Wireless Networks 14 740 Fundamentals of Computer Networks

Wireless Networks 14 -740: Fundamentals of Computer Networks Credit: Bill Nace Material from Computer Networking: A Top Down Approach, 6 th edition. J. F. Kurose and K. W. Ross

traceroute • Challenges and Opportunities of Wireless • CSMA/CA • 802. 11 • . . . and beyond! 14 -740: Fall 2017 2

Opportunities • Anytime, anywhere access • . . . often with a highly portable device • Chance to give up the cables 14 -740: Fall 2017 3

A View of the Infrastructure • A collection of wireless hosts served by an access point (AP) is known as a Basic Service Set (BSS) AP signal range 14 -740: Fall 2017 4

Infrastructure (2) • The AP is part of the link-layer infrastructure • It has a MAC address • It connects to a router • Or sometimes it is in • same box as a router • Entire BSS is in the same subnet • However, BSS ≠ subnet 14 -740: Fall 2017 5

Infrastructure (3) • More complex topologies are possible • Very common to connect multiple APs with a switch 14 -740: Fall 2017 6

Challenges • Mobility • Routing and addressing • How can a TCP flow sustain while user moves to a different subnet (and thus different IP address)? • Wireless • Link characteristics 14 -740: Fall 2017 7

Wireless Links • Radio signals, governed by Friis equation Pr = Pt • Gt. Gr(λ / 4πr)2 • thus, very little power at receiver • Receiver must be sensitive • . . . must shut off during transmission • Noise problems • Bit Error Rate = f(modulation scheme, signal to noise ratio, . . . ) • BER of 10 -4 to 10 -6 is common 14 -740: Fall 2017 8

Multipath Effects • Signals bounce around a lot • Receiver “sees” signal that is the sum of the line-of-sight signal plus those from all other paths

Hidden Terminal Problem • Two hosts may be prevented from hearing each other’s transmissions • By fading (i. e. excessive range) • By obstacle • They are still participants • in the network • Other hosts may hear transmissions from both • Collisions result

Exposed Terminal Problem • In networks without access point, like mesh networks • Node A has data to send to A’ • But, A hears transmission from neighbor (B) and does not send • But, node B is sending to a node outside range of A’ • Both transmissions could have proceeded without collision

traceroute • Challenges and Opportunities of Wireless • CSMA/CA • 802. 11 • . . . and beyond! 14 -740: Fall 2017 12

CSMA / CA • Remember Ethernet’s CSMA / CD? • Carrier Sense • Multiple Access • Collision Detection • CSMA / Collision Avoidance • Reduce potential for collision. . . • But, collisions are always a possibility 14 -740: Fall 2017 13

802. 11 CSMA/CA Algorithm 1. Listen for a specified time (DIFS) If medium is not free: 2. Exponential Backoff If medium is free: 2. Transmit entire frame 3. Await ACK frame 4. If no ACK, then Exponential Backoff 14 -740: Fall 2017 14

ACK? ? • Otherwise sender doesn’t know if a collision happened with a hidden terminal • Also, protects against bit errors • Receiver only sends ACK if frame passes CRC

Exponential Backoff • Similar to Ethernet algorithm • Each node chooses a random number • Max size increases from round to round • Can be modeled as a counter • Decremented during any idle time • Put on hold if another node transmits and for a short time afterwards (SIFS) • When zero, the counting station may transmit 14 -740: Fall 2017 17

Exponential Backoff (2) • In 802. 11, Backoff begins as soon as Carrier Sense is complete • Don’t want multiple waiting nodes to jump on the idle channel and collide

Backoff Waveform

Channel Reservations • A node can request access to the channel to ensure no collisions from hidden terminals • Node sends a Request to Send (RTS) frame to the AP • AP grants permission with a Clear to Send (CTS) frame • All nodes can hear the AP, even hidden terminals 14 -740: Fall 2017 20

RTS / CTS • RTS frame includes a duration requested for transmission and receipt of an ACK • RTS and CTS are small frames, so collisions don’t clog the channel for very long • But, RTS and CTS are overhead • Introduces delay • Consumes bandwidth • Reservations only used for long data frames 14 -740: Fall 2017 21

RTS / CTS Use

traceroute • Challenges and Opportunities of Wireless • CSMA/CA • 802. 11 • . . . and beyond! 14 -740: Fall 2017 23

802. 11 variants • A series of wireless standards • 802. 11 (1997): 1 or 2 Mbps, 2. 4 GHz • 802. 11 a (1999) • OFDM Physical Layer, 5 GHz band • 54 Mbs • Short range (~50 ft) due to 5 GHz freq • 802. 11 b (1999) • 11 Mbs • ~150 ft range, 2. 4 GHz band • Gained commercial acceptance 14 -740: Fall 2017 24

802. 11 variants • 802. 11 g (2003) • OFDM at 2. 4 GHz • 54 Mbs • Backwards compatible with. 11 b • Costs about 21% throughput • 802. 11 n (2009) • MIMO, 2. 4 and/or 5 GHz • ~300 ft range • 600 Mbps 14 -740: Fall 2017 25

802. 11 variants • 802. 11 ac (2013) • 5 GHz, 8 MIMO antennas • 500 Mbps per stream • multiple streams per AP • 802. 11 ad (2014) • 2. 4 / 5 / 60 GHz spectrum • 60 GHz doesn't penetrate walls • theoretical data rate of 7 Gbps 14 -740: Fall 2017 26

Operating Modes • Ad-hoc: Nodes create network, but unattached to internet • Infrastructure: Access Points (AP) connect wireless subnet to wired network • Point-to-point: Use directional antennas to extend range, restrict from multiple access 14 -740: Fall 2017 27

Security • Wi-Fi Protected Access (WPA) 2 • Uses AES with a pre-shared key • Excellent for home/small business use • Can be used with RADIUS server for enterprise use • Don’t use Wired Equivalent Privacy (WEP) 14 -740: Fall 2017 28

802. 11 Frame • 4 MAC addresses • Addr 1 is the receiver • Addr 2 is the sender • Addr 3 is the router connected to the AP and is used for converting to/from Ethernet frame • Addr 4 is used when forwarding in ad-hoc mode 14 -740: Fall 2017 29

3 Addresses? • A frame sent from a wireless host will get converted by the AP into an ethernet frame and sent to the router (Default Gateway) • In Wi. Fi Frame • Addr 1: AP • Addr 2: Host • Addr 3: Default Gateway • In Ethernet Frame • Src: Host • Dest: Default Gateway 14 -740: Fall 2017 30

802. 11 Frame • Frame Control (2 bytes) • Duration (2 bytes) • Sender can reserve the channel • Sequence Control (2 bytes) • Used like RDT sequence number • Necessary to add reliability 14 -740: Fall 2017 31

802. 11 Frame • Data (up to 2304 bytes, typically < 1500) • Holds an IP datagram, ARP packet, etc • CRC (4 bytes) • Especially important as the BER is high 14 -740: Fall 2017 32

Frame Control Field • Most of these fields are 1 -4 bits • Types include management frames, beacons, etc. , as well as data frames • Fragmentation is possible (More Frag) • In high BER environments, fragmenting improves throughput 14 -740: Fall 2017 33

802. 11 Other Stuff • Power Management • Node can tell AP that it will go to sleep for short (100 ms) time period • AP will buffer frames until it awakes • Rate Adaptation • Transmission rate can be tuned to the environment • As frames are lost, reduce rate 14 -740: Fall 2017 34

traceroute • Challenges and Opportunities of Wireless • CSMA/CA • 802. 11 • . . . and beyond! 14 -740: Fall 2017 35

. . . and beyond • Cellular Networks • Mobile Users • Mesh Networks • Wireless Sensor Networks • Personal Area Networks • Bluetooth 14 -740: Fall 2017 36

Interested in Learning More? • 14 -814: Wireless Network Security • 18 -758: Wireless Communications • 18 -759: Wireless Networks • 18 -843: Mobile and Pervasive Computing • 18 -847 A: Mobile Hardware 14 -740: Fall 2017 37

Glossary • AP: Access Point • OFDM: Orthogonal Frequency Division Multiplexing • DIFS: DCF Inter-Frame Space (sometimes given as Distributed Inter-Frame Space) • DCF: Distributed Coordination Function • SIFS: Short Inter-Frame Space • MIMO: Multiple-input multiple-output 14 -740: Fall 2017 38

Lesson Objectives • Now, you should be able to: • explain the challenges of a wireless subnet, including those caused by range limits, mobility, receiver shutdown, noise, multi-path, hidden terminal, and exposed terminal problems • describe the CSMA/CA algorithm and how it helps overcomes some of the wireless challenges 14 -740: Fall 2017 39

• You should be able to: • describe the use of channel reservations to avoid collisions • explain some features of the 802. 11 standard (do not memorize features of the variants). Such features include operating modes, security, frame format, power management and rate adaptation