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IEEE 802.16
Air Interface for Fixed Broadband Wireless Access Systems
Kwangho Kook
IEEE 802 Standard
• 802.3 : CSMA/CD (Ehernet)• 802.4 : Token Bus• 802.5 : Token Ring• 802.6 : MAN• 802.11 : Wireless LAN• 802.12 : Gigabit LAN• 802.16 : Fixed Broadband Wireless Access
System
802.3
Medium
Access
802.3
Physical
802.2 Logical Link
802.1 Bridging
802.4
Medium
Access
802.4
Physical
802.5
Medium
Access
802.5
Physical
802.6
Medium
Access
802.6
Physical
802.11
Medium
Access
802.11
Physical
802.12
Medium
Access
802.12
Physical
802.16
Medium
Access
802.16
Physical
Data
Link
Layer
Physical
Layer
Fig. 1 The relationship between the standard and other members of the family
• 802.16 consists of the access point, BS(Base Station) and SSs(Subscriber Stations)
• All data traffic goes through the BS, and the BS can control the allocation of bandwidth on the radio channel.
• 802.16 is a Bandwidth on Demand system.
IEEE 802.16
SS
SS
SS
BS
Figure 1. Wireless Access Network
IEEE 802.16 [1]
• Scope :– Specifies the air interface, MAC (Medium Access
Control), PHY(Physical layer)• Purpose :
– to enable rapid worldwide deployment of cost-effective broadband wireless access products
– to facilitate competition in broadband access by providing alternatives to wireline broadband access
• Main advantage : – fast deployment, dynamic sharing of radio resources
and low cost
• The spectrum to be used– 10 - 66 GHz licensed band
• Due to the short wavelength– Line of sight is required – Multipath is negligible
• Channels 25 or 28 MHz wide are typical• Raw data rates in excess of 120 Mbps
– 2 -11 GHz• IEEE Standards Association Project P802.16a• Approved as an IEEE standard on Jan 29, 2003
IEEE 802.16
IEEE 802.16 MAC layer function[2]• Transmission scheduling :
– Controls up and downlink transmissions so that different QoS can be provided to each user
• Admission control : – Ensures that resources to support QoS requirements of
a new flow are available• Link initialization
– Scans for a channel, synchronizes the SS with the BS, performs registration, and various security issues.
• Support for integrated voice/data connections– Provide various levels of bandwidth allocation, error
rates, delay and jitter
IEEE 802.16 MAC layer function
• Fragmentation : – Sequence number in the MAC header is used to
reassemble at the receiver
• Retransmission : – Implement an ARQ(Automatic Repeat Request)
Basic Services
• UGS(Unsolicited Grant Service)– Supports real-time service flows that generate fixed size
data packets on a periodic basis, such as T1/E1 and Voice over IP
– The BS shall provide fixed size slot at periodic intervals.
• rtPS(Real-Time Polling Service)– Supports real-time service flows that generate variable
size data packets on a periodic basis, such as MPEG video
Basic Services
• nrtPS(Non-Real-Time Polling Service)– Supports non real-time service flows that generate
variable size data packets on a regular basis, such as high bandwidth FTP.
• BE(Best Effort service)– Provides efficient service to best effort traffic
ZeroN.A.<250 msec< 1 KBTwo-wayTelnetData
ZeroN.A.<250 msec< 1 KBTwo-wayInteractive games
Data
ZeroN.A.<250 msec< 28.8 kbpsTwo-wayTelemetryTwo-way control
Data
<1% FER<150 msecPreferred <400 msecLip-sync<100 msec
32 - 384 kbpsTwo-wayVideophonevidio
<3% FER< 1msec<150 msecPreferred <400 msec
4 - 13 kbpsTwo-wayConversationalvoice
Audio
Information Loss
Delay variation
One-way Delay
Key performance parametersAnd target values
Data rateDegree ofSymmetry
ApplicationMedium
Table 1 End-user Performance Expectations – Conversational/Real-time Services
ZeroN.A.<250 msecTwo-wayInteractive gamesData
ZeroN.A.<250 msecTwo-wayTransactions Services High priority e-commerce, ATM
Data
ZeroN.A.<4 sec/pageTwo-wayWeb-browsing HTML
Data
<3% FER< 1 msec<1 secPlayback<2 secrecord
4 - 13 kbpsTwo-wayVoice messagingAudio
Information Loss
Delay variation
One-way Delay
Key performance parametersAnd target values
Data rateDegree ofSymmetry
ApplicationMedium
Table 2 End-user Performance Expectations – Interactive Services
ZeroN.A.<10 sec< 28.8 kbpsOne-wayTelemetry- monitoring
Data
ZeroN.A.<10 secOne-wayStill imageData
ZeroN.A.<10 secPrimarilyOne-way
Bulk dataTransfer/retrieval
Data
<1% FER<10 sec32 - 384 kbpsOne-wayOne-wayvidio
<1% FER< 1msec<10 sec32 - 128 kbpsPrimarilyOne-way
High qualityStreaming audio
Audio
Information Loss
Delay variation
One-way Delay
Key performance parametersAnd target values
Data rateDegree ofSymmetry
ApplicationMedium
Table 3 End-user Performance Expectations – Streaming Services
FDD based MAC protocol [3]
• Downlink – Broadcast phase : The information about uplink and
downlink structure is announced.– DL-MAP(Downlink Map)
• DL-MAP defines the access to the downlink information
– UL-MAP(Uplink Map)• UL-MAP message allocates access to the uplink channel
• Uplink– Random access area is primarily used for the initial
access but also for the signaling when the terminal has no resources allocated within the uplink phase.
MAC Frame MAC Frame MAC Frame
Broadcast Phase Downlink Phase Movable boundary
DownlinkCarrier
Uplink Carrier Uplink Phase Random Access Phase
Broadcast Reserved
Movable boundary
Reserved Contention
Figure 4. FDD based 802.16 MAC Protocol
Downlink Subframe
Uplink Subframe
DL-MAP n-1
UL-MAP n-1
Frame n-1 Frame n
Round trip delay + T_proc
Bandwidth request slots
Figure 3. Time relevance of PHY and MAC control information
802.11• Wireless LAN Medium Access Control (MAC)
and Physical Layer(PHY) Specifications• 802.11a : up to 54 Mbps in 5GHz band• 802.11b : up to 11 Mbps in 2.4GHz band• 802.11 MAC protocol supports two kinds of
access method– PCF(Point Coordinated Function)
• Based on the polling controlled by AP(Access Point)• Intended for transmission of real-time traffic as well as that of
asynchronous data traffic
– DCF(Distributed Coordinated Function)• Designed for asynchronous data transmission • Based on CSMA/CA(Carrier Sense Multiple Access with Collision
Avoidance
Beacon D1+poll
U1+ack
D2+ack +poll
U2+ack
SIFS SIFSSIFSSIFSSIFS SIFS
D3+ack +poll
CF_End
PICF
Contention free period Contention period
Contention free period repetition interval (super frame)
Figure 5. Point Coordinator Function in IEEE 802.11 Standard
Downlink/Uplink Scheduling
• Radio resources have to be scheduled according to the QoS(Quality of Service) parameters
• Downlink scheduling: – the flows are simply multiplexed – the standard scheduling algorithms can be used
• WRR(Weighted Round Robin)• VT(Virtual Time)• WFQ(Weighted Fair Queueing)• WFFQ(Worst-case Fair weighted Fair Queueing)• DRR(Deficit Round Robin)• DDRR(Distributed Deficit Round Robin)
111VCC 1 (Source 1)
22VCC 2 (Source 2)
333VCC 3 (Source 3) 3 3
2
1
3
123111 2 33333
WRR scheduler
Counter Reset Cycle
• It is an extention of round robin schedulingbased on the static weight.
WRR
VT• VT : aims to emulate the TDM(Time Division Multiplexing) system [4]
– connection 1 : reserves 50% of the link bandwidth– connection 2, 3 : reserves 20% of the link bandwidth
Connection 1 Average inter-arrival : 2 unitsConnection 1
Average inter-arrival : 2 units
Connection 2 Average inter-arrival : 5 units
Connection 3 Average inter-arrival : 5 units
First-Come-First-Served service order
Virtual times
Virtual Clock service order
WFQ and WFFQ
• FFQ(Fluid Fair Queue) : head-of-the line processor sharing service discipline
– : guaranteed rate to connection i– C : the link speed– : the set of non-empty queue– The service rate for a non-empty queue i
• WFQ : picks the first packet that would complete service in the corresponding FFQ system[4]
�i
CBj j
i
� � )(���
)(�B
• WFFQ : picks the first packet that would complete service among the set of packets that have started service in the corresponding FFQ system[4]
• Example– All packets have the same size 1 and link speed is 1
Guaranteed rate for connection 1 : 0.5Guaranteed rate for connection 2-11 : 0.05Connection 1 sends 11 back-to-back packets at time 0Connection 2-11 sends 1 packet at time 0
– The completion time of connection 1 :2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22
– The completion time of connection 2 – 11 : 20
Connection 1Connection 1
Connection 2
WFQ Service Order
Connection 11
… …
WFFQ Service Order
Figure 6. WFQ and WFFQ
VT and WFQ
• All packets are fixed size and require exactly one second to service
• Starting at time zero, 1000 packets from connection 1 arrive at a rate of 1 packet/second
• Starting at time 900, 450 packets from connection 2 arrive at a rate of 1 packet/second– The completion times of the 901, 902, 903, … packets
of connection 1 in FFQ system are 1802, 1904, 1806, …
– The completion times of the 1, 2, 3, … packets of connection 2 in FFQ system are 901, 902, 903, …
Connection 1Connection 1
Connection 2
Virtual Clock Service Order
WFQ Service Order
898 900 902 904
Figure 7. WFQ and Virtual Clock
898 900 902 904
Deficit Round Robin[5]
• Each connection is assigned a state variable called the DC(Deficit Counter).
• At the start of each round, DCi of queue i is incremented by a specific service share(quantum)
• If the length of the head of the line packet, Li, is less than or equal to DCi,, the scheduler allows the ith queue to send a packet.
• Once the transmission is completed DCi is decremented by Li.
• Deficit Round Robin Scheme
QiDCi
350035002800 7800 2000
1500
5000
700
1400
2800 7800 2000
2800 7800 2000
2800 7800 2000
2800 7800 2000
initializing(1st round)
serviced
Not serviced
serviced
(2nd round)
serviced
(3rd round)
(4th round)
Distrubuted Deficit Round Robin[6]
• Each connection is assigned a state variable called the DC(Deficit Counter)
• If the value of the DCi is positive then the scheduler allows the ith queue to send a packet.
• Once the transmission is completed DCi is decremented by Li, the length of the transmitted packet .
• At the start of the subsequent rounds, DCi is incremented by a specific service share(quantum)
• Distributed Deficit Round Robin Scheme
QiDCi
350035002800 7800 2000
1500
-6300
-2800
700
-2100
2800 7800 2000
2800 7800 2000
2800 7800 2000
2800 7800 2000
2800 7800 2000
initializing(1st round)
serviced
serviced
Not serviced
(2nd round)
Not serviced
(3rd round)
serviced
Uplink scheduling: – Responsible for the efficient and fair allocation of the
resources(time slots) in the uplink direction– Uplink carrier :
• Reserved slots• contention slots(random access slots)
– The standard scheduling algorithms can be used
Downlink/Uplink Scheduling
Bandwidth allocation and request mechanisms• The method by which the SS(Subscriber Station)
can get the bandwidth request message to the BS(Base Station)– Unicast
• When an SS is polled individually, no explicit message is transmitted to poll the SS.
• The SS is allocated, in the UP-MAP(Uplink Map), bandwidth sufficient for a bandwidth request.
– Multicast• Certain CID(Connection Identifier) are reserved for multicast
groups and for broadcast messages.• An SS belonging to the polled group may request bandwidth
during any request interval allocated to that CID in the UP-MAP
– Broadcast
Bandwidth allocation and request mechanisms
• UGS : – The BS provides fixed size bandwidth at periodic intervals
to the UGS.– The SS is prohibited from using any contention request
opportunities.– The BS shall not provide any unicast request opportunities.
• rtPS– The BS provides periodic unicast request opportunities.– The SS is prohibited from using any contention request
opportunities.
Bandwidth allocation and request mechanisms
• nrtPS– The BS provides timely unicast request opportunities.– The SS is allowed to use contention request opportunities.
• BE– The SS is allowed to use contention request opportunities.
Contention Resolution• Collisions may occur during Request intervals.• Contention resolution is based on a truncated
binary exponential backoff, with the initial backoff window and the maximum backoff window controlled by the BS.
• A truncated binary exponential backoff– The SS shall randomly select a number within its backoff
window.– This value indicates the number of contention
transmission opportunities that the SS shall defer before transmitting
– If the contention transmission fails, the SS increases its backoff window by a factor of two.
The 4Gmobile system• 4Gmobile system : Fourth-generation mobile
wireless communications• The vision of the 4Gmobile system
– Providing broadband wireless access– Providing Internet-based communications– Ensuring seamless services provisioning across a
multitude of wireless systems and networks– Providing optimum delivery of the user’s wanted service
via the most appropriate network available• IEEE 802.16e
– Air interface for Fixed and Mobile Broadband Wireless Access Systems
– Started at December 11, 2002
Future Study
• Study on the scheduling method– Downlink scheduling method– Uplink scheduling method
• Study on the relevant Fragment Size• Study on the criteria whether packing or non-
packing
References[1] IEEE Std 802.16-2001.[2] B. Larish, “The MAC layer in Broadband Wireless Access Networks,” http:
//www.eas.asu.edu/trace/eee459/Bryan%20Larish.doc[3] J. Bostie, G. Kandus, “MAC Scheduling for Fixed Broadband Wireless Access
Systems, COST263_v0_0.doc[4] Hui Zhang, “Service disciplines for guaranteed performance service in packet-
switching networks,” Proc. IEEE, vol. 83, Oct. 1995.[5] M. Shreedhar and G. Varghese, “Efficient Fair Queueing using deficit round
robin,” IEEE/ACM Transactions on Networking, Vol. 4, No. 3, June 1996, pp. 375-385.
[6] R.S. Ravindra, D. Everitt, and L.L.H. Andrew, “Fair Queueing Scheduler for IEEE 802.11 Based Wireless Multimedia Networks, http://www.ee.mu.oz.au/staff/lha/abstract/wlan_mmt99.html
[7] S. Lu, V. Bharghavan, and R. Srickant, “Fair Scheduling in Wireless Packet Networks,” IEEE Trans. on Networking, Vol. 7, No. 4 August 1999.
[8] Y. Cao and V.O.K. Li, “Scheduling Algorithms in Broad-band Wireless Networks,” Proc. IEEE, Vol. 89, No.1, January 2001, pp 76-87.
