1 / 93

INFO 331 Computer Networking Technology II

INFO 331 Computer Networking Technology II. Chapter 6 Wireless Networking Glenn Booker. Wireless & Mobile Networks. The number of mobile devices has grown immensely in the last few years 34 million cell phones worldwide as of 1993 2 billion [ ITU ] as of 2005, many Internet-aware

lang
Download Presentation

INFO 331 Computer Networking Technology II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. INFO 331Computer Networking Technology II Chapter 6 Wireless Networking Glenn Booker INFO 331 chapter 6

  2. Wireless & Mobile Networks • The number of mobile devices has grown immensely in the last few years • 34 million cell phones worldwide as of 1993 • 2 billion [ITU] as of 2005, many Internet-aware • 4 billion by the end of 2008 • Distinguish between wireless connectivity and the mobility that affords • Some wireless devices are stationary INFO 331 chapter 6

  3. Wireless & Mobile Networks • Challenges for this context include • Establishing and maintaining a wireless connection • Handing off a wireless client from one part of the network to another • Some terminology • Wireless host is the end user’s device connected to the network • Wireless links are analogous to the wired variety INFO 331 chapter 6

  4. Terminology • A base station communicates with the wireless hosts; e.g. cell towers for cell phones, and access points for wireless computers • Base stations connect to the rest of the network, either through wired or other wireless links • Infrastructure versus ad hoc mode • When a wireless host connects in infrastructure mode, it relies on the network for address resolution, routing, etc. • In ad hoc mode, the host performs those functions INFO 331 chapter 6

  5. Terminology • When a host changes from one base station to another, the change of attachment is a handoff • Can categorize wireless networks by the number of wireless hops (one or more), and whether it uses infrastructure (e.g. a base station) • Single hop, with infrastructure – is typical of a local wireless connection to a wired network INFO 331 chapter 6

  6. Terminology • Single hop, no infrastructure – like Bluetooth or ad hoc 802.11 networks • Multi-hop, with infrastructure – needs a wireless relay to get to the wired world, like a wireless mesh network • Multi-hop, no infrastructure – typically has mobile nodes as well as hosts; MANETs (mobile ad hoc networks) and vehicle versions, VANETs are in this category INFO 331 chapter 6

  7. Wireless Links • If a simple wired Ethernet link is replaced by a wireless connection • The hub or switch would be replaced by an access point • The host needs a wireless network card • The Ethernet cable goes in the closet • So how does this affect service? INFO 331 chapter 6

  8. Wireless Links Problems • Key impacts of changing to wireless are • Decreasing signal strength with distance from the access point • Interference from other sources in the same frequency range • Multipath propagation – signals can bounce around, giving echoes (like talking at edge of Grand Canyon) • This results in much higher, and more variable, bit error rates (BER) INFO 331 chapter 6

  9. Wireless Links Problems Images from Kurose’s slides INFO 331 chapter 6

  10. CDMA • Last term we covered three approaches to sharing links (multiple access) • Channel partitioning (TDM and FDM) • Random access protocols (ALOHA & CSMA) • Taking turns protocols (polling or token ring) • Here we need another type of multiple access protocol – Code Division Multiple Access (CDMA) INFO 331 chapter 6

  11. CDMA • In CDMA, the original data stream is multiplied by a code which changes much faster than the data, the chipping rate • In the following example, for every bit of incoming data, the code has eight values (11101000) • The data*code product is sent over the link • The receiver undoes the code, and recovers the original signal INFO 331 chapter 6

  12. CDMA Example INFO 331 chapter 6

  13. CDMA • So how does this help?? • Interfering signals add onto the signal you want to receive • If the code is chosen properly, the desired signal can be picked out of the sum of your signal plus garbage • It’s kind of like being able to follow one conversation in a crowded room INFO 331 chapter 6

  14. 802.11 LAN Protocols • The WiFi or 802.11 protocols are used for local wireless networks • 802.11a and 802.11g are most common • Both provide service at up to 54 Mbps • 802.11a operates at 5.8 GHz • 802.11g operates at 2.4 GHz • All use CSMA/CA as medium access protocol, have the same frame structure INFO 331 chapter 6

  15. 802.11 LAN Protocols INFO 331 chapter 6

  16. 802.11 LAN Protocols • All 802.11 protocols can slow themselves down for longer distances, or to deal with interference • All can use infrastructure or ad hoc mode • They differ at the physical layer INFO 331 chapter 6

  17. 802.11 LAN Protocols • Both 2.4 (for .11b and g) and 5.8 GHz (.11a) frequency ranges have disadvantages • 2.4 GHz has more interference from cell phones and microwave ovens • 5.8 GHz needs more power for a given distance, and suffers more from multipath propagation INFO 331 chapter 6

  18. 802.11 LAN Protocols • Notice each band is a range of frequencies (technically 2.4 – 2.485 and 5.1 – 5.8 GHz); typically have 11 channels in that range INFO 331 chapter 6

  19. 802.11 LAN Protocols • 802.11n is being standardized • Uses two or more antennae to send and receive, and should be over 200 Mbps • What wavelength are the 802.11 bands? • ln = c = 3E10 cm/s • l = c/n • For 2.4 GHz, l = 3E10 cm/s / 2.4E9 s-1 = 12.5 cm • For 5.8 GHz, l = 3E10 cm/s / 5.8E9 s-1 = 5.2 cm INFO 331 chapter 6

  20. 802.11 Architecture • A basic service set (BSS) is an access point (base station) and one or more wireless hosts • The access points for various BSSs are connected to each other via hubs, switches, or routers • Every wireless adapter has a 6 byte MAC address, and the access point has a MAC address • Again, MAC addresses are managed by IEEE INFO 331 chapter 6

  21. 802.11 LAN Architecture INFO 331 chapter 6

  22. 802.11 Architecture • In infrastructure mode, the access points are essential elements • In ad hoc mode, there are no access points, and wireless devices communicate independently • This could be used to network with another laptop directly, for example • The outside world isn’t visible in ad hoc mode INFO 331 chapter 6

  23. Channels & Association • In infrastructure mode, need to associate with an access point before data can be sent or received • Each access point is given a Service Set Identifier (SSID), and channel • The SSID is a readable name, like ‘sixflags’ • Channels 1-11 are available, but only channels 1, 6, and 11 are non-overlapping INFO 331 chapter 6

  24. It’s a jungle out there! • A Wi-Fi jungle is when you can choose from multiple access points (APs), possibly using the same channels • Could occur downtown, where many cafés and local networks could intersect • How tell the networks (APs) apart? • Each AP sends beacon frames periodically, with the AP’s SSID and MAC address • You choose which AP with which to associate INFO 331 chapter 6

  25. Passive vs Active scanning • When access points broadcast their presence, and you merely look for their beacon frames, this is passive scanning • A wireless host can also broadcast a signal to look for APs, this is active scanning INFO 331 chapter 6

  26. After association • Once an AP has been selected for association, generally DHCP is used to get an IP address, find DNS servers, etc. • To be allowed to associate, might have to authenticate the host • Can specify which MAC addresses are allowed to associate • May require logging into the network, verify identity with a Radius or Diameter server INFO 331 chapter 6

  27. 802.11 Multiple Access Control • Ethernet has been very successful • Recall it used CSMA/CD – carrier sense multiple access with collision detection • Wait for a pause in traffic before transmitting, and sense when a collision occurs • 802.11 uses a variation of this – CSMA/CA • Collision avoidance instead of detection • Also adds link-layer acknowledgement & retransmission (ARQ) INFO 331 chapter 6

  28. 802.11 Collision Avoidance • Why no collision detection? • It requires ability to send and receive at the same time - here the received signal is weak compared to the sent signal, so it’s expensive to make hardware to do this • The hidden terminal problem and fading make it impossible to detect all collisions • So 802.11 always transmits a full frame • Unlike Ethernet, it won’t stop mid-transmission INFO 331 chapter 6

  29. 802.11 ARQ • To transmit data from sender to receiver: • Sender waits a short time period DIFS (distributed inter-frame spacing) • Sender transmits the data using CSMA/CA • Data gets to receiver • Receiver validates integrity of data with CRC • Waits a time SIFS (short inter-frame spacing) • The receiver sends an ACK INFO 331 chapter 6

  30. 802.11 ARQ INFO 331 chapter 6

  31. 802.11 ARQ • 802.11 uses CRC to check for bit errors • You recall the cyclic redundancy check, right? • If channel is busy when a transmission is ready • Wait a random time of idle channel, and transmit when the channel is idle; don’t count down when the channel is busy • Why? This avoids collisions when multiple hosts are waiting for a clear channel INFO 331 chapter 6

  32. 802.11 ARQ • So in wireless communication, it’s all about AVOIDING COLLISIONS! • If the source doesn’t get an ACK within some time, it retransmits • If some number of retransmissions aren’t ACKed, discard the frame INFO 331 chapter 6

  33. 802.11 Reservation Scheme • There is an optional scheme to avoid collision even when there are hidden hosts • It’s very polite – each host asks for permission to transmit • Sort of like the polling protocols • Sender sends a Request To Send (RTS) frame to the AP • AP broadcasts a Clear To Send (CTS) frame to reserve use of channel by that sender INFO 331 chapter 6

  34. 802.11 Reservation Scheme • Sender then transmits exclusively during that time period – other hosts know from getting the CTS to be quiet • This is very effective at avoiding collisions, but has time overhead to exchange RTS and CTS messages • Often used for sending large data files • May establish threshold, so only files larger than threshold are allowed to use RTS/CTS INFO 331 chapter 6

  35. 802.11 point-to-point • Using directional antennae, the 802.11 protocols can be used up to 80 kilometers of distance • This was done in India, for example INFO 331 chapter 6

  36. 802.11 Frames • A frame in 802.11 consists of 34 bytes of header and trailer, plus 0 to 2312 bytes of data (payload) • Data generally limited to 1500 bytes due to Ethernet limit • Data is usually an IP datagram or ARP packet INFO 331 chapter 6

  37. 802.11 Frame Fields • Frame control (2 B, shown on next slide) • Duration (2 B) for timeout or CTS period • Address 1 (6 B) MAC of destination node • Address 2 (6 B) MAC of transmitting node • Address 3 (6 B) MAC of router leaving this BSS • Sequence control (2 B) just like in TCP • Address 4 (6 B) used only for ad hoc networks • Payload (data) (0-2132 B) • CRC code (4 B) [size verified here] INFO 331 chapter 6

  38. bytes bits 802.11 Frames INFO 331 chapter 6

  39. 802.11 Frame Fields • The sizes for frame control parts are in bits (total 16 bits = 2 bytes) • The Type field also distinguishes association frames from normal data frames • WEP is an encryption mode • The duration field can be the timeout interval, or time for a clear to send (CTS) • Address 3 is critical for communicating across wireless networks INFO 331 chapter 6

  40. 802.11 Frame Fields • Sequence numbers are also used to tell multipath echoes apart, in addition to detecting retransmissions • Address 4 is only used for ad hoc networks • The CRC field (4 B, not 2) is particularly important, since there is a large chance of bit errors • We’ll ignore the other fields for now INFO 331 chapter 6

  41. Mobility within IP subnet • If a host moves between BSS’ within the same subnet (i.e. they are not connected by a router), it’s relatively easy for the handoff from one AP to another to occur • If the BSS’ are connected by a hub, there’s no problem – the host disassociates from one AP and associates with another INFO 331 chapter 6

  42. Mobility within subnet • If the BSS’ are connected by a switch, the self-learning features of switches is too slow to keep up well • The new AP has to send a broadcast Ethernet message to update the switch with the new association • An 802.11f standards group was working on this issue – standard was withdrawn 2/06 INFO 331 chapter 6

  43. Advanced 802.11 Features • 802.11 hints at supporting added features • Adapt transmission rate, depending on the SNR (signal to noise ratio) and other channel characteristics (e.g. lost frames) • Power management, by limiting the time various functions are on; done by putting itself to sleep • It can tell its access point it’s asleep, so frames aren’t sent to it until it wakes up! INFO 331 chapter 6

  44. 802.15 WPAN • The 802.11 standards are designed for wireless communication up to 100 meters • The 802.15 wireless personal area network (WPAN) is for ad hoc wireless networking with a range of about 10 meters • Based on Bluetooth, it’s designed to handle up to eight ‘active’ local devices near a host in a piconet, controlled by a master node INFO 331 chapter 6

  45. 802.15 WPAN • The master node decides which devices are active or parked • Can have up to 255 parked devices • Operates at 2.4 GHz using TDM with slot of 625 ms, and 79 channels • Hops randomly across channels (frequency-hopping spread spectrum, or FHSS) • Data rates up to 4 Mbps INFO 331 chapter 6

  46. WiMAX • WiMAX is World Interoperability for Microwave Access, IEEE 802.16 • It uses a base station to coordinate sending and receiving packets, similar to 802.11 infrastructure mode, using TDM • Each frame defines the physical layer properties for later packets; hence the transmission approach can change to get the best reception possible INFO 331 chapter 6

  47. WiMAX • The transmission time allocated to each subscriber can be controlled • WiMAX uses a connection identifier in the packet to allow quality of service (QoS) to be customized • MAC addresses are mapped to the connection identifiers • WiMAX is a complex beast, and is changing rapidly INFO 331 chapter 6

  48. Cellular Internet Access • Since Wi-Fi is limited to about 100 meters, how do we connect to the Internet when far from an access point? • Use your cell phone! • Key concerns are: • Is it fast? • Is it reliable? • Is it going to be better than a long distance wireless LAN? INFO 331 chapter 6

  49. GSM Cellular Architecture • Cellular architecture is broken into … cells • Each cell is a geographic area served by a cell tower, which routes through a mobile switching center (MSC) • Acts like a switching center or central office • The center is connected to the Internet directly, and/or the phone system (Public Switched Telephone Network) INFO 331 chapter 6

  50. Cellular Architecture INFO 331 chapter 6

More Related