
The procedure for call setup is
aborted and the BSC releases
the seized SDCCH
T3107
Speech Channel
Assignment
BSC
Protection timer for TCH
assignment
Assignment
command is sent to
the MS
assignment complete
message is received
by the BSC
The assignment procedure is
aborted and radio resources
are released the BSC sends an
assignment failure message to
the MSC
T10
Speech Channel
Assignment
MSC
MSC supervisory timer for the
assignment procedure
The MSC sends an
Assignment Request
message to the BSS
The BSC sends an
Assignment
Complete message to
the MSC
The assignment procedure is
aborted and The MSC send a
Clear Command message to
the BSC
T11
Speech Channel
Assignment
BSC
The queue timer for
assignment
The BSC receives an
assignment request
and no channel is
available for
assignment, and the
BSC sends a Queuing
Indication message
to the MSC
The BSC sends an
Assignment
Complete message to
the MSC
the Assignment Request
message is removed from the
queue, and the BSC sends a
Clear Request message to the
MSC to clear the call
22. Timers List
Timer Category Network Element Description Start Event Stop Event Expiry Event
T305 (calling mobile) Call release MS Call disconnect protection timer
MS sends a Disconnect
message to the MSC
MS receives a Release
message
The MS sends a Release message to
the network
T308 (calling mobile) Call release MSC Call release protection timer
the MSC sends a Release
message to the MS
MSC receives a Release
Complete message
MSC retransmits the Release
message and restarts timer
T308, then sends Clear Command
T305 (called mobile) Call release MSC Call disconnect protection timer
The MSC sends a
Disconnect message
MSC receives a Release
message
The MSC sends a Release message to
the MS
T308 (called mobile) Call release MS Call release protection timer
the MS sends a Release
message to the network
MS receives a Release
Complete message
MS retransmits the Release message
and restarts timer
T308, then returns to Idle mode
T3109 Call release BSC
Channel release period, Its value
should be large enough to ensure that
the MS detects a radio link failure
BSC sends a Channel
Release message to the
MS
the BSC receives a
Release Indication
message from the BTS
The BSC sends an RF Channel Release
message to the BTS
T3111 Call release BSC
Channel deactivation delay used to
ensure that the radio link layer is
disconnected after the radio link layer
at the Air interface is released
The BSC receives a
Release Indication
message
There is no condition for
stopping this timer
The BSC sends an RF Channel Release
message to the BTS
T3103 Handover BSC Handover command protection timer
the BSC sends a
Handover Command
message to the MS
The BSC receives
Handover Complete
message from the new
cell or Handover Failure
message from the old cell
The BSC consider the MS lost and
release the radio resources in both
cells
T3124 Handover MS
The time for MS to receive the
physical information send by the
network
the MS sends the
Handover Access
message to target BTS
The MS receives a
physical information
message
The MS attempts to return to source
BTS then Then the MS sends the
Handover Failure message from the
old channel
T3105 Handover BSC
Duration for sending physical
information to MS
The network sends
physical information
message
The target BTS receives a
valid response from MS
The network resends physical
information, The maximum repeated
times is indicated by Ny1
T7 Handover BSC
Timer for waiting for
acknowledgment for Handover
request
The source BSC sends a
handover request to the
MSC
The source BSC receives
the Handover Command
message from MSC
Terminates the Handover Attempt
and the BSC imposes penalty on the
target external cell
T8 Handover BSC External handover protective time
The source BSC sends a
handover command to
the MS
The source BSC receives
Clear Command message
or Handover Failure
message from MSC
The source BSC releases the old
channel
T101 Handover MSC
Timer to supervise External Handover
request response
The MSC sends Handover
Request message to
target BSC
The MSC recieves
Handover Request Ack
from target BSC
The Handover request is resent, then
another cell is selected for handover
T102 Handover MSC
Timer to supervise external handover
completion
The MSC sends source
BSC a Handover
Command message
The MSC recieves
Handover Complete
message from target BSC
The MSC sends a handover failure
message to source BSC
Editor's Notes
RACH
AGCH btetb3t 3ala el SDCCH or TCH
Some msgs need acknowledgments
CCCH
DCCH
