1. MITP 413: Wireless Technologies Week 8
Michael L. Honig
Department of ECE
Northwestern University
May 2004

2. Cellular Call Setup 1. Call Request 2. Send numbers to switch
3. Page Receiver
4. Request Channel/Time slot/Code

3. Cellular Call Setup (cont.)
5. Switch assigns channels
6. Cellular conversation is set up

4. Multiple Access Control (MAC)
Fixed assignment access
Each user is assigned a dedicated channel, time slot, or code
Appropriate for circuit-switched traffic, transferring long data files
Random access: users contend for access to the channel
Users may collide, losing packets.
Sometimes can negotiate rate (bandwidth, time slots, codes) and power
Widely used in wired networks
Used in wireless networks for requesting channel/time slot/code

5. ALOHA-Based Random Accessno
Packet
arrives
Base station
Sends ack
transmit
Collision?
yes
Transmitter does not
Receive ack
Packet is rescheduled
With random delay
Simple: asynchronous
Low throughput under heavy loads (maximum is 18% of incoming packets)
Slotted ALOHA
Synchronous, maximum throughput increases to 36%
Used in GSM to reserve a time slot for voice connection
Reservation ALOHA
Contention period followed by reserved message slots

6. ALOHA Protocols

7. Carrier Sense Multiple Access (CSMA)no
Packet
arrives
Sense channel
Busy?
Transmit packet
yes
Delay transmission (non-persistent)
“Listen before talk” (LBT) protocol
Collisions occur if transmitters cannot sense the other transmission (e.g., due to large propagation delay)
Lower probability of collision/higher throughput than ALOHA
Long propagation times  more collisions
ALOHA preferred for wide area applications

8. CSMA Example

9. Carrier Sensing
Nonpersistent: After sensing a busy channel, the terminal senses the channel after a random waiting period
Persistent: The terminal senses the channel until the channel becomes free.
1-Persistent: After the channel becomes free, the terminal transmits immediately.
p-Persistent: The terminal transmits with probability p.

10. Binary Exponential Backoffno
Packet
arrives
Base station
Sends ack
transmit
Collision?
yes
Transmitter does not
Receive ack
Packet is rescheduled
After R time slots
After 1st collision:
R=0 or 1 with equal probability
After 2nd collision:
R=0,1,2, or 3 with equal probability
After ith collision (i=1,…,10):
R is selected between 0 and 2i-1
R is random
Time slot X (maximum round trip delay)
Maximum of 16 retries

11. Performance
Throughput (S): Average number of successful packet transmissions per unit time.
Normalized throughput: Percentage of successful packet transmissions
Average Delay (D): Average waiting time before successful transmission
Offered Traffic (G): Number of packet transmission attempts per packet time slot – includes both new arrivals and retransmissions.
Performance depends on the propagation delay across the network relative to the packet duration.

12. Throughput vs. Offered Load

13. Delay vs. Throughput

14. CSMA with Collision Detection (CSMA/CD)
Nodes detect a collision in progress, and stop transmitting before the entire packet is transmitted.
Assumes nodes can hear each other when they are transmitting.
Appropriate for wired channels.
Problems with wireless channels:
Nodes cannot transmit and receive at the same frequency at the same time.
Not all nodes may be in range of each other.

15. Hidden Terminal Problem
Station B
Station A
Station D
Station C
Coverage area for station A.
A is transmitting to B.
C wants to transmit to D.
C may not sense A’s transmission, causing a collision at B.

16. Exposed Terminal Problem
Station B
Station A
Station D
Station C
Coverage area for station B.
B is transmitting to A.
C wants to transmit to D.
C senses B’s transmission, and does not transmit even though it would not cause interference at A.

17. Basic Problem Carrier sensing determines whether or not there
are interfering sources near the transmitter, not the receiver.

18. Solutions Busy-tone multiple access (BTMA)
Separate control channel used to indicate that the channel is idle or busy.
An active station transmits a busy tone on the control channel.
Each receiver that senses a busy tone turns on its own busy tone.
Used in ad hoc networks.
Digital or Data Sense Multiple Access (DSMA)
Used in FDD cellular mobile data networks
Forward channel periodically broadcasts a busy/idle bit for the reverse link.
Mobile transmits if bit is in idle state; base station sets bit to busy.
Not carrier sensing: sensing is performed after demodulation.
Multiple Access with Collision Avoidance (MACA)

19. MACA Protocol (RTS/CTS)
Receiver
Transmitter
Request to Send (RTS), packet length
Clear to Send (CTS), packet length
Data
Ack
Terminals receiving either an RTS or CTS must not transmit for the duration of the packet. (What if the terminal hears RTS but not CTS?)
Collision occurs if multiple nodes transmit an RTS, or the CTS is not heard due to other interference.
Collision  binary exponential back-off

20. Wireless Local Area Networks (WLANs)
Low mobility, high data rates within confined region (building or campus)
Competitive with other wireless data systems (3G, fixed wireless access)
Unlicensed bands
Industrial, Scientific, Medical (ISM): 2.4 GHz
National Information Infrastructure (UNII): 5 GHz
Must accept interference, therefore uses spread spectrum signaling, or random access with collision avoidance.

21. Overview of 802.11 Standard IEEE 802.11 2.4 GHz 850 to 950 nm FHSS
DS-SS
Diffuse IR
2 Mbps
4GFSK
1 Mbps
2GFSK
2 Mbps
DQPSK
1 Mbps
DBPSK
IEEE b
Extension
5.5 Mbps
DQPSK-CCK
11 Mbps
DQPSK-CCK

22. 802.11b vs a
802.11a
802.11b
2.4—2.485 Ghz
Interference from Microwave, Bluetooth
3 non-overlapping channels (26 MHz)
Max Speed 11 Mbps
Direct Sequence Spread Spectrum (DSSS)
CSMA/CA
Many products available
5 Ghz range
5 channels
Max Speed 54 Mbps
Orthogonal Frequency Division Multiplexing (OFDM)
CSMA/CA
Some products available (Cisco, Proxim)
Look at passive

23. Peer-to-Peer Configuration
Single Cell
Station C
Station A
A Service Set is a Collection of stations
Station B
Mobile devices are referred to as Stations.
Each Station can communicate directly with another Station.
System referred to as Independent Basic Service Set (IBSS)

24. Infrastructure Configuration
Single Cell
Station A
Station B
Station C
Access Point
Access Point is analogous to a cellular Base Station
System referred to as Infrastructure Basic Service Set (BSS)

25. Extended BSS (EBSS) Configuration
Distribution System (DS)
BSS
Distribution System is a wired network (L2/L3 devices) connecting BSS
Connect APs via a wired network
DS consist of Layer 2, Layer 3 devices
System referred to as Extended BSS
APs have a BSSID
System has a SSID
Basic Service Area

26. Association/Disassociation
STA AP
Probe Request
Probe Response
Authenticate
Associate Request
Mention that this is Active Scanning. Where it can go around scanning different frequencies?
Passive Scanning is to wait for a Beacon.
On Authentication, STA gets an Association Identifier.
Associate Response
Disassociate
(No ack)

27. 802.11b: Physical Layer CCK (Complementary Code Keying):
Each 8-bit “symbol” is mapped to 8 four-phase (QPSK) symbols.
The signatures (codes) are orthogonal.

28. 802.11 Protocol Architecture
Logical link control
Contention-free
service
Contention service
Point Coordination Function (PCF)
MAC
Layer
Distributed Coordination Function (DCF)
Air interface (802.11/11a/11b)
DCF: CSMA/CA with additional virtual carrier sensing (ad hoc)
PCF: Point Coordination Control Function (infrastructure)
Access Point Becomes a Scheduler
Contention Free Mode
DCF is more prevalent
PCF and DCF can co-exist

29. IEEE 802.11 Medium Access Control Logic
Wait for frame
to transmit
Medium
idle? No
Wait until current
transmission ends
IEEE Medium Access Control Logic
Yes
Wait IFS
Wait IFS
Still
idle? No
Still
idle? No
Yes
Yes
Transmit frame
Exponential backoff
while medium idle
Transmit frame

30. Inter Frame Spacing (IFS)
Station has to detect a minimum idle time before transmit
Time depends on type of Frame, type of MAC
SIFS: Short IFS (used by ACKs, CTS)
PIFS: PCF IFS used by PCF Frames
DIFS: DCF IFS used by DCF Frames
SIFS < PIFS < DIFS
Relative values of IFS used to “prioritize” Medium Access

31. Basic Access Method DIFS Contention window PIFS DIFS Busy medium SIFS
Backoff window
Next frame
Time
Slot time
Select slot using binary
exponential backoff
Defer access

32. CSMA/CA

33. Distributed Control Function (DCF)
Network Allocation Vector:
Prevents transmissions by other
terminals.
“Virtual” carrier sensing

34. Point Coordination Function (PCF)
Alternative access method implemented on top of the DCF.
Stations are scheduled to transmit by AP.
Point coordinator (AP) uses PIFS when issuing polls.
Optional MAC feature – not widely available in products.

35. Frame Structure
Frame Duration Addr Addr Addr Sequence Addr Data CRC
Control Control
Total Header Size – 34 bytes
Variable packet size
Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses)
Frame Control: indicates if frame is data, RTS, CTS, or other type of control.
Duration: indicates time in microseconds that the channel will be allocated for transmission (virtual carrier sensing)
Sequence Control: identifies place in packet sequence for fragmentation and reassembly
CRC: Cyclic Redundancy Check (for error detection)
Source and Destination address are as in regular Ethernet packets
Receiver address is AP’s BSSid when message is sent to the DS
Transmitter address is AP’s BSSSid when message is sent from the DS
Duration is specified in Microseconds

36. Power Management
Most of the time mobile devices receive data in bursts and then become idle.
To maintain session:
AP buffers download data while mobile “sleeps”
Mobiles are synchronized, and “wake up” at designated times.
AP transmits beacon announcing mobiles with buffered data
Mobile checks for buffered data; if listed, then goes active.
AP transmits buffered data.
Different from power control in cellular networks – geared towards burst data.