802 QoS Architectures
Osama Aboul-Magd
Nortel
Outline
• QoS Components and Architectures
– Reservation model
– Differentiating model
• IEEE 802 QoS Models
–
–
–
–
–
IEEE 802.1 QoS
IEEE 802.11 QoS
IEEE 802.15 QoS (no data is provided)
IEEE 802.16 QoS
IEEE 802.17 QoS
• Inter-working Model
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QoS Components
Policy
Server
Control
Plane
Data
Path
classification
signaling
Routing
Outpout
I/F
Traffic
Conditioning
Scheduling
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Admission
Control
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Shaping
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QoS Architectures
• Reservation Model
– Require signaling for resource reservation
• Require per session state
– Usually includes defined services
– Possible to support applications with stringent requirements
– Examples include ATM QoS, IP Integrated Services (Intserv)
• Differentiating Model
– Different treatment of frames based on information carried in the
frame header
– Engineering is the main tool for assuring application
performance
– Examples includes IP Differentiated Services (Diffserv)
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Reservation Model
Request
Message
Response
Message
• Request message includes elements related to session
identification, service class, and performance objectives
• Response message indicates acceptance or rejection of
the request
• The signaling protocol can be a “soft” or a “hard” state
protocol
– Soft state requires refreshing the state from time to time
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Reservation Model Examples
• IETF Integrated Service (IntServ)
– Uses RSVP (soft state) for signaling
– Two services are defined
• Guaranteed Service (GS): provides mathematical upper bound on
packet delays
• Controlled Load Service (CLS): the service offered is the same as
that seen by best-effort service on a lightly loaded netwok
• ATM Forum Traffic Management
– Uses PNNI (hard state) for signaling and routing
– A number of service categories are defined. Among them:
• Constant Bit Rate (CBR)
• Variable Bit Rate (VBR)  both real-time and non-real-time
• Unspecified Bit Rate (UBR) similar to best effort service
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Differentiating Model
PHB1
PHB2
• No end-to-end signaling is required
• Only edge switches need to maintain per flow state ad possibly
perform policing and/or shaping
• Core switches need only to forward packets according to their per
hop behavior (PHB) information in the frame header.
– No per-flow state allows scalability to a large number of flows
• End-to-end services are constructed by combing edge rules and
nodal behavior
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Differentiating Model Examples
• IP Differentiated Service (Diffserv)
– PHB information is carried in the IP header (ToS byte
in IPv4 or class field in IPv6)
– A number of PHBs were specified and standardized
by the IETF
• Expedited Forward (EF): ensure a limit on the time a packet
stays in the queue
• Assured forwarding (AF):
• Class Selector (CS): introduced to support legacy routers.
Introduces 8 straight priority levels
• Ethernet (802.1) User Priority bits
– Introduces 8 straight priority levels
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IEEE 802.1 QoS
• Follows a differentiated model with no resource
reservation
• 3 User priority bits were introduced in 1999(??)
as part of 802.1Q Tag.
– Provides up to 8 straight priority levels similar to the
differentiated service class selector PHB.
• Recently augmented (802.1ad) to support drop
precedence in a number of possible ways
– 7x1, 6x2, and 5x3 (five transmission classes with 3
discard levels)
– Brings 802.1 QoS very close to IP differentiated
services
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Possible Implementation
E-EF
Ethernet Frame
E-AF2x
Class-based or Weighted
Fair Queuing (WFQ)
Scheduler
E-AF1x
PHB = PSC + Drop Precedence
P-bits
PHB
PSC
Link
Drop
Prece
dence
111
EF
EF
Low
110
AF41
AF4
Low
101
AF42
AF4
High
110
AF31
AF3
Low
011
AF32
AF3
High
010
AF21
AF2
Low
001
AF22
AF2
High
000
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DF
DF
High
DF
E-DiffServ Classes
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IEEE 802.11 (WLAN) QoS
• Supports both differentiating and
reservation models.
– EDCA (Enhanced Distributed Channel
Access) supports four levels (Access
Categories) of differentiated access
– HCCA (HCF Controlled Channel Access) is
centrally controlled and allocates TXOP
(Transmission Opportunities) using polling
and based on some scheduler.
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IEEE 802.11 Access Categories
(AC)
• IEEE 802.11 defines 4 Access Categories (AC) for use with EDCA.
• The priority of an AC to access the WM is determined by the
Arbitration Inter-frame Spacing AIFS[AC], and congestion window,
CWmin[AC] and CWmax[AC]
• One-to-one mapping between UP and AC
Mapping to
AC
Transmit
Queues
Per Queue
Channel Access
Function
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UP
Designation
AC
001
010
000
011
100
BK
BK
BE
BE
CL
AC_BK
AC_BK
AC_BE
AC_BE
Background
Video
101
110
VI
VO
NC
AC_VI
AC_VI
AC_V0
AC_VO
111
Background
Best Effort
Best Effort
Video
Voice
Voice
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IEEE 802.11 EDCA
AIFS[j]
AIFS[i]
DIFS/AIFS
Busy
Medium
Contention Window
DIFS
PIFS
Next Frame
SIFS
•
•
•
•
Each QoS station has a separate channel access function per AC.
Access rules are similar to those of DCF (CSMA/CA)
The TXOP duration is advertised by the AP in the EDCA Parameter Set IE.
The QoS station ensures that its transmission does not exceed the TXOP limit
–
•
A “continuation” TXOP is granted if there is a frame available for transmission that fits
in the remaining TXOP duration
–
•
Fragmentation may be employed
A “continuation” TXOP is granted to the same AC that initially won the TXOP.
Internal collisions are handled as if they were external collision.
–
The higher priority AC gains access to the WM.
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IEEE 802.11 HCCA
•
HCCA is a polling scheme that is centrally controlled by Hybrid Coordinator (HC)
–
•
TXOP are assigned by the HC to a QSTA at a regular interval and for a specified
duration
–
•
•
HC resides in the AP.
TXOP duration and frequency are determined based on Traffic Specifications (TSPEC IE)
Traffic Streams (TS) are locally identified using TSID (part of TID)
HC may generate CFP. However it is mandatory for HC to use CFP for QoS data
transfers
–
Element
ID
Controlled access phase (CAP) cab be initiated at anytime by the HC
Length
Traffic
Type
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TSID
TS
INFO
Nominal Maximum Minimum Maximum Inactivity Suspension Service
Start
Service Service
MSDU
MSDU
Interval
Interval
Time
Interval
Size
Size
Interval
Access
Direction
Policy
TSInfo
User
AggregationPriority APSD Ack Policy Schedule Rsvd
802 QoS Architectures
Minimum
Data
Rate
Mean
Data
Rate
Peak
Data
Rate
Maximum Delay
Burst
Bound
Size
Minimum
Surplus
Physical Bandwidth
Rate
Allowance
Medium
Time
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IEEE 802.11 TSPEC Procedure
(Admission Control)
QAP
QSTA
ADDTS Request (TSPEC)
ADDTS Response (TSPEC, Schedule)
Elements
ID
Length
Schedule
Info
Service
Start Time
Service
Interval
Maximum
Specification
TXOP
Interval
Duration
• The AP uses the traffic parameters to perform admission control on
the incoming request
• Service Interval is the time between two successive service periods
(SP)
– Directly related to bandwidth reserved
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IEEE 802.15 QoS
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IEEE 802.16 QoS
DownLink
UpLink
Subscriber Station
(SS)
Base Sstation
(BS)
• IEEE 802.16 MAC is connection oriented. Each
connection, upon establishment, subscribes to one of the
scheduled services
• Resource allocation, admission, and scheduling is
controlled by the base station (BS)- centralized control
architecture
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IEEE 802.16 QoS
• Scheduled services includes:
– Unsolicited Grant Services (UGS): supports services that
generates fixed units of data periodically (CBR service).
– Real-time Polling Service (rtPS): supports real-time data streams
with variable size data such as VoIP and video
– Extended rtPS: supports real-time applications such as voice
with silence suppresion.
– Non-real-time Polling Service (nrtPS): supports delay tolerant
data streams
– Best Effort: supports data stream with no requirements on
minimum service levels.
• Traffic parameters include: maximum sustained traffic
rate, minimum reserved traffic rate, maximum latency,
and tolerated jitter.
• Scheduled services and traffic parameters are similar to
those defined for ATM.
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IEEE 802.16 Scheduled Class
Summary
Class
Possible Applications
Expected
Application Traffic
Traffic Paramters
UGS
CBR Voice (no silence
submission), circuit emulation
Fixed-size packets at
fixed intervals
Sustained maximum
traffic rate
rtPS
MPEG Video
Variable-size packets at
fixed intervals
Maximum sustained
traffic rate, minimum
reserved traffic rate
Extended
rtPS
Voice with silence suppression
Variable-size packets at
fixed interval (I don’t
think this is correct)
Same as rtPS
nrtPS
Data application with minimum
rate requirements, e.g. FTP
Variable-size packets,
variable intervals
Same as rtPS
BE
Data applications with no
minimum rate requirements
Who cares
Maximum sustained
traffic rate
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IEEE 802.17 QoS
• IEEE 802.17 (RPR) defines three traffic classes, class A,
class B, and class C.
– Class A is divided into classes A0 and A1
– Class B is divided into B-CIR and B-EIR.
– Class C is best effort service
• Bandwidth allocated for A0 traffic is called reserved and
can only be used by the station holding the reservation
• Bandwidth allocated for A1 and B-CIR traffic is called
reclaimable and may be used for other transmissions
• Bandwidth reservation requests are broadcast on the
ring using topology messages
– Each station calculates how much bandwidth it can reserve.
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Some Remarks
• IEEE 802 models are not homogeneous and
follows different architectures
– IEEE 802.1 and IEEE 802.11 (EDCA) follows mainly a
differentiating model
– IEEE 802.11 (HCCA), IEEE 802.16, and IEEE 802.17
follows reservation model with defined service
classes for .16 and .17
• QoS inter-working between different IEEE 802
technologies could follow rules established for
other inter-working, e.g. between ATM service
categories and IP differentiated services.
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Inter-working Scenraio
IEEE 802.3
IP Network
IEEE 802.11
•
•
•
QoS inter-working is needed
– IEEE 802.11 <--> IEEE 802.1 (within 802 scope)
– IEEE 802.1 and IP (out of scope)
Inter-working between IEEE 802.11 (EDCA) and IEEE 802.1 is straightforward
– EDCA utilizes the same UP bits as in IEEE 802.1
Other inter-working scenarios may be more interesting, e.g. IEEE 802.11 (HCCA) to
IEEE 802.11.
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