CECS 474 Computer Network Interoperability Wireless (IEEE 802.11) Supplement Tracy Bradley Maples, Ph.D. Computer Engineering & Computer Science Cal ifornia State University, Long Beach Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)
IEEE 802.11 Wireless Networks The designers of IEEE 802.11 wanted to use a protocol as close to Ethernet as possible. Unfortunately, the characteristics of wireless transmission requires that WiFI to use a modified Ethernet protocol for it’s Media Access Control (MAC). In the Ethernet protocol, a station must listen to the transmission media before they send and continue listening during transmission to determine whether a collision occurs. Unfortunately, in WiFi, it cannot be guaranteed that a station can hear the transmissions of all other stations on the network.
IEEE 802.11 MAC Protocol: Collision Avoidance • Two issues arise which cause a direct CSMA/CD implementation impossible. These problems result in a modification of the CSMA/CD to CSMA/CA: • The hidden node problem (A cannot see D in Figure 2) • Using the standard Ethernet protocol, the hidden node problem will result in collisions that the sender cannot recognize. • (2) The exposed node problem (B can send to A; simultaneously with C sending to D) • Using the standard Ethernet protocol, these simultaneous transmissions would not be allowed and we want them! FIGURE 1: Example Wireless Network with operating in an ad hoc fashion.
Multiple Access with Collision Avoidance (CSMA/CA) • 802.11 uses CA to solve these two issues: • The sender and receiver exchange control frames before the sender actually transmits data so that all nearby nodes know that transmission is about to begin. • Sender sends a request to send (RTS) frame to the receiver. The frame includes the length of time the sender needs to transmit its data frame (or “hold the medium”). • Receiver replies with a clear to send (CTS) frame that echoes the length field back to the sender and anyone else who receives it. • If a node sees the CTS frame, it must not transmit during the time interval indicated in the length field (solving the hidden node problem).
Multiple Access with Collision Avoidance (CSMA/CA) (cont’d) • If a node sees the RTS frame but not the CTS frame, it is free to transmit (solving the exposed node problem). • The receiver sends an ACK to the sender after successfully receiving a frame. All nodes wait for this ACK before trying to transmit. • If two senders transmit RTS frames at the same time, a collision happens. Collisions cannot be detected, but because the RTS frames are destroyed no CTS frames are ever sent. The sender’s time-out while awaiting the CTS frames, and must retransmit.
Ad hoc vs. Infrastructure Modes In ad hoc mode, all nodes communicate directly with each other. In infrastructure mode, nodes communicate via an access point (AP). Access points can be connected to a wired network infrastructure. Access points can also be connected by a distribution system. The distribution system runs at Layer 2 forming a Wireless LAN (WLAN). FIGURE 2: Access points connected to a distribution network.
Scanning • Nodes can roam from cell to cell selecting the access point they need to use. • Active scanningoccurs when the node sends probe frames searching for an access point. • Passive scanningoccurs when the access point sends out a beacon periodically to advertise its presence. FIGURE 4: Node Mobility.
802.11 Frame Format • FIGURE 5: 802.11 frame format. • Source Addr—48 bits • Destination Addr–48 bits • Data—up to 2312 bytes • 32-bit CRC • Addr3—48 bits (described below) • Addr4—48 bits (described below) • Not shown are three subfields: • 6-bit Type Field that specifies whether the frame carries data, RTS, CTS, or is being used by ToDS or FromDS • 1-bit field called ToDS • 1-bit field called FromDS • DS Field Examples: • If ToDS=0 and FromDS=0,the node is being sent directly from the source to the destination. (Addr1=target node; Addr2=source node) • If ToDS=1 and FromDS=1,the node is being sent through the distribution system (DS). (Addr1=ultimate destination; Addr2=immediate sender; Addr3=intermediate destination; Addr4=original source)
Physical Properties • 802.11 was designed to run over three different types of physical media: • Spread spectrum using frequency hopping • Spread spectrum using direct sequence • Infrared signals • Spread spectrum spreads the signal over a wider frequency band than normal so that interference from other devices will not have as great an impact. • 1. Frequency Hopping • The signal is transmitted over a (apparently) random sequence of frequencies…first one, then a second, then a third, etc. • The sequence of frequencies is computed algorithmically using a pseudorandom number generator. The sender and the receiver use the same seed value to calculate synchronized frequencies. • 802.11 defines a physical layer standard that uses frequency hoping (over 79 1-Mhz-wide frequency bandwidths). It runs in the 2.4 Ghz frequency band of the elecromagnetic spectrum.
Physical Properties • 2.Direct Sequence • Each bit in the frame is represented by multiple bits in the transmitted signal. • The sender sends the XOR of the bit it wants to send and n random bits. This is known as an n-bit chipping code. The sender and receiver random number streams match. • This spreads the signal across a frequency band that is n times wider than is required. FIGURE: Example 4-bit chipping sequence. 802.11 defines a physical layer standard that uses direct sequence (using an 11-bit chipping sequence). It runs in the 2.4 Ghz frequency band of the elecromagnetic spectrum.
Physical Properties (Cont’d) • Infrared Signals • The third physical layer standard for 802.11 uses infrared signals that are diffused (the sender and receiver do not have to be aimed at each other). • This technology works for about 10 meters inside buildings only. See the Information about HedyLamarr posted on the CECS 474 Web Page. "Any girl can be glamorous," HedyLamarr once said. "All she has to do is stand still and look stupid." The film star belied her own apothegm by hiding a brilliant, inventive mind beneath her photogenic exterior. In 1942, at the height of her Hollywood career, she patented a frequency-switching system for torpedo guidance that was two decades ahead of its time. www.Inventions.org