ETSI TS 103 325 V1.1.1 (2015-04)

Smart Body Area Network (SmartBAN); Low Complexity Medium Access Control (MAC) for SmartBAN

DTS/SmartBAN-005

General Information

• Status: Published
• Publication Date: 27-Apr-2015
• Technical Committee: SmartBAN - Smart Body Area Network
• Current Stage: 12 - Completion
• Due Date: 12-May-2015
• Completion Date: 28-Apr-2015

Standards Content (Sample)

TECHNICAL SPECIFICATION
Smart Body Area Network (SmartBAN);
Low Complexity Medium Access Control (MAC) for SmartBAN

2 ETSI TS 103 325 V1.1.1 (2015-04)

Reference
DTS/SmartBAN-005
Keywords
health, interface, interoperability, protocol
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3 ETSI TS 103 325 V1.1.1 (2015-04)
Contents
Intellectual Property Rights . 6
Foreword . 6
Modal verbs terminology . 6
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definitions, symbols and abbreviations . 8
3.1 Definitions . 8
3.2 Symbols . 9
3.3 Abbreviations . 9
4 Introduction and Background . 10
5 General MAC Framework . 10
5.0 Different device types . 10
5.1 Frequency Spectrum . 11
5.2 Channel Format . 11
5.2.1 Control Channel (CCH) Format . 11
5.2.2 Data Channel (DCH) Format . 11
5.2.2.1 Scheduled Access Slot Structure . 12
5.2.2.2 Control and Management Slot Structure . 13
5.2.2.3 Multi-use Access Slot Structure . 13
5.3 User Priorities . 14
5.4 Node IDs . 14
5.5 Information Units . 14
6 Frame Formats . 15
6.1 MAC General Frame Format. 15
6.1.0 General description . 15
6.1.1 MAC Header . 16
6.1.1.0 MAC Header Format. 16
6.1.1.1 Frame Control . 16
6.1.1.1.0 Frame Control Format . 16
6.1.1.1.1 Protocol Version . 16
6.1.1.1.2 Acknowledgement (ACK) Policy . 16
6.1.1.1.3 Frame Type . 16
6.1.1.1.4 Frame Subtype . 17
6.1.1.1.5 Sequence Number . 17
6.1.1.1.6 Fragment Number . 17
6.1.1.1.7 Non-final Fragment . 17
6.1.1.1.8 Command Acknowledgement . 18
6.1.1.2 Recipient ID . 18
6.1.1.3 Sender ID . 18
6.1.1.4 BAN ID . 18
6.1.1.5 FCS . 18
6.1.2 MAC Frame Body . 18
6.1.3 Frame Parity. 19
6.2 Management Frames . 19
6.2.1 Control Channel Beacon (C-Beacon) . 19
6.2.1.0 Control Channel Beacon Frame Format . 19
6.2.1.1 Hub Address . 19
6.2.1.2 Slot Length . 19
6.2.1.3 Time Slots . 20
6.2.1.4 Interference Mitigation . 20
6.2.1.5 Duty Cycling . 20
6.2.1.6 DCH Channel Number . 20
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4 ETSI TS 103 325 V1.1.1 (2015-04)
6.2.1.7 Initial State . 20
6.2.1.8 Time Stamp . 20
6.2.2 Data Channel Beacon (D-Beacon) . 20
6.2.2.0 Data Channel Beacon Frame Format . 20
6.2.2.1 Hub Address . 21
6.2.2.2 Inter-Beacon Interval . 21
6.2.2.3 Control and Management Period Start Slot . 21
6.2.2.4 Inactive Period Start Slot . 21
6.2.2.5 Function Indicator . 22
6.2.2.5.0 Function Indicator field . 22
6.2.2.5.1 Downlink Data Indicator . 22
6.2.2.5.2 Slot Reassignment Indicator . 22
6.2.2.5.3 Channel Migration Indicator . 22
6.2.2.6 Multi-use Access . 22
6.2.2.7 Time Stamp . 22
6.2.2.8 Downlink/Slot Reassignment (D/SR) List . 22
6.2.2.9 Slot Reassignment Timing . 22
6.2.2.10 Channel Migration Information . 22
6.2.2.10.1 Migration Timing . 22
6.2.2.10.2 Channel Number . 23
6.2.3 Connection Request (C-Req) . 23
6.2.3.0 Connection Request Frame Format . 23
6.2.3.1 Recipient Address . 23
6.2.3.2 Sender Address . 23
6.2.3.3 Multi-use Access Capability . 23
6.2.3.4 PHY Capability . 24
6.2.3.5 Requested Wakeup Phase . 24
6.2.3.6 Requested Wakeup Period . 24
6.2.3.7 Uplink Allocation Request . 24
6.2.3.8 Downlink Allocation Request . 24
6.2.4 Connection Assignment (C-Ass) . 24
6.2.4.0 Connection Assignment Frame Format . 24
6.2.4.1 Recipient Address . 25
6.2.4.2 Node ID . 25
6.2.4.3 Assigned Wakeup Phase . 25
6.2.4.4 Assigned Wakeup Period . 25
6.2.4.5 Uplink Assignment . 25
6.2.4.6 Downlink Assignment . 26
6.2.5 Slot Reassignment (S-Ras) . 26
6.2.6 Disconnection Request (D-Req) . 26
6.2.6.0 Disconnection request command . 26
6.2.6.1 Length . 26
6.2.6.2 Recipient ID . 26
6.2.7 Disconnection Response (D-Res) . 26
6.3 Control Frames (C-Frame) . 27
6.3.1 Acknowledgement/Negative Acknowledge Frame (ACK/NACK) . 27
6.4 Data Frames (D-Frame) . 27
7 MAC Functions . 27
7.1 General . 27
7.2 SmartBAN Creation and Connection Initialization . 27
7.2.1 SmartBAN Creation . 27
7.2.2 Connection Initialization . 27
7.3 Channel Access . 28
7.3.1 Scheduled Channel Access . 28
7.3.1.0 Scheduled Channel Access . 28
7.3.1.1 Starting Scheduled Access . 28
7.3.1.2 Using Scheduled Access . 28
7.3.1.3 Modifying Scheduled Access . 29
7.3.1.4 Ending Scheduled Access . 29
7.3.2 Slotted Aloha Channel Access . 29
7.3.2.0 Description . 29
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5 ETSI TS 103 325 V1.1.1 (2015-04)
7.3.2.1 Starting Slotted Aloha Channel Access. 29
7.3.2.2 Using Slotted Aloha Channel Access . 29
7.3.2.3 Ending Slotted Aloha Channel Access . 30
7.3.3 Multi-use Channel Access . 30
7.3.3.0 Description . 30
7.3.3.1 Starting Multi-use Channel Access . 30
7.3.3.1.0 Description . 30
7.3.3.1.1 Backoff Algorithm . 30
7.3.3.1.2 Re-use Channel Access (RCA) . 30
7.3.3.1.3 Priority Channel Access (PCA) . 31
7.3.3.2 Using Multi-use Channel Access . 31
7.3.3.2.0 Description . 31
7.3.3.2.1 Scheduled Access Period . 31
7.3.3.2.2 Control and Management Period . 31
7.3.3.3 Ending Multi-use Channel Access . 31
7.4 Supplementary Downlink Data Transmission . 32
7.5 Slot Re-assignment . 32
7.6 Data Channel Migration . 33
8 MAC Parameters . 34
Annex A (normative): Multi-use Channel Access . 35
History . 36

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6 ETSI TS 103 325 V1.1.1 (2015-04)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://ipr.etsi.org).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This Technical Specification (TS) has been produced by ETSI Technical Committee Smart Body Area Network
(SmartBAN).
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
ETSI
7 ETSI TS 103 325 V1.1.1 (2015-04)
1 Scope
The present document specifies low complexity medium access control (MAC) for SmartBAN.
The present document applies to short range, wireless communication between wearable sensors devices and the hub
coordinator. The present document specifies MAC protocol designed to facilitate spectrum sharing with other devices.
The specification describes:
• Channel Structure.
• MAC Frame Formats.
• MAC functions.
The devices are capable of operating in all or any part of the frequency band shown in Table 1.
Table 1: Industrial, Scientific and Medical (ISM) frequency band
Direction of Transmission Industrial, Scientific and Medical (ISM) frequency band
Transmit / Receive 2,4 GHz to 2,4835 GHz

2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
reference document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI TS 103 326 (V1.1.1) (2015-03): "Smart Body Area Network (SmartBan); Enhanced Ultra-
Low Power Physical Layer".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
reference document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] ETSI EN 300 328-1 (V1.3.1) (2001-12): "Electromagnetic compatibility and Radio spectrum
Matters (ERM); Wideband Transmission systems; Data transmission equipment operating in the
2,4 GHz ISM band and using spread spectrum modulation techniques; Part 1: Technical
characteristics and test conditions".
[i.2] IEEE™ Std. 802.15.6-2012: "IEEE Standard for Local and metropolitan area networks - Part 15.6:
Wireless Body Area Networks".
ETSI
8 ETSI TS 103 325 V1.1.1 (2015-04)
[i.3] IEEE™ Std. 802.15.4-2011: "IEEE Standard for Local and metropolitan area networks -
Part 15:4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specification for
Low-Rate Wireless Personal Area Networks".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
active period: period within the superframe period that is ready for frame reception and transmission
active state: internal power management state that is ready for the frame reception and transmission
allocation: one or more time intervals that a node or a hub obtains using an access method for initiating one or more
frame transactions
beacon: frame transmitted by a hub to facilitate network management, such as the coordination of medium access and
power management of the nodes in the SmartBAN, and to facilitate clock synchronization therein
beacon period: duration when a beacon is transmitted
connection: relation between a node and a hub in a body area network (BAN), substantiated by an identification
assigned to the node by the hub and by access arrangement between them
device: entity conforming to the SmartBAN medium access control and physical interface to the wireless medium
downlink: communication link for transfer of management and data traffic from a hub to a node
frame: uninterrupted sequence of octets delivered by the medium access control (MAC) sublayer to the physical (PHY)
layer, or vice versa, within a node or a hub
hub: entity that possesses a node's functionality and coordinated the medium access and power management of the
nodes in the SmartBAN
inactive period: period in time following an active transmission sequence during which the equipment does not
transmit or receive
medical device: any instrument, apparatus, appliance, software, material or other article, whether used alone or in
combination, together with any accessories, including the software intended by its manufacturer to be used specifically
for diagnostic and/or therapeutic purposes and necessary for its proper application, intended by the manufacturer to be
used for human beings for the purpose of:
• diagnosis, prevention, monitoring, treatment or alleviation of disease,
• diagnosis, monitoring, treatment, alleviation of or compensation for an injury or handicap,
• investigation, replacement or modification of the anatomy or of a physiological process,
• control of conception,
and which does not achieve its principal intended action in or on the human body by pharmacological, immunological
or metabolic means, but which may be assisted in its function by such means
multi-use channel access mode: mode of operation where the slot structure during the scheduled and control and
management periods is accessible by multiple different priorities based on a temporal order
node: entity conforming to the SmartBAN medium access control and physical interface to the wireless medium
operating frequency: frequency at which the equipment can be operated
priority channel access: highest priority access during multi-use channel access
re-use channel access: lowest priority access during multi-use channel access enables re-use of scheduled but not
utilized slots
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9 ETSI TS 103 325 V1.1.1 (2015-04)
scheduled access: one or more scheduled reoccurring time intervals that a node and a hub obtains using scheduled
access for initiating frame transactions
NOTE: A scheduled allocation is an uplink or downlink allocation suitable for servicing high or low duty cycle
periodic or quasi-periodic traffic on a committed schedule.
star network: logical network partition comprising a hub and zero or more nodes whose medium access and power
management are coordinated by the hub
uplink: communication link for transfer of management and data traffic from a node to a hub
3.2 Symbols
For the purposes of the present document, the following symbols apply:
× Mathematical multiplication of the term immediately preceding the symbol and the term
immediately following the symbol
CP Maximum Contention Probability
max
CP Minimum Contention Probability
min
GHz Gigahertz
L Number of time slots in Inter-Beacon Interval
D
L Length of MAC Frame Body (bits)
F
MHz Megahertz
N Number of time slots in Control and Management Period
CM
N Number of time slots in Schedule Period
S
T Interval between control channel beacons
C
T Inter-Beacon Interval
D
T Total duration of sensing period in Multi-use Channel Access
MUA
T Duration of a time slot
S
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
ACK Acknowledgement
BAN Body Area Network
C-Ass Connection Assignment
C-Beacon Control Channel Beacon
CCH Control Channel
C-Frame Control Frame
CRC Cyclic Redundancy Check
C-Req Connection Request
D/SR Downlink/Slot Reassignment
D-Beacon Data Channel Beacon
DCH Data Channel
D-Frame Data Frame
DL Downlink
D-Req Disconnection Request
D-Res Disconnection Response
EUI-48™ Extended Unique Identifier-48 bits
FCS Frame Check Sequence
FEC Forward Error Correction
IFS Inter-Frame Spacing
IM Information Module
ISM Industrial, Scientific and Medical
IU Information Units
MAC Medium Access Control
NACK Negative Acknowledgement
NID Node ID
PHY Physical Layer
REP Repetition Coding
Rx Receive
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10 ETSI TS 103 325 V1.1.1 (2015-04)
S-Ras Slot Reassignment
Tx Transmit
UL Uplink
4 Introduction and Background
Modern medical and health monitoring equipment are moving towards the trend of wireless connectivity between the
data collection or control centre and the medical devices or sensors. Therefore, the need for a standardized
communication interface and protocol between the actors are required. This network of actors performing some medical
monitoring or functions is called a Smart Body Area Network (SmartBAN).
A SmartBAN is a simple, low complexity, low energy communication network that allows wireless connectivity
between the devices and a hub. The distinct features of the SmartBAN are ease of access, minimal listening, reliable
data transfer, provision of additional control messages (in the form of C-Beacons) for the low duty cycling nodes while
maintaining simple and flexible protocol. SmartBAN also has provision for multi-use channel access mechanism for
emergency and other high priority access and for improved channel utilization.
The distinct characteristics of the SmartBAN are:
i) asymmetrical relation between hub and the device, where the hub performs most scheduling and computations;
ii) minimized listening period for the node;
iii) additional provisioning of beaconing messages and thus reliable and enhanced connectivity.
5 General MAC Framework
5.0 Different device types
This clause provides the basic MAC framework for the nodes and hubs.
Two different device types can participate in SmartBAN: medical sensor device (node) and coordinator device (hub). A
hub is a device that acts as a SmartBAN coordinator. A node is any device that acts as an information source or an
information sink. One hub and at least one node constitute a SmartBAN.
A SmartBAN shall be organized into a star topology consisting of at least one node communicating directly with the
hub.
Figure 1: SmartBAN Topology
The hub and nodes shall communicate using communication media known as channels. A SmartBAN shall use two
different channel entities to enable communication between the hub and nodes. The channel entities are assigned the
following names:
• Data channel (DCH)
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11 ETSI TS 103 325 V1.1.1 (2015-04)
• Control channel (CCH)
Each SmartBAN shall utilize one control channel (CCH) and one data channel (DCH) at any one time.
5.1 Frequency Spectrum
The frequency of operation shall fall within 2 401 - 2 481 MHz. The channels shall be arranged in blocks of 2 MHz
with centre frequencies:
f = 2 402 + 2×n MHz, where n = 0 to 39.
c
The channels are categorized into data and control channels with:
• 3 Control channels, where control frames (in the form of Control Channel Beacon) from the hub is transmitted;
• 37 Data channels, where data, control, and management frames are transmitted.
The list of channels can be found in ETSI TS 103 326 [1].
5.2 Channel Format
5.2.1 Control Channel (CCH) Format
Only hub devices shall transmit on control channels. A hub shall select one control channel from the list of control
channels in ETSI TS 103 326 [1], Table 1 and transmit one control beacon frame (C-Beacon) on the chosen control
channel (CCH) every T seconds. The format of the C-Beacon is set out in clause 6.1.
C
.
T
C
Figure 2: Control Channel
5.2.2 Data Channel (DCH) Format
5.2.2.0 Data channel description
Both hub and node devices may transmit on the data channels. A hub shall select one data channel from the list of data
channels in ETSI TS 103 326 [1], Table 1 on which both hub and node devices in the associated SmartBAN may
transmit. For any SmartBAN, the data channel is partitioned into time intervals of T seconds, known as the Inter-
D
Beacon Interval. The boundaries of each Inter-Beacon Interval shall be marked by the transmission of a data channel
beacon (D-Beacon). A hub shall transmit a D-Beacon at the beginning of each Inter-Beacon Interval.
Each Inter-Beacon Interval shall be partitioned into L distinct time epochs known as slots. The duration of each time
D
slot is T . The duration of each Inter-Beacon Interval shall be L × T seconds. Any device transmitting in a time slot
S D S
shall ensure that the transmission takes place within the duration of that time slot.
Each Inter-Beacon Interval shall consist of four distinct periods:
• Beacon Period, consisting of one single time slot, where the D-Beacon frame shall be transmitted by the hub.
No nodes shall transmit in this period;
• Scheduled Period, consisting of N time slots, where scheduled transmissions and acknowledgements occur;
S
• Control and Management Period, consisting of N time slots, where unscheduled access, and management
CM
and control signalling occur;
ETSI
C-Beacon
C-Beacon
12 ETSI TS 103 325 V1.1.1 (2015-04)
• Inactive Period, where no transmission occurs.
The time slots shall be identified by a 10 bit sequence denoting the position of the time slot in an Inter-Beacon Interval.
The Beacon Period, consisting of 1 time slot shall have the sequence number 0000000000. Subsequent time slots shall
have sequence numbers incremented by the number of time slots following the Beacon Period.
Control and
Management Period
Scheduled Access Period (N * T ) (N * T )
S S CM S Inactive Period
…
…
…
T
S
Inter-Beacon Interval (T )
D
Figure 3: Access Periods in Data Channel
The Scheduled Access Period shall begin on the slot boundary immediately following the Beacon. The Scheduled
Access Period may be of zero length, in which case, the Control and Management Period shall begin immediately
following the Beacon Period. The Control and Management Period shall begin on the slot boundary immediately
following the Scheduled Access Period. The Inactive Period shall begin on the slot boundary immediately following the
Control and Management Period.
Three types of channel access mechanisms can be used in the access periods:
• Scheduled Channel Access, in the Scheduled Access Period.
• Slotted Aloha Channel Access, in the Control and Management Period.
• Multi-use Channel Access, in both Scheduled Access and Control and Management Period.
The hub shall support all three types of channel access mechanisms. Nodes shall always support Scheduled Channel
Access and Slotted Aloha Channel Access, and may support Multi-use Channel Access. Multi-use Channel Access may
only be used when every node in the SmartBAN supports it. Each channel access mechanism shall adhere to their
respective slot structure as described in the following clause.
5.2.2.1 Scheduled Access Slot Structure
Scheduled Access Slot
Data Frame Transmission Time IFS ACK Frame Transmission Time IFS

Figure 4: Scheduled Access Slot Structure
The Scheduled Access time slots shall follow the structure as illustrated in Figure 4. Any Scheduled Access time slots
allocated by the hub shall be in the Scheduled Access Period.
The Scheduled Access slot shall consist of at most 2 transmission periods:
• Data Frame Transmission: the device allocated the time slot shall transmit;
• ACK Frame Transmission: If ACK policy of the received frame defined in clause 6.1.1.1.2 is '0' and the
transmission is successful, the receiving device shall transmit an Acknowledgement Frame. If ACK policy is
'1' and the transmission is not successful, the receiving device shall transmit a Negative Acknowledgement
(NACK) Frame. The ACK Frame Transmission period shall commence 1 IFS after the end of the Data
Transmission period and end at least 1 IFS before the end of the time slot.
ETSI
D-Beacon
D-Beacon
13 ETSI TS 103 325 V1.1.1 (2015-04)
5.2.2.2 Control and Management Slot Structure
Control and Management Slot
ACK Frame
Data/Management Frame Transmission Time IFS IFS
Transmission Time
Figure 5: Control and Management Slot Structure

The Control and Management time slots shall follow the structure as illustrated in Figure 5.
The Control and Management slot shall consist of 2 transmission periods:
• Data/Management Frame Transmission: any device wishing to transmit either data or management frames may
transmit;
• ACK Frame Transmission: If ACK policy of the received frame defined in clause 6.1.1.1.2 is '0' and the
transmission is successful, the receiving device shall transmit an Acknowledgement Frame. If ACK policy is
'1' and the transmission is not successful, the receiving device shall transmit a Negative Acknowledgement
(NACK) Frame. The ACK Frame Transmission period shall commence 1 IFS after the end of the Data
Transmission period and end at least 1 IFS before the end of the time slot.
5.2.2.3 Multi-use Access Slot Structure
Multi-use Access Slot
Sensing Data Frame Transmission Time IFS ACK Frame Transmission Time IFS
T
MUA
Figure 6: Multi-use Access Slot Structure
The Multi-use Access time slots shall follow the structure as illustrated in Figure 6. The Multi-use Access time slots
shall be used when Multi-use Access is in operation.
The Multi-use Access slot shall consist of 1 sensing period, and at most 2 transmission periods:
• Sensing; the sensing period, T , shall depend on the traffic type. The values of T are defined in Table 2,
MUA MUA
where T is the minimum sensing period as defined in clause 8. Any device wishing to transmit shall sense
sch
the channel for a period of T . If the channel is busy, they shall wait for the next available Multi-use Access
MUA
time before another attempt is made. If the channel is found to be idle, the device may commence data
transmission.
• Data Frame Transmission: any device that has sensed the channel for the sensing period above may transmit if
the channel is found to be idle.
• Acknowledgement period; the receiving device may transmit an Acknowledgement frame if the transmission
is successful, depending on the acknowledgement policy.
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14 ETSI TS 103 325 V1.1.1 (2015-04)
Table 2: Values of T
MUA
Type of User Type of traffic Access Period
TMUA
Any User Emergency traffic (User priority 3) Scheduled Access 0
Slot Owner Any traffic Scheduled Access T
sch
Non-Slot Owner Any traffic Scheduled Access 2 × T
sch
Any User Emergency traffic (User priority 3) Control & Management 0
Access
Any User Any traffic Control & Management T
sch
Access
5.3 User Priorities
Four user priority levels shall be defined in the operation of a SmartBAN. The user priorities are defined in Table 3. The
user priority shall determine the contention probability of the node in Slotted Aloha Channel Access. The range of
contention probabilities for different user priority levels is listed in Table 4.
Table 3: List of User Priorities
User Priority Data Type
0 Low priority
1 Mid priority
2 High priority
3 Very High (Emergency)
Table 4: Contention Probabilities for Different User Priorities
User Priority Contention Probability
CP CP
max min
...