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Mobile and Wireless Networking

Mobile and Wireless Networking. Lecture 15 Dr. Xinbing Wang. WLANs – Advantages. Buildings with large open areas, such as manufacturing plants, stock exchange trading floors, and warehouses

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Mobile and Wireless Networking

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  1. Mobile and Wireless Networking Lecture 15Dr. Xinbing Wang

  2. WLANs – Advantages • Buildings with large open areas, such as manufacturing plants, stock exchange trading floors, and warehouses • Historical buildings with insufficient twisted pair and where drilling holes for new wiring is prohibited • Small offices where installation and maintenance of wired LANs is not economical • Very flexible within the reception area • Users can access high speed multimedia applications anywhere at anytime, with easy implementation, low cost, and wide user acceptance • Generally works in industrial, scientific, and medical (ISM) band, which is un-licensed and available for public. • (Almost) no wiring difficulties (e.g. historic buildings, firewalls) Dr. Xinbing Wang

  3. Disadvantages of WLANs • Typically very low bandwidth compared to wired networks (1-10 Mbit/s) • Many proprietary solutions, especially for higher bit-rates, standards take their time. • Products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions. • Interference Problems Dr. Xinbing Wang

  4. Family of Wireless LAN (WLAN) Standards 802.11 • 802.11a - 5GHz- Ratified in 1999 • 802.11b - 11Mb 2.4GHz- ratified in 1999 • 802.11d - Additional regulatory domains • 802.11e - Quality of Service • 802.11f - Inter-Access Point Protocol (IAPP) • 802.11g - Higher Data rate (>20MBps) 2.4GHz • 802.11h - Dynamic Frequency Selection and Transmit Power Control mechanisms • 802.11i - Authentication and security • 802.11n Dr. Xinbing Wang

  5. WLANs –Current Use • Home wireless networks. • Enterprise wireless networks. • Public access. • Hospitals. • Warehouses. • Consulting and audit teams • Dynamic environments, ad agencies, etc. • Universities • Historic buildings, older buildings. • Meeting rooms. • Retail stores • Restaurants and car rental agencies • Data backup. Dr. Xinbing Wang

  6. Some Facts • By 2005, more than 1/3rd of Internet users will have Internet connectivity through a wireless enabled device (750 million users)!!! (Source: Intermarket group) • By the end of 2001, more than half of the workforce in the US uses a wireless net device – primarily cellular phones! (Source: Cahners Intat Group) • By the year 2004 revenue from wireless data will reach $34B, and by the year 2010 the number of wireless data subscribers will hit 1B!! Dr. Xinbing Wang

  7. Real Time Traffic Normal Data Traffic (Asynchronous) Point Coordination Function (PCF) MAC DistributedCoordination Function (DCF) Physical Layer (PHY) WLANs – 802.11 Protocol Architecture Dr. Xinbing Wang

  8. WLAN (802.11) Classification • Infrared (IR) LANs • An individual cell of an IR LAN is limited to a single room, since infrared light does not penetrate opaque walls. • Narrowband Microwave LANs • These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing; others use one of the unlicensed ISM bands. • Spread Spectrum LANs • In most cases these LANs operate in the ISM (industrial, scientific, and medical) bands, so no FCC licensing is required for their use in the United States. Dr. Xinbing Wang

  9. Infrared • Infrared signals used to transmit data (similar to TV remotes!) • Higher data rates possible (than spread spectrum) • Line of sight point-to-point configuration required (or reflection surface that reflects signals) • Too sensitive to obstacles, line-of-sight requirement, etc. • 850-950 nm, diffuse light (to allow point-to-multipoint communication) • 10 m maximum range with no sunlight or heat interfere Dr. Xinbing Wang

  10. Narrowband Microwave • Typically used to link two WLANs together (for example, to link WLANs in two buildings) • Microwave dishes required at both ends of link • Unlike spread spectrum which operates in the unlicensed ISM band, narrowband microwave requires FCC licensing • Exclusive license typically effective within a 17.5 mile radius Dr. Xinbing Wang

  11. Spread Spectrum • Distributed signals over multiple frequencies (to avoid eavesdropping or jamming) • Frequency Hopping Spread Spectrum (FHSS) • Sender transmits over a seemingly random series of frequencies • Intended receiver aware of sequence of frequencies and hops accordingly • Allows the coexistence of multiple networks in the same area by using different hopping sequences • Direct Sequence Spread Spectrum (DSSS) • Sender transmits redundant information called “chips” between actual data bits • Intended receiver aware of spread removes redundant information accordingly • Preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s Dr. Xinbing Wang

  12. Wireless LAN MAC • CSMA as Wireless MAC? • Hidden and Exposed TerminalProblems make the use of CSMA an inefficient technique Dr. Xinbing Wang

  13. Review of CSMA/CD/CA • In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place). • Wasting medium for an entire packet time. • CSMA/CD Step 1: If the medium is idle, transmit Step 2: If the medium is busy, continue to listen until the channel is idle then transmit Step 3: If a collision is detected during transmission, cease transmitting Step 4: Wait a random amount of time and repeats the same algorithm Dr. Xinbing Wang

  14. CSMA/CD (2) T0 A begins transmission A B T0+- B begins transmission A B T0+ B detects collision A B T0+2 - A detects collision just before end of transmission A B Time ( is the propagation time) Dr. Xinbing Wang

  15. CSMA/CA • All terminals listen to the medium same as CSMA/CD. • Terminal ready to transmit senses the medium. • If medium is busy it waits until the end of current transmission. • It again waits for an additional predetermined time period DIFS (Distributed inter frame Space). • Then picks up a random number of slots (the initial value of backoff counter) within a contention window to wait before transmitting its frame. • If there are transmissions by other terminals during this time period (backoff time), the terminal freezes its counter. • It resumes count down after other terminals finish transmission + DIFS. The terminal can start its transmission when the counter reaches to zero. Dr. Xinbing Wang

  16. CSMA/CA (2) Node A’s frame Node B’s frame Time Delay: B Delay: C Nodes B & C sense the medium Nodes B responses the medium and transmits it frame Nodes C responses the medium but defers to Node B Dr. Xinbing Wang

  17. Interframe Space (IFS) Values • Short IFS (SIFS) • Shortest IFS • Used for immediate response actions • Examples: Acknowledgment (ACK), Clear to send (CTS), and Poll response • Point coordination function IFS (PIFS) • Midlength IFS • Used by centralized controller in PCF scheme when using polls • Takes precedence over normal contention traffic • Distributed coordination function IFS (DIFS) • Longest IFS • Used as minimum delay of asynchronous frames contending for access Dr. Xinbing Wang

  18. CSMA/CA (3) Contention window DIFS DIFS Medium Busy Next Frame Time Slot Defer access Backoff after defer DIFS – Distributed Inter Frame Spacing Dr. Xinbing Wang

  19. CSMA/CA with ACK • Immediate Acknowledgements from receiver upon reception of data frame without any need for sensing the medium. • ACK frame transmitted after time interval SIFS (Short Inter-Frame Space) (SIFS <DIFS) • Receiver transmits ACK without sensing the medium. • If ACK is lost, retransmission done. Dr. Xinbing Wang

  20. CSMA/CA/ACK DIFS Time Data Source SIFS ACK Destination DIFS Contention window Next Frame Other Defer access Backoff after defer SIFS – Short Inter Frame Spacing Dr. Xinbing Wang

  21. CSMA/CA with RTS/CTS • Transmitter sends an RTS (request to send) after medium has been idle for time interval more than DIFS. • Receiver responds with CTS (clear to send) after medium has been idle for SIFS. • Then Data is exchanged. • RTS/CTS is used for reserving channel for data transmission so that the collision can only occur in control message. Dr. Xinbing Wang

  22. CSMA/CA with RTS/CTS (2) DIFS SIFS Time Data RTS Source SIFS SIFS CTS ACK Destination DIFS Contention window Next Frame Other Defer access Backoff after defer Dr. Xinbing Wang

  23. IEEE 802.11: Medium Access Control • Point Coordination Function • PCF is an alternative access method implemented on top of the DCF. • The operation consists of polling with the centralized polling master (point coordinator). • The point coordinator makes use of PIFS when issuing polls. • Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses. Dr. Xinbing Wang

  24. IEEE 802.11-- Medium Access Control (2) • Point Coordination Function (Cont.) • A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic contends for access using CSMA. • The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. • When a poll issued, the polled station may respond using SIFS. • If the point coordinator receives a response, it issues another poll using PIFS. • If no response is received during the expected turnaround time, the coordinator issues a poll. Dr. Xinbing Wang

  25. 802.11 -- MAC Layer DFWMAC • Method 1:DFWMAC-DCF CSMA/CA (mandatory) • Distributed Foundation Wireless (DFW) MAC • collision avoidance via randomized “back-off“ mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) • Method 2: DFWMAC-DCF w/ RTS/CTS (optional) • avoids hidden terminal problem • Method 3: DFWMAC- PCF (optional) • access point polls terminals according to a list Dr. Xinbing Wang

  26. Method 1: DFWDCF-MAC CSMA/CA • Priorities • defined through different inter frame spaces • no guaranteed, hard priorities • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS Dr. Xinbing Wang

  27. Method 1: DFWMAC-DCF CSMA-Example DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 boe busy boe bor station4 boe bor boe busy boe bor station5 t medium not idle (frame, ack etc.) busy boe elapsed backoff time packet arrival at MAC bor residual backoff time Dr. Xinbing Wang

  28. Method 2 - DFWMAC-RTS/CTS • station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) • acknowledgement via CTS after SIFS by receiver (if ready to receive) • sender can now send data at once, acknowledgement via ACK • other stations store medium reservations distributed via RTS and CTS -- check network allocation vector (NAV) value. DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t contention defer access Dr. Xinbing Wang

  29. Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency Free 2.4 GHz ISM-band Security Limited, WEP (Wired Equivalent Privacy) insecure, SSID (Service Set Identifier) Availability Many products, many vendors Connection set-up time Connectionless/always on Quality of Service Typ. Best effort, no guarantees (unless polling is used, limited support in products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM-band (Industrial, Scientific, Medical band), no service guarantees, slow relative speed only WLAN: IEEE 802.11b Dr. Xinbing Wang

  30. Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbit/s mandatory Transmission range 100m outdoor, 10m indoor E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security Limited, WEP insecure, SSID Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all 802.11 products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency, no QoS WLAN: IEEE 802.11a Dr. Xinbing Wang

  31. WLAN: IEEE 802.11 – Future (grouper.ieee.org/groups/802/11) • 802.11d: Regulatory Domain Update – completed • 802.11e: MAC Enhancements – QoS – ongoing • Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol. • 802.11f: Inter-Access Point Protocol – ongoing • Establish an Inter-Access Point Protocol for data exchange via the distribution system. • 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM – ongoing • 802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing • 802.11i: Enhanced Security Mechanisms – ongoing • Enhance the current 802.11 MAC to provide improvements in security. • Study Groups • 5 GHz (harmonization ETSI/IEEE) – closed • Radio Resource Measurements – started • High Throughput – started Dr. Xinbing Wang

  32. WLANs - Challenging Issues • Relatively low data rate. • Some can achieve very high data rate, e.g., the rate of IEEE 802.11 WLANs can be as high as 11Mbps, while some products such as Bluetooth can only achieve medium speed data rate. • Lack of support for real time services. • IEEE 802.11 products, which are based on the CSMA/CA protocol, are unable to provide QoS guarantees for voice, video, and other real time services. (IEEE 802.11 working group E is still working QoS enhancement) • Interference between different types of WLANs • Lack of Interoperability between WLANs Dr. Xinbing Wang

  33. After Class • Reading materials • Chapter 13.1-13.4 • Chapter 14.1-14.2 • Exercises • What is the difference between a single-cell and a multiple-cell wireless LAN? • Describe infrared, narrowband, and spread spectrum wireless LANs • What are DCF and PCF in 802.11? • What are SIFS, PIFS, and DIFS? How they are used? • Explain the first two methods in 802.11. Dr. Xinbing Wang

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