Transcript [slides] Wireless networks

The Two Successful Domains
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Wireless networks (Cellular)
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Supports voice
Total coverage in many countries
Decreasing cost
The boon – user mobility
Wireless extension to the Internet (Wi-Fi)
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Information content
Supports multimedia services
Global penetration – millions of nodes
Decreasing cost
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IEEE 802.16 based WiMax
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LTE (Long Term Evolution)
General Problems in Wireless Networks
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Resource scarcity
– Limited bandwidth
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Unreliable wireless link
– Error prone channels (BER 10-4 to 10-3)
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Varying channel conditions
– Channel models fluctuates
In spite of all these problems, voice services are well supported.
Can it support multimedia services?
Characteristics of Multimedia Services
A picture is worth thousand words
Combination of various medium – text, audio/video, graphics
– Audio/video conferencing, shared whiteboard, surfing, email, etc.
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Varied requirements
– Low bit error rate
– High bandwidth
– Low delay
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Synchronization of multiple data types
– Proper scheduling
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Different coding schemes for different types
– Source coding
Data on Wireless Networks!
What are the Problems?
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True characterization of data traffic is yet
unknown
– Traffic modeling needs to be done
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Data services cannot tolerate bit errors
– Corrupt packets need to be recovered
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Unpredictable nature of wireless medium
– QoS provisioning becomes difficult
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Bottleneck due to the bandwidth limitation
– Proper buffering / filtering required
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No differentiated service plans for customers
– Class based services required
What is QoS?
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Specified by <bandwidth, delay, reliability>
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Ability of a network element (e.g. an application, host or router)
to have some level of assurance that its traffic and service
requirements can be satisfied
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Predictable service for the traffic from the network
e.g., CPU time, bandwidth, buffer space
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Acceptable end-to-end delay and minimum delay jitter
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What is QoE (Quality of Experience)?
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Human subjectivity associated with quality
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How happy is a user with respect to the service he gets
End-to-End QoS
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Requires cooperation of all network layers from top-to-bottom, as
well as every network element
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Knowledge of application at end points decides QoS functions
implemented at every layer of the network protocol stack
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Type of Services
- Best-effort: the Internet (lack of QoS)
- Differentiated service (soft QoS) : partial to some traffic but most
effective
- Guaranteed service (hard QoS) : absolute reservation of
resources (RSVP), more expensive
Wireless QoS Challenges
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A limited spectral bandwidth to be shared, causes interference
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Communication links are time varying, frequency selective channels
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User mobility in wireless networks makes QoS provisioning complex
because routes from source to destination cells are different, thus causing
varying packet delays and delay jitters
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Error rate of wireless channel is higher due to mobility, interference from
other media, multi-path fading. So mobile hosts may experience different
channel rates in the same or different cells
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Different applications have different requirements for bandwidth, delay,
jitter (e.g., 9.6Kbps for voice and 76.8Kbps for packetized video)
Wireless QoS: Desirable Features
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Adapt to dynamically changing network and traffic conditions
Good performance for large networks and large number of
connections (like the Internet)
Higher data rate
Modest buffer requirement
Higher capacity utilization
Low overhead in header bits/packet
Low processing overhead/packet within network and end
system
Bandwidth Requirement for
Multimedia Traffic
Application bandwidth requirements on log-scale axis in bits per second (bps)
Vertical dashed lines show the bandwidth capability of a few network technologies
Multi-rate Traffic Scenario
Base Station
C channels
Mobile Users
Real-time traffic (voice, video)
 Non real-time traffic (TCP/IP packets)
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Evolution of Wireless Data Networks
 2G wireless systems ( voice-centric, data loss unimportant)
- IS-95 CDMA, TDMA, GSM
 2.5G systems (voice and low data rate)
- CDPD, GPRS, HSCSD, IS-99 CDMA, IS-136+
- Date rates: CDPD (19.2Kbps), HSCSD (76.8Kbps), GPRS (114Kbps)
 3G proposed standards (data-centric, high data rate)
- UMTS, EDGE, W-CDMA, cdma2000, UWC 136, IMT-2000
- Data rates: EDGE (384Kbps), cdma2000 (2Mbps), W-CDMA (10Mbps)
Last Hop Communication
ISDN/PSTN/Internet
WIRELINE
NETWORK
Cell
Base Station (BS)
Mobile unit
Wireless Links
Wired Links
Mobile Switching Center (MSC)
Cellular Framework
HLR
BSC
MSC/VLR
MSC/VLRBSC
BTS
Mobile
Terminal
Air Link
Cellular Network
Local Switch
Terms to remember
MSC: Mobile Switching Center
VLR: Visiting Location Register
HLR: Home Location Register
BSC: Base Station Controller
BTS: Base Transmitter Station
Mobile Terminal
Air Link
BTS
PSTN Network
Cell: geometric representation of areas. Geographic area is divided into
cells, each serviced by an antenna called base station (BS)
Mobile Switching Center (MSC) controls several BSs and serves as
gateway to the backbone network (PSTN, ISDN, Internet)
WHY CHANNEL REUSE?
 Limited number of frequency spectrum allocated by FCC and
remarkable growth of mobile (wireless) communication users
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Frequency band allocated by FCC to the mobile telephone system is
824-849 MHz for transmission from mobiles (uplink) and 869-894 MHz
for transmission from base stations (downlink)
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With a channel spacing of 30 KHz, this frequency band can
accommodate 832 duplex channels
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Frequency Reuse: use same carrier frequency or channel at different
areas (cells) avoiding co-channel interference
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Number of simultaneous calls (capacity) greatly exceeds the total
number of frequencies (channels) allocated
Hand-off Problem
 Hand-off is the process of switching from one frequency channel to another by
the user in midst of a communication
 Normally induced by the quality of the ongoing communication channel
parameters: Received Signal Strength (RSS), Signal-to-Noise Ratio (SNR) and
Bit Error Rate (BER)
 RSS attenuates due to the distance from BS, slow fading (shadow or lognormal
fading), and fast fading (Rayleigh fading)
 Hand-offs are triggered either by the BS or the mobile station itself
BS-1
BS-2
Handoff Types
Intra-Cell
Inter-Cell
Soft Handoff
Hard Handoff
Hand-off: Who Triggers?
 The quality of the RSS from the mobile station is monitored by the BS. When the
RSS is below a certain threshold. BS instructs the mobile station to collect signal
strength measurements from neighboring BSs
 Case 1: mobile station sends the collected information to the BS.
BS conveys the signal information to its parent MSC (mobile switching center)
which selects the most suitable next BS for the mobile station
Both the selected BS and the mobile station are informed when new BS assigns
an unoccupied channel to the mobile station
 Case 2: mobile station itself selects the most suitable BS.
The mobile station informs the current BS, who conveys information about the
next BS to its MSC
The selected BS is informed by the MSC which assigns a new channel
Hand-off Policies
 BS handles hand-off requests in the same manner as originating calls
- Disadvantage: Ignores the fact an ongoing call has higher priority for a new
channel than originating calls
- Solution: Prioritize hand-off channel assignment at the expense of tolerable
increase in call blocking probability
 Guard channel concepts (Prioritizing Handoffs)
- Reserve some channels exclusively for hand-offs. Remaining channels shared
equally between hand-offs and originating calls
- For fixed assignment. Each cell has a set of guard channels. While for dynamic
assignment, channels are assigned during hand-off from a central pool
- Disadvantages:
-- Penalty in reduction of total carried traffic. Since fewer channels are available for
originating calls. Can be partially solved by queuing up blocked originating calls
-- Insufficient spectrum utilization – need to evaluate an optimum number of guard
channels.
Capacity Improvement and Interference Reduction
 There is a close correspondence between the network capacity
(expressed by N) and the interference conditions (expressed by C/I)
 Cell sectoring reduces the interference by reducing the number of co-
channel interferers that each cell is exposed to. For example, for 60
degrees sectorization, only one interferer is present, compared to 6 in
omnidirectional antennas. But, cell sectorization also splits the channel
sets into smaller groups
 Cell splitting allows to create more smaller cells. Thus, the same
number of channels is used for smaller area. For the same probability
of blocking, more users could be allocated
Cell Splitting: Example
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 Advantages: more capacity, only local redesign of the system
 Disadvantages: more hand-offs, increased interference levels, more
infrastructures