The Two Successful Domains

1. The Two Successful Domains • Wireless networks (Cellular) • Supports voice • Total coverage in many countries • Decreasing cost • The boon – user mobility • Wireless extension to the Internet (Wi-Fi) • Information content • Supports multimedia services • Global penetration – millions of nodes • Decreasing cost • IEEE 802.16 based WiMax • LTE (Long Term Evolution)

2. General Problems in Wireless Networks • Resource scarcity • Limited bandwidth • Unreliable wireless link • Error prone channels (BER 10-4 to 10-3) • Varying channel conditions • Channel models fluctuates In spite of all these problems, voice services are well supported. Can it support multimedia services?

3. 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. • Varied requirements • Low bit error rate • High bandwidth • Low delay • Synchronization of multiple data types • Proper scheduling • Different coding schemes for different types • Source coding

4. Data on Wireless Networks! What are the Problems? • True characterization of data traffic is yet unknown • Traffic modeling needs to be done • Data services cannot tolerate bit errors • Corrupt packets need to be recovered • Unpredictable nature of wireless medium • QoS provisioning becomes difficult • Bottleneck due to the bandwidth limitation • Proper buffering / filtering required • No differentiated service plans for customers • Class based services required

5. What is QoS? • Specified by <bandwidth, delay, reliability> • 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 • Predictable service for the traffic from the networke.g., CPU time, bandwidth, buffer space • Acceptable end-to-end delay and minimum delay jitter • What is QoE (Quality of Experience)? • Human subjectivity associated with quality • How happy is a user with respect to the service he gets

6. End-to-End QoS • Requires cooperation of all network layers from top-to-bottom, as well as every network element • Knowledge of application at end points decides QoS functions implemented at every layer of the network protocol stack • 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

7. Wireless QoS Challenges • A limited spectral bandwidth to be shared, causes interference • Communication links are time varying, frequency selective channels • 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 • 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 • Different applications have different requirements for bandwidth, delay, jitter (e.g., 9.6Kbps for voice and 76.8Kbps for packetized video)

8. Wireless QoS: Desirable Features • 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

9. 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

10. Base Station C channels Mobile Users Multi-rate Traffic Scenario • Real-time traffic (voice, video) • Non real-time traffic (TCP/IP packets)

11. 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)

12. WIRE-LINE NETWORK Cell Base Station (BS) Mobile unit Wireless Links Wired Links Mobile Switching Center (MSC) Last Hop Communication ISDN/PSTN/Internet

13. Cellular Framework HLR BTS Mobile Terminal BSC BSC MSC/VLR MSC/VLR BTS 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 PSTN Network

14. 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 • 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) • With a channel spacing of 30 KHz, this frequency band can accommodate 832 duplex channels • Frequency Reuse: use same carrier frequency or channel at different areas (cells) avoiding co-channel interference • Number of simultaneous calls (capacity) greatly exceeds the total number of frequencies (channels) allocated

15. 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-2 BS-1

16. Handoff Types Intra-Cell Inter-Cell Soft Handoff Hard Handoff

17. 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 stationBoth 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 MSCThe selected BS is informed by the MSC which assigns a new channel

18. 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.

19. 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

20. 2 1 3 7 6 4 5 Cell Splitting: Example 2 2 3 1 3 1 7 7 4 6 4 6 5 5 • Advantages: more capacity, only local redesign of the system • Disadvantages: more hand-offs, increased interference levels, more infrastructures