A Scheduling Technique for Bandwidth Allocation in Wireless Personal Communication Networks

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A Scheduling Technique for Bandwidth Allocation
in Wireless Personal Communication Networks
Nikos Passas , Nikos Loukas , and Lazaros Merakos
Nikos Passas , Nikos Loukas , and Lazaros
Merakos Future generation wireless personal
communication networks (PCN) are expected to
provide multimedia capable wireless extensions of
fixed ATM/B-ISDN.
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Paper presents A method for transmission
scheduling in PCN (similar to the technique of
virtual leaky bucket developed for fixed ATM
networks) Introduces two alternative priority
mechanisms for the sharing of the available
bandwidth
Goals Fair and efficient treatment of various
types of traffic on the air interface
Supporting two kinds of sources ? Constant -
bit - rate (CBR) voice and ? Variable - bit -
rate (VBR) video ensuring that
bandwidth allocation is consistent with
their declarations at connection
setup with different traffic characteristics
and service requirements
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The PCN terminal of the future ?will be able to
integrate voice, video and data services ?will
have to coexist with fixed ATM / B-ISDN

• Design objectives
• ? flexible multiservice capability (voice, data
  and multimedia)
• ? good QoS for many service types
• ? compatibility with future ATM/B-ISDN networks
• ? low terminal cost/complexity/power consumption
  and
• ? efficient, scalable and moderate cost network
  architecture
• An important system design issue in PCN is the
  selection of a suitable bandwidth sharing
  technique

4
Advanced techniques must be employed
including ?Call Admission Control (CAC) (PCN user
requirements and available resources are of the
same nature as in fixed ATM) ? Bandwidth
enforcement and sharing mechanisms (must be
incorporated in the medium access control (MAC)
protocol of the wireless environment) Limitation
of the radio medium make efficiency and fairness
of such techniques more critical than in fixed
ATM To avoid inconsistencies and provide a
common platform to users, regardless of their
connection point, it is essential to consider
compatibility with future fixed ATM networks
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Cellular environment ? each cell consists
of one BS and a number of MSs ? the number of
MSs in cell changes dynamically as they move
from one cell to another Sources ? voice
sources transmitting at constant rate when they
are active, and ? video sources transmitting
at variable rate

• MSs can be thought of as advanced mobile
  telephones (e.g., videophones) equipped with
  micro-cameras and mini displays, capable for
  voice-video applications

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Operate on Uplink channel ?
from MSs to the BS of their cell
different bands Downlink channel ? from BS
to MSs

• Uplink channel multiple access control protocol
  used in conjunction with the scheduling technique
• Downlink channel is not a multiple access channel
• The access control protocol used for controlling
  transmissions on the uplink channel not only has
  to enable MSs to share it efficiently with high
  statistical multiplexing gain, but also to
  provide MSs with QoS guarantees similar to those
  in fixed ATM networks.

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QoS guarantees are accomplished through the
combined use of ?an appropriate connection
admission control (CAC) scheme, which ensures
that no new MS connections are admitted if by
doing so the QoS of already existing connections
cannot be guaranteed, and ? A scheduler,
located at the BS , which is responsible for
allocating the uplink channel to the MSs in
accordance with the QoS agreed upon admission.
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Framework of the uplink access control protocol
within which the scheduler will operate

• Uplink channel is organized as a TDMA-based
  system
• Each cell is a hub-based system since all
  communications between MSs are done through their
  BS (the hub)
• Channel time is subdivided into fixed length
  TDMA slot frames.
• Slots in each frame are dynamically allocated
  by the BS to the MSs on the basis of transmission
  requests received from active MSs during the
  previous frame, and the QoS agreed upon at
  connection setup.
• ?Each TDMA frame is subdivided into Nr request
  slots and Nd data slots (Nr, Nd assumed constant
  - in general they may vary depending on the
  number and the kind of active sources)
• ? The length of the data slots is selected to
  be equal to an ATM cell (48 bytes data, 5 bytes
  header), plus an additional radio - specific
  header, which depends on the specific physical
  and MAC layer protocols used on the radio
  interface

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Request slots in one uplink frame? are used
by the MS sources to inform the BS about the data
slots they need in the next uplink frame? are
expected to be short, compared to data slots,
since the only information they have to include
is the sources ID and the number of the
requestsed data slots? Nr request slots per
frame are shared by the active MSs, in accordance
with a random access protocol (e.g., the slotted
ALOHA protocol , or the stack protocol)? Nr can
be chosen large enough so that the probability of
an allocation request being transmitted
successfully on its first attempt is close to
unity, without substantial overhead
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Requests and allocation of data slots ?A
source transmits its allocation request for the
next frame to the BS in a request slot, and
waits for an acknowledgement on the downlink
before the beginning of the next frame?If a
collision occurs, the source will not receive the
acknowledgement and, if its request does not
correspond to a packet that has already expired,
it will attempt to retransmit it in the next
frame? After receiving all the request slots of
a frame, the BS must decide on how to allocate
the Nd data slots of the next uplink frame
?Before the beginning of the next frame , BS
sends an allocation acknowledgement to all
sources , notifying them about the slots that
they have been assigned
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Voice data traffic is considered CBR and is given
priority over VBR video traffic? Requests from
voice sources are satisfied first without
competition from video traffic requests? For
requests that come from video sources a mechanism
similar to the leaky bucket and the virtual leaky
bucket is used. ? In order to enter a fixed ATM
network, an ATM cell must first obtain a token
from a token pool. ? A token pool for each
video source is located at the BS.? If there are
no available tokens, in the leaky bucket, the
cell must wait until a new token is generated .
?Tokens are generated at a fixed rate equal to
the mean cell rate of the source ? The size of
the token pool depends on the burstiness of the
source ? The state of each pool gives an
indication about how much of the declared
bandwidth, the corresponding source has consumed
at any instance of time

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Difference between the leaky bucket and the
virtual leaky bucketIn the virtual leaky bucket
when the pool is empty, an arriving cell, rather
than waiting (as in the leaky bucket) is
permitted to enter the network with violation tag
in its header. Violating cells are the first to
be discarded if they later arrive at a congested
network node.A major difference between the
leaky bucket and the mechanism of this paper when
each source has its own connection line with the
network, ? is that in TDMA system the traffic
from all sources is multiplexed in a common radio
channel ?all requesting packets cannot enter the
network, at least not immediately.

• The acceptance rate of video sources in the
  channel is limited to the number of slots per
  frame minus those slots dedicated to the voice
  sources.
• ? A priority mechanism must be introduced, to
  decide how the available channel capacity will be
  allocated to the competing requests from
  different video sources.
• ? The unaccepted requests will have to wait until
  their priority becomes higher or until they
  become expired.

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The paper introduces two mechanisms, which are
based on the state of the token pools and the
current requests from all sources. The main
objectives of these mechanisms are ?to
guarantee fair treatment of all sources under
heavy traffic conditions, based on declarations
at connection setup, and ?to permit sources to
transmit over their negotiated throughput, when
capacity is available

• Priority Mechanism A
• ? is based on the philosophy that the source
  which has more tokens compared to its requests
  has higher priority, since it is below its
  declarations, and therefore the system should try
  to satisfy its requests as soon as possible.
• Si ? source
• Pi ? the state that the token pool of Si will be
  in , if all of its requests are satisfied
• Ti ? the number of tokens in the pool of Si at
  the time a request slot from source Si arrives
• Ri ? the number of requests declared in that slot
• Pi Ti - Ri

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Let assume that M sources have requested slots
for the next frame , with priorities P1, P2, .,
PM and let P1 ? P2 ? . ? PM The mechanism
will first try to allocate slots in the next
frame for all requests of source S1, since it has
the highest priority. When all requests of source
S1 are satisfied and if there are still available
slots in the next frame, source S2 will be
selected , then S3 and so on, until the requests
of all sources are satisfied, or until all the
available slots of the next frame have been
allocated.In case the priorities of some sources
are equal , the source with the most requests is
serviced first.

• Example
• If for source Sk and Sl,
• Pk Pl and RkgtRl
• the mechanism will first allocate slots for all
  requests of source Sk and then for all requests
  of source Sl.
• In the special case where Pk Pl and Rk Rl
  (leading to TkTl),
• the mechanism randomly chooses one source to
  service first.

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Priority Mechanism A, seems reasonable since
it is based on both negotiations made at
connection setup, expressed by the token pools,
and current needs, expressed by the request slots
of each source. A possible weakness is that ,
when a source becomes active after a long idle
period, it will probably take all the slots it
requests, since its token pool is almost full,
resulting in many temporary denials for other
sources.

• Priority Mechanism B
• Tries to solve the abovementioned problem of
  Priority Mechanism A , by gradually allocating
  slots, based on the state of the token pool of
  each source.
• The available slots of the frame are spread to
  more sources avoiding abrupt denials , which can
  affect the QoS offered to the end user.
• Let S1, S2, . SM be the sources requesting slots
  in one frame and T1 ?T2 ?. ?TM the
  corresponding tokens. The mechanism starts by
  gradually allocating T1 - T2 slots to source S1
  (with the assumption that there are that many
  requests and available slots). If T1 T2 no
  slots are allocated at this state. Then, it
  allocates T2 - T3 slots to source S1, and T2 - T3
  slots to source S2 (allocating one slot at a time
  to sources S1 and S2 in a round robin fashion),
  T3 - T4 slots to source S1, S2 and S3 and so on,
  until all requests are satisfied, or until all
  the available slots of the next frame have been
  allocated.
• 
  continue

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For every slot allocated to a source , the
corresponding token variable is decremented by
one ? ensures the fair treatment of all sources
since, even if a source is assigned many slots in
one frame, it will have lower priority in the
following ones.

• Results
• In both mechanisms no request is blocked if slots
  are available. Even if a source s priority
  (mechanism A) or token variable (mechanism B) is
  negative, available slots are allocated to the
  source, according to mechanisms A and B .
• The proposed technique is more similar to the
  virtual leaky bucket method, than of the leaky
  bucket.
• Simulation Model
• Channel speed C 1,92 Mb/sec
• Frame length L 12 msec
• Data slot size 53 bytes (48 bytes payload)
  ? to fit an ATM cell
• A frame can contain

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The length of request slots was set to 12 bits,
? 6 bits for the sources ID, and ? 6 bits for
the number of requests

• There are two kind of sources
• ?CBR voice sources, producing 32 Kb/sec (1
  slot/frame) on active state
• ?VBR video sources, with mean rate µ 128 Kb/sec,
  peak rate 512 Kb/sec, deviation 64 Kb/sec
  and autocovariance C(T) 2 e-aT (a3.9 sec
  -1)
• To model the traffic from video sources
  independent discrete-time batch Markov arrival
  process (D-BMAP) was used.
• Time-of-expiry
• ? for both voice and video packets was chosen to
  be between 2 and 3 frames
• In all examples
• ? the number of voice sources was equal to the
  number of video sources, since we have to do with
  MSs as videophones, each having one voice and one
  video source.

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Ploss is ? long term average fraction of
packets lost (due to time-of-expiry violation)
from all sources combinedThe two mechanisms
induce the same Ploss since the total number of
slots allocated per frame is the same (in both) ,
and packets are lost if the corresponding
requests are not granted in the next frame.

• Equivalent bandwidth is
• ? a unified metric representing the effective
  bandwidth of a connection based on its parameters
  declared at connection setup
• For Example
• With Ploss 10 -3 and the previous mentioned
  parameters
• Equivalent bandwidth 349.44 Kb/sec or 10.92
  slots/frame
• For 5 active sources (CBR) ? 53-5 48 slots
• 48/10.92 4,39 video sources (VBR)

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The utilization of the available bandwidth was
found identical for both mechanisms, because ?
no request blocking is performed when slots are
available

• How lost packets are spread in time ?
• The variance of denials of source Si is
  considered as the variance in time of
• the number of requests that are denied.
• Di,k ?the number of slots requested by Si to be
  allocated in frame k but denied by the scheduler
  due to slot unavailability
• Di(n) ? the mean variance of Di,k
• V Di(n) ? the sample variance of Di,k

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Priority Mechanism B results in milder variations
of denials compared to those of Priority
Mechanism A. This is because Mechanism B tries
to spread the slots of a frame to more sources
than Mechanism A . Smaller denials can more
easily absorbed by the end user. Large denials
of Mechanism A can result in temporary
degradation in quality, which can be rather
annoying to the end user

• A promising idea towards combining the two
  mechanisms is a method that gradually allocates
  slots for each source as in mechanism B and uses
  the priorities in Mechanism A.