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

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

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

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

ALOHA-Based Random Access no 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

ALOHA Protocols

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

CSMA Example

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.

Binary Exponential Backoff no 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 2 X (maximum round trip delay) Maximum of 16 retries

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.

Throughput vs. Offered Load

Delay vs. Throughput

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.

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.

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.

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

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)

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

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.

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 802.11b Extension 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK

802.11b vs. 802.11a 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

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)

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)

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

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)

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.

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

IEEE 802.11 Medium Access Control Logic Wait for frame to transmit Medium idle? No Wait until current transmission ends IEEE 802.11 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

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

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

802.11 CSMA/CA

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

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.

802.11 Frame Structure 2 2 6 6 6 2 6 0-2312 4 Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 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

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.