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Technical White Paper

Advantages and Challenges of Multi-Role
Multi-Channel versus Single-Channel

Guy Mishol
Yoav Ben-Yehezkel
Uri Weinrib
ABSTRACT
Modern Wireless (Wi-Fi™ and Bluetooth®) connectivity networks is composed of several heterogeneous devices
such as internet of things (IoT) sensors, HD (high definition) streaming devices, or low latency gaming consoles.
It is common practice to functionally separate high-end devices network and low-end devices network, byusing multiple radios to reduce collisions between those network; which would be costly for an overall system
implementation.
This white paper describes how Wi-Fi multi-role capability within a single radio can enable functional networks
separation. More specifically it shows how a multi-role multi-channel solution provides an affordable way to alsoachieve collision domains separation which dramatically increases system bandwidth utilization, especially incomparison to multi-role single channel solution.
This is demonstrated by measurements done on the TI WiLink™ Wi-Fi™ and Bluetooth® combo transceivers(WL18xx), which shows up to 118% throughput improvement in a typical separated network deployment.

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Table of Contents

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Table of Contents
Acronyms................................................................................................................................................................................... 3
1 Introduction.............................................................................................................................................................................4
2 Single-Channel Versus Multi-Channel ................................................................................................................................. 5
2.1 MRSC Single Radio Network Topology..............................................................................................................................5
2.2 MRMC Single Radio Network Topology............................................................................................................................. 5
2.3 Experiment and Results .................................................................................................................................................... 6
3 Multi-Channel Challenges and Requirements......................................................................................................................9
3.1 802.11 Spec Enablers for Role Switch Protections............................................................................................................ 9
3.2 Role Switch Protection Implementations............................................................................................................................9
3.3 Single Radio Multi-Channel Challenges...........................................................................................................................10
3.4 Single Radio Multi-Channel Key Requirements............................................................................................................... 11
4 WL18xx™ Multi-Channel Implementations........................................................................................................................ 12
4.1 Arranging the Role Activity Requests by Priority..............................................................................................................12
4.2 Scheduling the Role Activities..........................................................................................................................................12
4.3 Leveraging MRMC to Improve WLAN-BT Coexistence................................................................................................... 12
5 Conclusions.......................................................................................................................................................................... 14
Appendix...................................................................................................................................................................................14

List of Figures
Figure 1-1. Multi Role Example................................................................................................................................................... 4
Figure 2-1. Multi-Role Single-Channel Network Topology........................................................................................................... 5
Figure 2-2. Multi-Role Multi-Channel Network Topology............................................................................................................. 6
Figure 2-3. Test Setup Diagram...................................................................................................................................................6
Figure 2-4. Multi-Channel Throughput Improvement...................................................................................................................8
Figure 3-1. Role Switch Protection - Role AP..............................................................................................................................9
Figure 3-2. Role Switch Protection - Role STA.......................................................................................................................... 10
Figure 4-1. High Level Scheduler Generated Plan Guidelines.................................................................................................. 12
Figure 4-2. Scheduling with WLAN-Bluetooth Coexistence.......................................................................................................13

List of Tables
Table 2-1. Multi-Channel Versus Single-Channel Results........................................................................................................... 7
Table 2-2. Multi-Channel Throughput Improvement.................................................................................................................... 7
Table 3-1. Multi-Role Multi-Channel Single-Radio Implication of Unmanaged Role Absence...................................................10

Trademarks
Wi-Fi™ is a trademark of Wi-Fi Alliance.
WiLink™, WL18xx™, and AM335x™ are trademarks of Texas Instruments.
Bluetooth® is a registered trademark of Bluetooth SIG.
All trademarks are the property of their respective owners.

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Acronyms

Acronyms
ACK

Acknowledgment

AP

Access Point

BT

BluetoothBW

BandwidthCH

ChannelCL

ClientDUT

Device Under Test

DTIM

Delivery Traffic Indication Message

GO

Group Owner

MIMO

Multiple Input Multiple Output

MRSC

Multi Role Single Channel

MRMC

Multi Role Multi Channel

P2P

Peer to Peer

PS

Power Save

SISO

Single Input Single Output

STA

StationSP

Service Period

WLAN

Wireless Local Area Network

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Introduction

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1 Introduction
Legacy WLAN implementations assume a single WLAN role operation (station, access point or any other WLAN
role). Multi Role means that several WLAN roles (two or more) are operating concurrently, on the same or evendifferent channels and/or bands.
A typical example for Multi Role can be a gateway device where one role is a WLAN station connected to homeaccess point which provides the access to the internet, and the other role is a WLAN access point connected to
one or more sensors scattered around the house. In such a system a user can access the gateway device toeasily monitor and control each one of the sensors instead of accessing one by one.

Figure 1-1. Multi Role Example
One approach is to implement concurrency using two or more completely separate radio systems (for example,
one per each role). This fits the high-end, highest-performance, and most expensive solution.
On the other hand, for more cost-effective solutions – a single radio system (for example, radio and SW
controller) manages transitions between multiple roles. With the single radio approach, the concurrency can be
implemented using time-division sharing of system resources and thus achieve a more cost-effective solution.
In such single radio solutions, the Multi Role implementation can be limited to operate all roles on the same
physical channel (“Single-Channel” Multi Role) or allow the different roles to operate on different channels(“Multi-Channel” Multi Role).
This paper presents the multi-channel approach advantages over a single-channel approach of such multirole solutions from an overall system performance point-of-view. This paper also discusses the challengesof achieving a cost-effective multi-role multi-channel solution with a single radio, and describes the different
solutions that ensure a stable, high performing and reliable system as implemented in TI WL18xx™.

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Single-Channel Versus Multi-Channel

2 Single-Channel Versus Multi-Channel
As mentioned, a Multi Role solution implements several WLAN roles operating concurrently. There are different
multi role types and each one has different properties: The single channel type limiting the WLAN roles tooperate on same band and channel (for example AP and STA operating on channel 1 on the 2.4-GHz band).
The multi-channel type, on the other hand, can avoid this by allowing the WLAN roles to operate on different
channels and even bands.
Another aspect is the amount of radios used for the multi-role solution. The multiple radios solution can achieve
full concurrency where each WLAN role gets its own resources and full radio access. The single radio solution
can achieve similar system efficiency and performance by dividing the radio access and resources in a managed
way and overcome some of the challenges.

2.1 MRSC Single Radio Network Topology
The following example will demonstrate the Multi-Role Single-Channel use case and how system efficiency issignificantly impacted when roles are operating on same channel (and band): As can be seen in the Figure 2-1
below the traffic between the IoT STAs connected to the Multi-Role device Access Point overlaps and collides
with the traffic running from the Remote Station to the Remote Access Point running on the same channel. Thus,
overall system throughput is degraded. In addition, the system’s overall power consumption is higher as theamount of re-transmissions required from all the devices is higher.

Figure 2-1. Multi-Role Single-Channel Network Topology

2.2 MRMC Single Radio Network Topology
The following example will demonstrate the Multi-Role Multi-Channel use case and how system efficiency issignificantly improved when roles are operating on different channels or bands. As can be seen in the Figure 2-2
below, the traffic does not overlap and no collisions between the IoT STAs connected to the Multi-Role device
Access Point (“Red network”) and the Remote Station connected to the Remote Access Point (“Blue network”).
Thus, overall system throughput is higher (very important for heavy data consumers). In addition, the system’s
overall power consumption is lower as the amount of re-transmissions required from all the devices is lower
compared to a single channel (very important for IoT nodes).

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Figure 2-2. Multi-Role Multi-Channel Network Topology

2.3 Experiment and Results
The test below compares the overall system efficiency (throughput) of a multi-role single-channel solution where
both of STA and AP roles are running on the same channel as the external AP network versus a multi-role
multi-channel solution where the AP role is running on a different band than the STA role and the external AP
network. Please refer to the Appendix for the test setup details. See Figure 2-3 for a diagram of the test setup.

Figure 2-3. Test Setup Diagram
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Single-Channel Versus Multi-Channel

In the multi-role single-channel case the traffic running in the Multi-Role as an AP network (red network) impacts
the traffic running in the External AP network where the External STA is also connected (blue network) and viceversa.
In the multi-role multi-channel case, the Multi-Role as an AP is operating on a different channel (and band)
compared to the External AP. This actually creates a full separation between the two networks. It means thatthe traffic running in the Multi-Role as an AP network (red network) does not impacts the traffic running in theExternal AP network (blue network) and vice versa.
The test was repeated with increasing number of remote STAs from 1 to 5. The external STA was configured tosend UDP of 50 Mbps while each Remote STA was configured to send UDP of 1 Mbps. For multi-channel versus
single-channel results, refer to Table 2-1
The throughput results of this comparison are summarized in Table 2-2 and Figure 2-4.
Table 2-1. Multi-Channel Versus Single-Channel Results
Configuration

Blue Network

Multi-Role AP(Red Network)

External AP(Blue Network)

Band

CH

Band

CH

MRSC

2.4 GHz11

2.4 GHz

MRMC

5 GHz

36

MRSC

2.4 GHzMRMC

Red Network

Bandwidth

External
STA

MultiRole
STA

Remote
STA#1

11

SISO2027.6
Mbps

1 Mbps

1.0 Mbps

2.4 GHz11

SISO2027.8
Mbps

1 Mbps

1.0 Mbps

11

2.4 GHz11

SISO2024.8
Mbps

2 Mbps

1.0 Mbps 1.0 Mbps

5 GHz

36

2.4 GHz11

SISO2027.9
Mbps

2 Mbps

1.0 Mbps 1.0 Mbps

MRSC

2.4 GHz11

2.4 GHz11

SISO2022.2
Mbps

3 Mbps

1.0 Mbps 1.0 Mbps 1.0 Mbps

MRMC

5 GHz

36

2.4 GHz11

SISO2027.5
Mbps

3 Mbps

1.0 Mbps 1.0 Mbps 1.0 Mbps

MRSC

2.4 GHz11

2.4 GHz11

SISO2018.6
Mbps

4 Mbps

1.0 Mbps 1.0 Mbps 1.0 Mbps 1.0 Mbps

MRMC

5 GHz

36

2.4 GHz11

SISO2026.0
Mbps

4 Mbps

1.0 Mbps 1.0 Mbps 1.0 Mbps 1.0 Mbps

MRSC

2.4 GHz11

2.4 GHz11

SISO2011.3
Mbps

4.6 Mbps 1.0 Mbps 1.0 Mbps 1.0 Mbps 0.6 Mbps 1.0 Mbps

MRMC

5 GHz

36

2.4 GHz11

SISO2024.7
Mbps

5 MbpsTest
Case

Remote
STA#2

Remote
STA#3

Remote
STA#4

RemoteSTA#5

1.0 Mbps 1.0 Mbps 1.0 Mbps 1.0 Mbps 1.0 Mbps

Table 2-2. Multi-Channel Throughput Improvement
Number of remote STAs

External STA throughput inSingle-Channel

External STA throughput
Multi-Channel

127.6 Mbps

27.8 Mbps

0.7%

224.8 Mbps

27.9 Mbps

12.5%

3

22.2 Mbps

27.5 Mbps

23.87%

4

18.6 Mbps

26.0 Mbps

39.78%

5

11.3 Mbps

24.7 Mbps

118.58%

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Figure 2-4. Multi-Channel Throughput Improvement
As can be seen from the results, when each network was operating on a different channel and band (multichannel) the overall throughput of the external STA was better for any number of remote STAs used. The
throughput improvement is significantly higher when number of remote STAs in increased and can get to morethan 100% with 5 remote STAs. This is directly related to the networks separation between bands which helped
to avoid the interference between the Remote STAs and the External STA even though they were not connectedto the same access point.

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Multi-Channel Challenges and Requirements

3 Multi-Channel Challenges and Requirements
Implementing a cost-effective single radio multi-role multi-channel system has many challenges and should meetsome mandatory requirements in order to overcome those challenges. This chapter will first describe basic
concepts of a multi-role system like what is role switch protection and what are the tools provided by the IEEE
802.11 specification to enable it. Then it will elaborate on how those tools are used and related to the multi-role
multi-channel challenges and requirements.

3.1 802.11 Spec Enablers for Role Switch Protections
The MRMC solution is based on time division between the roles. When these roles operate on different
channels, it means that for each role there are portions of time in which it is absent (not transmitting andunable to receive) on its operating channel. The typical WLAN operation (and in the original spirit of the 802.11
specification) the rest of the network will normally assume a device (especially an AP) is present on the channel
at all time. Therefore, these absences from the channel need to be protected, in terms of their timing and theprocedures taken in order to leave the channel.
The IEEE 802.11 WLAN specification provides several tools which can be utilized as protection mechanisms forseamlessly switching between the active roles:••

CTS-To-Self – no transmissions on channel for a period of time defined by transmitter on this CTS frame(hold peers TX allowing AP role to leave the channel)
Power Save Mode – an AP will buffer packets for a STA during PS (hold RX by others to allow STA role toleave the channel)

3.2 Role Switch Protection Implementations
3.2.1 AP Based Roles••

A CTS frame destined to our own MAC-Address is sent without an RTS frame preceding it (CTS-to-Self)
Other wireless devices in the network will refrain from accessing the medium for the duration mentioned in
the CTS frame and wait to access the medium, while in fact the AP based role is leaving the channel as
illustrated by Figure 3-1 below:

Figure 3-1. Role Switch Protection - Role AP

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3.2.2 STA Based Roles••Entering power save indicates to the remote AP to start buffering packets destined to role STA. Because ofthis, a STA can be absent from the channel for a period of time, knowing that it is not missing any packets
that it is supposed to receive, and once it returns the AP will send all buffered packets.
Role STA remains active (based on hangover learning algorithm) to handle the case where AP continue tosend packets to avoid possible rate drops or disconnection. As shown in Figure 3-2 below:

Figure 3-2. Role Switch Protection - Role STA

3.3 Single Radio Multi-Channel Challenges
The MRMC uses a time division solution between the roles and therefore it is required to manage each of therole’s absences from their channel in order to protect it. An unmanaged concurrent operation of multiple WALN
role on a single radio chip can result in an unstable and unreliable system and even expose the system todisconnections, low performance and bad user experience. For example, the implications of unmanaged AP
absence from its operating channel will cause the connected STAs to have many data transmission retries orpoll for data without receiving the ACK or data back from the AP. The system can then suffer from rate drops
or even disconnections. Similarly, new STAs might fail to connect. Another example for a challenge in MRMC
single radio is handling unmanaged STA absence from the channel. In this case, the connected AP may senddata and receive no ACK which might lead to rate drop, disconnections and lost beacons, DTIMs and multicast
data. Table 3-1 summarizes the challenges:
Table 3-1. Multi-Role Multi-Channel Single-Radio Implication of Unmanaged Role Absence
Implication / WLAN Role

Unmanaged AP absence

Unmanaged STA absence

Rate drops

Connected STAs try to send or poll for data
and receive no ACK/data results in rate drop

Connected AP try to send data and receive
no ACK results in rate drop

Disconnections

Connected STAs lost too many beacons anddisconnect from the AP

Connected AP try to send data and receive
no ACK results in disconnection

Network Integrity

New STAs fail to complete the connection
process

STA might lose channel switch indication

Low Performance10

Bi-directional traffic is lower due to multiple retries and low data rate

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Multi-Channel Challenges and Requirements

3.4 Single Radio Multi-Channel Key Requirements
The above mention challenges make any MRMC single radio system a complex one to implement, and as suchthere are some requirements which it is expected to meet in order to ensure a reliable and a stable solution. Thissection elaborates the key requirements expected from any solution to consider when coming to implement a
MRMC single radio system.
First is the role’s availability. Each one of the active roles’ availability must be handled carefully and its absencefrom the operating channel must be taken in account. As will be further elaborate in Section 3, there are someactions that can be taken by the system (some are defined by the 802.11 specification) which allows the activerole to indicate its peers he is about to leave the operating channel and avoid any of the implication describedearlier.
Second is the role’s activities timing. Each one of the active roles has its own unique timing and priority
requirements for each of its activities (for example, activity starting point, its duration, minimum time required,expiry time). The solution is required to handle multiple of those hard-timing activities and requests coming fromeach one of the roles simultaneously and asynchronously. To do that the solution might include some kind ofscheduling mechanism that is able to suspend and resume each role activities on time and thus minimize user
experience impact and keep system efficiency high.
Failing to handle the above requirements properly can cause unstable system behavior, periodic disconnectionsand very low unbalanced throughput.

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4 WL18xx™ Multi-Channel Implementations
The Texas Instruments WL18xx™™ has implementations that can address the previously mentioned challenges
and provide great performance, robustness, and stability. Some of them are mentioned in the IEEE 802.11
specification and used as is or with some modifications. Other implementations are unique.

4.1 Arranging the Role Activity Requests by Priority
One of the key methods that allows meeting the hard-timing activity requests coming from each one of the roles
is prioritization. Each activity request can be submitted at any point in time representing the rough requirementsof their activity. The priority can then be dynamically changed to increase the system reliability (for example due
to new higher priority requests). This activity prioritization is the baseline for the scheduling decision for eachrole.

4.2 Scheduling the Role Activities
Based on the up-to-date activity requests status a plan is generated (and re-generated with every new requestor based on timeout). The plan is always open for changes and the scheduler manages suspension andresumption of those activities to utilize the system as much as possible. For this, the current activity can bepre-empted by another higher priority activity and as a consequence when it resumed it might be compensated.
To suspend or resume role’s activities is not immediate and takes some time. When a new activity starts, thetime needed to complete it is computed in advance. This allows to estimate the point in time to be ready to startsuspending the current activity and be ready to resume the next one just in time. A high-level multi-role activity
scheduling scheme is presented in Figure 4-1.

Figure 4-1. High Level Scheduler Generated Plan Guidelines

4.3 Leveraging MRMC to Improve WLAN-BT Coexistence
Bluetooth is another wireless technology commonly used with WLAN. Bluetooth shares the same radio band
(2.4 GHz) with the WLAN roles and can co-exist with them making the scheduling even more complex. From ascheduling perspective, Bluetooth's role is considered equivalent to WLAN, which needs to be scheduled andprotected as well. The same principles mentioned before can applied, with some hard-timing Bluetooth activities
getting priority and bypassing the scheduler to avoid a bad user experience.
This is further illustrated in Figure 4-2 below.
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WL18xx™ Multi-Channel Implementations

Figure 4-2. Scheduling with WLAN-Bluetooth Coexistence

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Conclusions

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5 Conclusions
Multi-role solutions which allows the concurrent operation of one or more WLAN or Bluetooth® roles are
becoming more and more commonly used in many products. The multi-channel multi-role is clearly more
complex and challenging to implement than the single-channel one, however it has significant advantages over itlike higher throughput and lower power consumption.
WL18xx™ is a great implementation of multi-role multi-channel that meets all challenges and provide great
system performance. The results suggest that using the cost-effective WL18xx™ multi-role multi-channel
solution with a single radio has several advantages over the multi-role single-channel: Improved throughput forthe stations connected to the access point and improved power consumption for the entire system (not coveredin the results section)
For more information on the WL18xx™ products, TI’s Wi-Fi and Bluetooth Low Energy combo transceivers, visit
www.ti.com/wifi.

Appendix
The setup described in Section 2.3 was placed in an anechoic clean room with the following devices:••••

Multirole device is Sitara AM335x™ with WL18xx™ running R8.8 release and configured as a bridge between
the roles.
Remote STAs are Sitara AM335x™ with WL18xx™ running R8.8 release limited to 1Mbps.
External AP is Sitara AM335x™ with WL18xx™ running R8.8 release.
External STA is Linksys AE1000 dongle.

The two different network topologies are as follows:••

The blue network includes the External AP, External STA and WL18xx™ STA role.
The red network includes WL18xx™ AP role and 5 WL18xx™ STAs.

Throughput was measured using iPerf tool running UDP for 5 minutes.

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