IROISE: A New QoS Architecture for IEEE 802.

16
and IEEE 802.11e Interworking
Kamal Gakhar, Annie Gravey and Alain Leroy
Department Of Computer Science, ENST Bretagne, 29238 Brest Cedex 3, France
Email: firstname.lastname@enst-bretagne.fr Fax: +33 2 29 00 12 82

Abstract— This article proposes a new architecture, which from the project in which it is conceived. In section II we
once implemented, would help in achieving end-to-end quality discuss the mechanisms proposed in IEEE 802.11e to support
of service (QoS) requirements of an application which is being the QoS followed by the details of how IEEE 802.16 presumes
served in an interworking system of IEEE 802.16/WiMAX
and IEEE 802.11e/WiFi networks. Our approach strives at to support QoS for different traffic types in section III. Section
mapping the QoS requirements of an application originating IV introduces QoS architecture for interworking including
in IEEE 802.11e network to a serving IEEE 802.16 network a scrutiny of proposed mapping. The paper concludes with
and assuring the transfer of data having appropriate QoS back perspectives in section V. The full details of various terms
to the application in IEEE 802.11e network. We discuss how and procedures discussed in this article can be found in the
an application flow specifies its QoS requirements, either in an
IEEE 802.11e or IEEE 802.16 network and the mechanisms standards [1] [2].
that ensure that these requirements are known to the serving
network. We identify the necessary parameters, as per advice II. Q O S SUPPORT IN IEEE 802.11 E
in the standards, that could stipulate the QoS requirements for
an application depending upon traffic type it represents. We Basic IEEE 802.11a/b/g standards offer only the Best
propose the mapping of various parameters for different kinds
of flows which would ultimately make sure that an application Effort (BE) service to an application flow using the chan-
receives the QoS it requested. The resulting architecture would nel access functions like Distributed Coordination Function
work as a hybrid of two different kinds of networks. (DCF) or Point Coordination Function (PCF). However, the
IEEE 802.11e draft [1] proposes new enhanced mechanisms,
I. I NTRODUCTION which once implemented, promise to ensure good QoS to
IEEE 802.11e [1] and IEEE 802.16 [2] networks, while in an application flow depending upon its traffic category/type.
operation, employ base station(s) (BS), subscriber station(s) The first improvement is the introduction of enhanced channel
(SS), QoS access point(s) (QAP), non-access point QoS access mechanisms, namely, Enhanced Distributed Channel
station(s) (non-AP QSTA). The need for efficient interworking Access (EDCA), which is a contention-based channel access,
between IEEE 802.16 and IEEE 802.11e networks arises in enriching the existing Distributed Coordination Function, and
order to support quality of service (QoS) for delay sensitive, the Hybrid Coordination Function Controlled Channel Access
bandwidth intensive applications such as VoIP, video trans- (HCCA), a controlled channel access, which improves upon
missions, large volume FTP etc. With the introduction of Point Coordination Function. These two entities are managed
IEEE 802.11n, which promises at least 100Mbps, some time by a centralized controller called Hybrid Coordinator (HC)
soon in the future, an interworking between two standards which is a module found in QoS Access Point (QAP).
should result in true mobility for users demanding good QoS. The second significant proposition is the facility of traffic
However, QoS for these applications can only be maintained differentiation via utilizing Traffic Specification (TSPEC).
when the serving network somehow knows and understands A TSPEC describes the traffic characteristics and the QoS
the requirements of the requesting application(s) and the im- requirements of a traffic stream (TS) to and from a non-
plied architecture could assure that the application would get AP QSTA (a STA that supports the QoS facility but is not
the best possible service. The WiMAX forum envisages this an AP). The frame format of a TSPEC element gives us an
interworking in the not so distant future [3]. A detailed study idea about traffic and QoS specifications, as in Fig. 1. The
had been done regarding interworking between HiperLAN/2 relevant details of these parameters have been discussed in the
and HiperMAN and extending this approach to IEEE 802.11 draft standard [1] as well as in [5] [6]. It offers a means of
networks [4]. However, it fell short of purposing any final admission control and reservation signaling for an application
architecture which, in the end, could support end-to-end QoS flow at MAC level between mobile terminals and the AP in a
for an application being served in an interworking system network. Admission control is performed by the HC, included
between IEEE 802.16 and IEEE 802.11 networks. In this in QAP. It serves to administer policy or regulate the available
paper, we introduce a new QoS architecture that aims at bandwidth resources. It is also needed when a QSTA desires
supporting this interworking. It is named IROISE, inspired guarantee on the amount of time that it can access the channel.
 As per the draft standard, there are two ways in which QoS Element ID Length TS Info Nominal Maximum Minimum Service
MSDU Size MSDU Size Interval
can be characterized:
1 1 3 2 2 4
• prioritized QoS: The service provisioning is such that
the MAC protocol data units (MPDUs) with higher
Maximum Service Inactivity Suspension Service Start Minimum Data
access category/priority are treated with preference over Interval Interval Interval Time Rate
MPDUs with a lower priority. This provisioning is 4 4 4 4 4
provided through the EDCA mechanism which, in turn,
provides aggregate QoS service. Peak Burst Delay Minimum Surplus Bandwidth Medium
Data Size Bound PHY Allowance Time
• parameterized QoS: MPDUs are treated depending Rate Rate
upon the parameters associated with them. It is mainly 4 4 4 4 2 2
provided through the HCCA mechanism but may also
be provided by the EDCA mechanism when used with Fig. 1. Traffic Specification Element Format
a TSPEC for admission control. The applications are
provided with par-flow QoS service.
In order to provide QoS support in an IEEE 802.11e resource reservation within an HC and is also responsible
network the following phases are passed by: for its scheduling policy. The Traffic Specification allows a
more extensive set of parameters than may be needed, or
• Computing a TSPEC according to a given application
may be available, for any particular instance of parameterized
flow ( traffic and QoS requirements).
QoS traffic (thus remains implementation dependent). It also
• Setting up a network facility (i.e setup of a traffic stream
allows other parameters to be specified that are associated
(TS)).
with the traffic stream, such as traffic classifier and Ack
• Handling the MPDUs according to whatever has been
policy. TSPECs are constructed at the station management
negotiated during the setup phase.
entity (SME), from application requirements supplied via
Lets see these steps in detail. the SME, and with information specific to the MAC layer.
Although the construction process of a TSPEC is beyond
A. Setup Process
the standard’s specification some “Admissible” TSPECs are
Setting up a TS (Fig. 2), a virtual connection, is the basic discussed in the standard to facilitate the admission control
process to ensure that the QoS requirements of an application process. This represents a set of necessary parameters in order
are entertained. TS is a set of MSDUs to be transferred subject for TSPEC to be admitted. However it is not sufficient to
to the QoS requirements of an application flow to the MAC. guarantee TSPEC admittance, which depends upon channel
The non-AP QSTA SME (station management entity) decides conditions and other factors. The complete table can be
that a TS needs to be created for an application flow and referred to in the draft [1].
assigns it a traffic stream identity (TSID). The SME gen-
erates an MLME-ADDTS.request (MAC layer management B. QoS Traffic Handling
entity request) containing a TSPEC. A TSPEC may also be
generated autonomously by the MAC without any initiation The QoS Control field in the MAC frame format (Fig. 3)
by the SME. The SME in the HC decides whether to accept facilitates the description of QoS requirements of a particular
the TSPEC as requested or not, or to suggest an alternative application flow. It is a 4-bit field that identifies the traffic
TSPEC and sends its response to the requesting non-QAP category (TC) or TS to which a frame belongs and various
STA. Once the request for TS setup is accepted, a traffic other QoS-related information about the frame that varies
stream is created, identified within the non-AP QSTA by the by frame type and subtype. The bits 0-3 are used as traffic
TSID and a direction assigned to it. In the HC at QAP, the identifier (TID). TID is a value used by higher-layer entities to
same TS is identified by a combination of TSID, direction and distinguish MSDUs to MAC entities that support QoS within
non-AP QSTA address. The TSID is assigned to an MSDU the MAC data service. There are 16 possible TID values, 8 of
in the layers above the MAC in the QAP containing the HC. them identify TCs (0-7) and the other 8 identify parameterized
Once traffic arrives at QAP, a traffic classification (TCLAS) TSs (8-15) and are assigned traffic stream identities (TSIDs).
specifies certain parameters to identify the MSDUs belonging The TID is assigned to an MSDU in the layers above the
to a particular TS. The classification is performed above the MAC.
MAC SAP at a QAP. The QAP uses the parameters in the 1) QoS using EDCF: Mapping of user priorities (UPs) in
TCLAS elements to filter the MSDUs belonging to a TS so EDCF is shown in Fig. 4. Each QSTA has 4 queues (ACs)
that they can be delivered with the QoS parameters that have and supports 8 UPs as defined in 802.1D [7]. These priorities
been set up for the TS. Traffic classification could also take vary from 0 to 7 and are identical to IEEE 802.1D priority
place at non-AP QSTA with multiple streams, however, it is tags. An MSDU with a particular UP is said to belong to a
beyond the scope of the draft standard. TSPEC coordinates TC with that UP.
 non-QAP STA non-QAP HC MAC HC SME Upto 8 User Priorities (UPs) per QoS-enhanced STA (QSTA)
SME MAC
Mapping 8 User Prioities to 4 Access Categories (ACs)

MLME-ADDTS.request
ADDTS QoS Action
Request
AC0 AC1 AC2 AC3

MLME-ADDTS.indication

ADDTS timer Backoff Backoff Backoff Backoff

MLME-ADDTS.response AIFSN0 AIFSN1 AIFSN2 AIFSN3
CWmin0 CWmin1 CWmin2 CWmin3
ADDTS QoS Action CWmax0 CWmax1 CWmax2 CWmax3
Response

Scheduler (Virtual collisions are resolved by granting TXOP to highest priority)

MLME-ADDTS.confirm

Transmission Attempt

Fig. 2. TS Setup in IEEE 802.11e Fig. 4. Mapping by EDCF in IEEE 802.11e

Octets: 2 2 6 6 6 2 6 2 0-23424 4
base station (BS) and a subscriber station (SS) MAC peers for
Frame Duration Sequence Frame
Address 1 Address 2 Address 3 Address 4 QoS Control FCS transporting a service flow’s traffic. Each traffic flow is a part
Control ID Control Body of some service flow. When a subscriber station is introduced
in a network it is assigned up to three dedicated connection
Fig. 3. The 802.11e MAC Frame identities (CIDs) for the purpose of sending and receiving
control messages. These are namely, Basic (short, time-urgent
MAC management messages), Primary (longer and delay-
2) QoS using HCF: Designed for parameterized QoS sup- tolerant management messages) and Secondary (optional) (de-
port, HCF can start the controlled channel access mechanism lay tolerant, standards-based like DHCP, TFTP) management
in both contention-free period (CFP) and contention-period connections, reflecting the fact there are inherently three
(CP) intervals. During the CP, a new contention-free period different levels of QoS for management traffic between an
named controlled access phase (CAP) is introduced which SS and the BS.
is the combination of several intervals during which frames
are transmitted using HCF-controlled channel access (HCCA)
A. Connection Setup for QoS
mechanisms. The QAP scheduler computes the duration
of polled-TXOP (transmission opportunity) for each QSTA The connection setup is the first crucial step to ensure
based upon the TSPEC parameters of an application flow. QoS as it paves the way to transfer the QoS requirements
The scheduler in each QSTA then allocates the TXOP for for different application flows that are being assigned to
different TS queues according to priority order. Similar to some service flow. The QoS parameter negotiations for an
the process as in Fig. 4, frames with TID values from 8 to application flow, before it enters the MAC SSCS, are done
15 are mapped into eight TS queues using HCF controlled by the network management entity, the details of which are
channel access rules. The reason for seperating TS queues beyond the scope of the standard (the suggested parameters
from AC queues is to support strict parameterized QoS at TS are discussed later). The BS and SS(s) provide this QoS
queues whereas prioritized QoS is supported at AC queues. according to the QoS Parameter Set negotiated by the
network management entity for the service flow. Service flows
III. Q O S SUPPORT IN IEEE 802.16
exist in both uplink and downlink directions and may also
The IEEE 802.16 MAC is connection-oriented (Fig. 5). exist without actually being activated to carry traffic. All
All traffic is carried on connection(s), even for flows of service flows (provisioned ) have a 32-bit service flow ID
connectionless protocols, such as IP. MAC layer functioning (SFID) whereas admitted and active service flows also have
is divided in three parts: Service Specific Convergence Sub- a 16-bit connection ID (CID), which in particular, is provided
layer (SSCS), MAC Common Part Sublayer (MAC CPS), and by the BS. Once AdmittedQoSParamSet is accepted by
Security Sublayer. For the implementation of the convergence the Authorization Module at the BS, a service flow becomes
sublayer two standard options are available, namely, ATM active and is provided a CID which is then used (in addition
convergence sublayer, and Packet convergence sublayer (con- with SFID) for further transfer of data. As discussed by
sidered here). As defined in the standard, a service flow (SF) the authors [8] the management messages make it possible
is a unidirectional flow of MAC service data units (MDUs) on to setup a connection and are demonstrated in Fig. 6. In
a connection that is provided a particular QoS. On the other fact, dynamic connection changes as well as deletion of a
hand, a connection is a unidirectional mapping between the connection follows the same cycle.
 Requesting Side Responding Side
CS SAP

Service Specific Convergence CS CPS CPS CS

Sublayer (CS)

MAC_CREATE_CONNECTION.
MAC SAP
request (1)
DSA-REQ (2) MAC_CREATE_CONNECTION.
MAC Common Part Sublayer indication (3)
(MAC CPS)
MAC
CID is assigned by the BS (4)

Security Sublayer
MAC_CREATE_CONNECTION.
response (5)
PHY SAP
MAC_CREATE_CONNECTION. DSA-RSP (6)
confirmation
Physical Layer
PHY
(PHY) DSA-ACK (7)

Data/Control Plane management messages

Fig. 5. The MAC in IEEE802.16 Fig. 6. Connection Setup

TABLE I
B. QoS Assurance and Mechanisms M ANDATORY Q O S PARAMETERS FOR T RAFFIC C ATEGORIES

The principal method for assuring QoS is to associate

packets entering the MAC SSCS to a service flow as identified UGS Maximum Sustained Traffic Rate, Maximum Latency,
by its SFID and CID. The SSCS performs classification of an Tolerated Jitter, and Request/Transmission Policy
incoming MAC SDU and associates it either to an existing rtPS Minimum Reserved Traffic Rate, Maximum Sus-
service flow or demands creation of a new connection for tained Traffic Rate, Maximum Latency, and Re-
quest/Transmission Policy
transmission between MAC peers. Several classifiers may
nrtPS Minimum Reserved Traffic Rate, Maximum Sus-
each refer to the same service flow (as many application
tained Traffic Rate,, Traffic Priority, and Re-
flows may be associated to a service flow). The classifier quest/Transmission Policy
priority is used for the ordering of classifiers to packets. BE Maximum Sustained Traffic Rate, Traffic Priority, and
Uplink classifiers are applied at the SS whereas downlink Request/Transmission Policy
classification is done at the BS. The MAC CPS facilitates
the core functionality of system access, bandwidth allocation,
connection establishment and connection maintenance. To
facilitate different types of traffic, four scheduling services in achieving QoS requirements for an application. However,
are discussed in the standard. These are: (a) Unsolicited this similitude does not mean a direct conversion of traffic
Grant Service (UGS) for service flows that generate fixed category from one system into another and vice-versa.
size data packets on a periodic basis, like, VoIP without Our proposition for a hybrid architecture can be seen
silence suppression. (b) Real-Time Polling Service (rtPS) in Fig. 7. The radio gateway (RG), as perceived, works
to support service flows that generate variable data packets as a Subscriber Station for the IEEE 802.16 network and
on a periodic basis, like, MPEG video, and probably VoIP also as a QAP for the IEEE 802.11e network. In order
with silence suppression. (c) Non-Real Time Polling Service to address the goals set for the project work (providing
(nrtPS) that handles service flows that require variable size real-time audio/video, audio/video on demand, precious data
data grant burst types on a regular basis, like, high bandwidth transfer) we have identified the following traffic classes which
FTP. (d) Best Effort (BE) for the rest of data. could be made up of various traffic parameters found in
The QoS for the above discussed traffic categories is the drafts/standards. These classes are worked out depending
attributed through QoS parameters. The service flow manage- upon a traffic type and its QoS requirements and should not
ment encodings, as found in the standard [2], designate a list be conflicted with categorized traffic services in Tab. I
of parameters associated with uplink/downlink scheduling for • CBR with Real-Time Traffic (C1): Applications like
a service flow. A list of mandatory QoS parameters associated real-time audio/video fall into this class. The desirable
with each traffic category can be seen in Tab. I. characteristics for this class are very limited packet
losses, minimum latency delays and very little jitter.
IV. I NTERWORKING : Q O S A RCHITECTURE AND M APPING
• VBR with Real-Time Traffic (C2): Examples of traffic
In this section we address the “matching” between traffic for this class include video on demand (streaming) and
parameters as found in IEEE 802.16 and in IEEE 802.11e variable rate VoIP. Again, packet losses, minimum la-
systems. In IEEE 802.16, we deal with various application tency delays and jitter limits apply to such traffic though
flows by handling them via various scheduling services as their values could be more tolerable compared to those
in Tab. I. In IEEE 802.11e, the access mechanisms help of class C1.
 Radio Gateway (RG)
802.16
Radio Gateway T
R
SS
S
MM
MM Subscriber
Station (SS)

802.11e Mapping
Table
QAP
BS non-AP QSTA QoS Access
Point (QAP)

R T
T: Transmitter
S: Scheduler
R: Receiver
S

Fig. 8. RadioGateway supporting QoS

non-AP QSTA non-AP QSTA
BS: Base Station
SS: Subscriber Station lier) requesting QoS for an application flow is executed by
QAP: QoS Access Point the SS module present on the RG. As discussed in [2],
non-AP QSTA: QoS Stations
MM: Mapping Module
the QoS requirements for an application flow can be sent
in MAC CREATE SERVICE FLOW.request along with the
Fig. 7. IROISE: The Proposed Architecture scheduling required. However, whether the request is served
or not depends upon the resources available to the BS.
Similarly, for the downlink, once the SS receives an appli-
• VBR with Precious Data (C3): This class addresses cation flow it is forwarded to the MM. The MM identifies
traffic type like large data files. However in this case the incoming flow with its SFID and associates it with the
traffic characteristics are more delay tolerant with a need corresponding TID that it received with the request from a
of minimum packet loss. non-AP QSTA. This mapping between SFID and TID would
• Unspecified Type (C4): This class contains simple best then be used until the completion of data transmission for
effort type traffic such as web access, emails etc. So the an application flow. Obviously, during this whole process we
traffic is purely BE. will need to buffer the incoming traffic at the RG being used.
We now discuss the proposed mapping in detail. For this
Lets zoom-in on the radio gateway as in Fig. 8. The QAP sort of mapping to work the traffic characteristics pertaining
module, after receiving a request from a non-AP QSTA, for- to a class, as seen in the mapping table, in one system (say
wards the traffic identifier (TID) of an application flow along IEEE 802.11e) should be interchangeable with the similar
with the priorities/parameters that convey QoS requirements traffic characteristics in the other system (say IEEE 802.16).
of an application to the mapping module (MM). The MM
• Both Maximum Sustained Traffic Rate and Peak Data
then maps the incoming traffic parameters to the ones that are
Rate specify the peak information rate of the service in
supported for an IEEE 802.16 application flow. Based upon
bits per second. They do not include MAC overhead such
the traffic priorities discussed in an IEEE 802.11e network
as MAC and PHY headers.
(first 8-bits of TID) as well as the traffic classes (per-flow
• Maximum Latency and Delay Bound asserts the max-
traffic characteristics) worked out above, we propose two
imum latency periods within their respective networks,
different kinds of mappings. The first kind of mapping is
representing a service commitment, starting at the time
“prioritized mapping”. In this mapping, the traffic priorities
of arrival of a packet at the local MAC sublayer till
for an application flow, as in 802.1D [7], coming from a
the time of successful transmission of the MSDU to its
WiFi network are mapped to the corresponding traffic class
destination.
in an IEEE 802.16 network and vice-versa. The second
• The following terms are used in the following equations:
kind of mapping is per-flow “parameterized mapping” as
D : Delay, max D : Delay Bound, Dq : “Queueing”
illustrated in Fig. 9. It depends upon optional/mandatory
Delay, Dt : Transmission Delay, J : Tolerated Jitter. We
traffic parameter requirements for an application flow though
consider that Dq includes all types of delay (buffering,
more optional parameters (found in the drafts/standards) could
scheduling, retransmission) except transmission delay.
be used depending upon the technical and/or the financial re-
Note that max D and data rate are not independent and
quirements of a network. However the handling for these two
also Dt is proportional to data rate. Indeed, we observe
kinds of mappings remain MM implementation dependent.
that:
Following this mapping the whole process of connec-
tion setup in an IEEE 802.16 network (as discussed ear- D = Dq + Dt , min D ≥ min Dq + min Dt
 IEEE 802.11e IEEE 802.16 vice flows identical in all QoS parameters except priority.
Traffic Class C1 Traffic Class C1
However for class C3/C4 type traffic from an IEEE
802.11e network, traffic priority is mapped onto by user
Peak Data Rate Maximum Sustained Traffic priority (UP) assigned to an application flow. So UP
Rate
Delay Bound Maximum Latency and Traffic Priority play a similar role when it comes
to mapping.
( DataRate + Delay Tolerated Jitter
Bound)
• Burst Size specifies the maximum burst of the MSDUs
belonging to a TS which arrives at the MAC SAP at peak
Traffic Class C2 Traffic Class C2
rate. Maximum Traffic Burst describes the maximum
Minimum Data Rate Minimum Reserved Traffic continuous burst the system should accommodate for a
Rate service. It also assumes that the service is not currently
Peak Data Rate Maximum Sustained Traffic
Rate using any of its available resources i.e. the instant when
Delay Bound Maximum Latency an MSDU arrives at MAC.
Burst Size Maximum Traffic Burst
V. C ONCLUSION AND P ERSPECTIVE
Traffic Class C3 Traffic Class C3 The proposed mapping may evolve further in details but
Minimum Data Rate Minimum Reserved Traffic
is not the only factor that will count when it comes to
Rate ensure QoS. The process of establishing data transmission
Peak Data Rate Maximum Sustained Traffic including buffering, proposed mapping , setting up of a
Rate
User Priority Traffic Priority new connection etc. will take some initial “setup” time. A
synchronization should be ensured between the arrival of data
Burst Size Maximum Traffic Burst
at the RG and transmission opportunity (TxOP) available to
Traffic Class C4 Traffic Class C4 QAP module. That will largely depend upon the behavior of
the mapping module (MM) which ensures the mapping. The
Peak Data Rate Maximum Sustained Traffic
Rate
role of MM is multifold: It has to ensure the integrity of the
User Priority Traffic Priority incoming and the outgoing traffic (in either direction). The
scheduling policy for the traffic inside the RG has to make
Fig. 9. Parameterized Mapping sure that application flows are channeled to the corresponding
connections (real/virtual). Two possible outlooks could be
either to do this in an aggregate or per-flow handling of
traffic. Besides the traffic handling inside the RG, scheduling
and max D ≤ max Dq + max Dt policies within the individual networks should ensure that
Also, we can say that QoS sensitive applications get served in time along with the
bandwidth constraints which in turn would also depends upon
max Dq ≤ max D − min Dt and min Dq = 0 the dimensioning of such a system. These concerns are under
study and are subject to further research.
Jitter for an application can be defined as:
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J = max D − min D
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