1. Mobile and Wireless Networking
Lecture 15 Dr. 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. WLANs – 802.11 Protocol Architecture
Physical Layer (PHY)
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Normal Data Traffic
(Asynchronous)
Real Time Traffic
MAC
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 Higher data rates possible (than spread spectrum)
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.
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 Terminal Problems 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) A B A B A B A B Time T0 A begins transmission 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) Medium Busy Next Frame Contention window DIFS DIFS Time
Defer access
Slot
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 Data ACK Next Frame DIFS Time Source SIFS 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
busy
medium not idle (frame, ack etc.)
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
defer access
contention
Dr. Xinbing Wang

29. WLAN: IEEE 802.11b Data rate Transmission range Frequency Security
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
Dr. Xinbing Wang

30. WLAN: IEEE 802.11a Data rate Transmission range Frequency Security
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 , , 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 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
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 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 a (DCS, TPC) – ongoing
802.11i: Enhanced Security Mechanisms – ongoing
Enhance the current 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 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 products, which are based on the CSMA/CA protocol, are unable to provide QoS guarantees for voice, video, and other real time services. (IEEE 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 Exercises Chapter 13.1-13.4
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 ?
What are SIFS, PIFS, and DIFS? How they are used?
Explain the first two methods in
Dr. Xinbing Wang