Chapter 6


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Chapter 6

  1. 1. INFO 331 Computer Networking Technology II Chapter 6 Wireless Networking Glenn Booker INFO 331 chapter 6
  2. 2. Wireless & Mobile Networks <ul><li>The number of mobile devices has grown immensely in the last few years </li></ul><ul><ul><li>34 million cell phones worldwide as of 1993 </li></ul></ul><ul><ul><li>2 billion [ ITU ] as of 2005, many Internet-aware </li></ul></ul><ul><ul><li>4 billion by the end of 2008 </li></ul></ul><ul><li>Distinguish between wireless connectivity and the mobility that affords </li></ul><ul><ul><li>Some wireless devices are stationary </li></ul></ul>INFO 331 chapter 6
  3. 3. Wireless & Mobile Networks <ul><li>Challenges for this context include </li></ul><ul><ul><li>Establishing and maintaining a wireless connection </li></ul></ul><ul><ul><li>Handing off a wireless client from one part of the network to another </li></ul></ul><ul><li>Some terminology </li></ul><ul><ul><li>Wireless host is the end user’s device connected to the network </li></ul></ul><ul><ul><li>Wireless links are analogous to the wired variety </li></ul></ul>INFO 331 chapter 6
  4. 4. Terminology <ul><ul><li>A base station communicates with the wireless hosts; e.g. cell towers for cell phones, and access points for wireless computers </li></ul></ul><ul><ul><ul><li>Base stations connect to the rest of the network, either through wired or other wireless links </li></ul></ul></ul><ul><li>Infrastructure versus ad hoc mode </li></ul><ul><ul><li>When a wireless host connects in infrastructure mode , it relies on the network for address resolution, routing, etc. </li></ul></ul><ul><ul><li>In ad hoc mode , the host performs those functions </li></ul></ul>INFO 331 chapter 6
  5. 5. Terminology <ul><li>When a host changes from one base station to another, the change of attachment is a handoff </li></ul><ul><li>Can categorize wireless networks by the number of wireless hops (one or more), and whether it uses infrastructure (e.g. a base station) </li></ul><ul><ul><li>Single hop, with infrastructure – is typical of a local wireless connection to a wired network </li></ul></ul>INFO 331 chapter 6
  6. 6. Terminology <ul><ul><li>Single hop, no infrastructure – like Bluetooth or ad hoc 802.11 networks </li></ul></ul><ul><ul><li>Multi-hop, with infrastructure – needs a wireless relay to get to the wired world, like a wireless mesh network </li></ul></ul><ul><ul><li>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 </li></ul></ul>INFO 331 chapter 6
  7. 7. Wireless Links <ul><li>If a simple wired Ethernet link is replaced by a wireless connection </li></ul><ul><ul><li>The hub or switch would be replaced by an access point </li></ul></ul><ul><ul><li>The host needs a wireless network card </li></ul></ul><ul><ul><li>The Ethernet cable goes in the closet </li></ul></ul><ul><li>So how does this affect service? </li></ul>INFO 331 chapter 6
  8. 8. Wireless Links Problems <ul><li>Key impacts of changing to wireless are </li></ul><ul><ul><li>Decreasing signal strength with distance from the access point </li></ul></ul><ul><ul><li>Interference from other sources in the same frequency range </li></ul></ul><ul><ul><li>Multipath propagation – signals can bounce around, giving echoes (like talking at edge of Grand Canyon) </li></ul></ul><ul><li>This results in much higher, and more variable, bit error rates (BER) </li></ul>INFO 331 chapter 6
  9. 9. Wireless Links Problems Images from Kurose’s slides INFO 331 chapter 6
  10. 10. CDMA <ul><li>Last term we covered three approaches to sharing links (multiple access) </li></ul><ul><ul><li>Channel partitioning (TDM and FDM) </li></ul></ul><ul><ul><li>Random access protocols (ALOHA & C S MA) </li></ul></ul><ul><ul><li>Taking turns protocols (polling or token ring) </li></ul></ul><ul><li>Here we need another type of multiple access protocol – Code Division Multiple Access (CDMA) </li></ul>INFO 331 chapter 6
  11. 11. CDMA <ul><li>In CDMA, the original data stream is multiplied by a code which changes much faster than the data, the chipping rate </li></ul><ul><ul><li>In the following example, for every bit of incoming data, the code has eight values (11101000) </li></ul></ul><ul><ul><li>The data*code product is sent over the link </li></ul></ul><ul><ul><li>The receiver undoes the code, and recovers the original signal </li></ul></ul>INFO 331 chapter 6
  12. 12. CDMA Example INFO 331 chapter 6
  13. 13. CDMA <ul><li>So how does this help?? </li></ul><ul><ul><li>Interfering signals add onto the signal you want to receive </li></ul></ul><ul><ul><li>If the code is chosen properly, the desired signal can be picked out of the sum of your signal plus garbage </li></ul></ul><ul><li>It’s kind of like being able to follow one conversation in a crowded room </li></ul>INFO 331 chapter 6
  14. 14. 802.11 LAN Protocols <ul><li>The WiFi or 802.11 protocols are used for local wireless networks </li></ul><ul><li>802.11a and 802.11g are most common </li></ul><ul><ul><li>Both provide service at up to 54 Mbps </li></ul></ul><ul><ul><li>802.11a operates at 5.8 GHz </li></ul></ul><ul><ul><li>802.11g operates at 2.4 GHz </li></ul></ul><ul><li>All use CSMA/CA as medium access protocol, have the same frame structure </li></ul>INFO 331 chapter 6
  15. 15. 802.11 LAN Protocols INFO 331 chapter 6
  16. 16. 802.11 LAN Protocols <ul><li>All 802.11 protocols can slow themselves down for longer distances, or to deal with interference </li></ul><ul><li>All can use infrastructure or ad hoc mode </li></ul><ul><li>They differ at the physical layer </li></ul>INFO 331 chapter 6
  17. 17. 802.11 LAN Protocols <ul><li>Both 2.4 (for .11b and g) and 5.8 GHz (.11a) frequency ranges have disadvantages </li></ul><ul><ul><li>2.4 GHz has more interference from cell phones and microwave ovens </li></ul></ul><ul><ul><li>5.8 GHz needs more power for a given distance, and suffers more from multipath propagation </li></ul></ul>INFO 331 chapter 6
  18. 18. 802.11 LAN Protocols <ul><li>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 </li></ul>INFO 331 chapter 6
  19. 19. 802.11 LAN Protocols <ul><li>802.11n is being standardized </li></ul><ul><ul><li>Uses two or more antennae to send and receive, and should be over 200 Mbps </li></ul></ul><ul><li>What wavelength are the 802.11 bands? </li></ul><ul><ul><li> = c = 3E10 cm/s </li></ul></ul><ul><ul><li> = c/  </li></ul></ul><ul><ul><ul><li>For 2.4 GHz,  = 3E10 cm/s / 2.4E9 s -1 = 12.5 cm </li></ul></ul></ul><ul><ul><ul><li>For 5.8 GHz,  = 3E10 cm/s / 5.8E9 s -1 = 5.2 cm </li></ul></ul></ul>INFO 331 chapter 6
  20. 20. 802.11 Architecture <ul><li>A basic service set (BSS) is an access point (base station) and one or more wireless hosts </li></ul><ul><li>The access points for various BSSs are connected to each other via hubs, switches, or routers </li></ul><ul><li>Every wireless adapter has a 6 byte MAC address, and the access point has a MAC address </li></ul><ul><ul><li>Again, MAC addresses are managed by IEEE </li></ul></ul>INFO 331 chapter 6
  21. 21. 802.11 LAN Architecture INFO 331 chapter 6
  22. 22. 802.11 Architecture <ul><li>In infrastructure mode, the access points are essential elements </li></ul><ul><li>In ad hoc mode, there are no access points, and wireless devices communicate independently </li></ul><ul><ul><li>This could be used to network with another laptop directly, for example </li></ul></ul><ul><ul><li>The outside world isn’t visible in ad hoc mode </li></ul></ul>INFO 331 chapter 6
  23. 23. Channels & Association <ul><li>In infrastructure mode, need to associate with an access point before data can be sent or received </li></ul><ul><li>Each access point is given a Service Set Identifier (SSID), and channel </li></ul><ul><ul><li>The SSID is a readable name, like ‘sixflags’ </li></ul></ul><ul><ul><li>Channels 1-11 are available, but only channels 1, 6, and 11 are non-overlapping </li></ul></ul>INFO 331 chapter 6
  24. 24. It’s a jungle out there! <ul><li>A Wi-Fi jungle is when you can choose from multiple access points (APs), possibly using the same channels </li></ul><ul><ul><li>Could occur downtown, where many cafés and local networks could intersect </li></ul></ul><ul><li>How tell the networks (APs) apart? </li></ul><ul><ul><li>Each AP sends beacon frames periodically, with the AP’s SSID and MAC address </li></ul></ul><ul><ul><li>You choose which AP with which to associate </li></ul></ul>INFO 331 chapter 6
  25. 25. Passive vs Active scanning <ul><li>When access points broadcast their presence, and you merely look for their beacon frames, this is passive scanning </li></ul><ul><li>A wireless host can also broadcast a signal to look for APs, this is active scanning </li></ul>INFO 331 chapter 6
  26. 26. After association <ul><li>Once an AP has been selected for association, generally DHCP is used to get an IP address, find DNS servers, etc. </li></ul><ul><li>To be allowed to associate, might have to authenticate the host </li></ul><ul><ul><li>Can specify which MAC addresses are allowed to associate </li></ul></ul><ul><ul><li>May require logging into the network, verify identity with a Radius or Diameter server </li></ul></ul>INFO 331 chapter 6
  27. 27. 802.11 Multiple Access Control <ul><li>Ethernet has been very successful </li></ul><ul><ul><li>Recall it used CSMA/CD – carrier sense multiple access with collision detection </li></ul></ul><ul><ul><li>Wait for a pause in traffic before transmitting, and sense when a collision occurs </li></ul></ul><ul><li>802.11 uses a variation of this – CSMA/CA </li></ul><ul><ul><li>Collision avoidance instead of detection </li></ul></ul><ul><ul><li>Also adds link-layer acknowledgement & retransmission (ARQ) </li></ul></ul>INFO 331 chapter 6
  28. 28. 802.11 Collision Avoidance <ul><li>Why no collision detection? </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>The hidden terminal problem and fading make it impossible to detect all collisions </li></ul></ul><ul><li>So 802.11 always transmits a full frame </li></ul><ul><ul><li>Unlike Ethernet, it won’t stop mid-transmission </li></ul></ul>INFO 331 chapter 6
  29. 29. 802.11 ARQ <ul><li>To transmit data from sender to receiver: </li></ul><ul><ul><li>Sender waits a short time period DIFS (distributed inter-frame spacing) </li></ul></ul><ul><ul><li>Sender transmits the data using CSMA/CA </li></ul></ul><ul><ul><li>Data gets to receiver </li></ul></ul><ul><ul><li>Receiver validates integrity of data with CRC </li></ul></ul><ul><ul><li>Waits a time SIFS (short inter-frame spacing) </li></ul></ul><ul><ul><li>The receiver sends an ACK </li></ul></ul>INFO 331 chapter 6
  30. 30. 802.11 ARQ INFO 331 chapter 6
  31. 31. 802.11 ARQ <ul><li>802.11 uses CRC to check for bit errors </li></ul><ul><ul><li>You recall the cyclic redundancy check, right? </li></ul></ul><ul><li>If channel is busy when a transmission is ready </li></ul><ul><ul><li>Wait a random time of idle channel , and transmit when the channel is idle; don’t count down when the channel is busy </li></ul></ul><ul><ul><li>Why? This avoids collisions when multiple hosts are waiting for a clear channel </li></ul></ul>INFO 331 chapter 6
  32. 32. 802.11 ARQ <ul><li>So in wireless communication, it’s all about AVOIDING COLLISIONS! </li></ul><ul><li>If the source doesn’t get an ACK within some time, it retransmits </li></ul><ul><li>If some number of retransmissions aren’t ACKed, discard the frame </li></ul>INFO 331 chapter 6
  33. 33. 802.11 Reservation Scheme <ul><li>There is an optional scheme to avoid collision even when there are hidden hosts </li></ul><ul><li>It’s very polite – each host asks for permission to transmit </li></ul><ul><ul><li>Sort of like the polling protocols </li></ul></ul><ul><li>Sender sends a Request To Send (RTS) frame to the AP </li></ul><ul><li>AP broadcasts a Clear To Send (CTS) frame to reserve use of channel by that sender </li></ul>INFO 331 chapter 6
  34. 34. 802.11 Reservation Scheme <ul><li>Sender then transmits exclusively during that time period – other hosts know from getting the CTS to be quiet </li></ul><ul><li>This is very effective at avoiding collisions, but has time overhead to exchange RTS and CTS messages </li></ul><ul><ul><li>Often used for sending large data files </li></ul></ul><ul><ul><li>May establish threshold, so only files larger than threshold are allowed to use RTS/CTS </li></ul></ul>INFO 331 chapter 6
  35. 35. 802.11 point-to-point <ul><li>Using directional antennae, the 802.11 protocols can be used up to 80 kilometers of distance </li></ul><ul><ul><li>This was done in India , for example </li></ul></ul>INFO 331 chapter 6
  36. 36. 802.11 Frames <ul><li>A frame in 802.11 consists of 34 bytes of header and trailer, plus 0 to 2312 bytes of data (payload) </li></ul><ul><ul><li>Data generally limited to 1500 bytes due to Ethernet limit </li></ul></ul><ul><ul><li>Data is usually an IP datagram or ARP packet </li></ul></ul>INFO 331 chapter 6
  37. 37. 802.11 Frame Fields <ul><ul><li>Frame control (2 B, shown on next slide) </li></ul></ul><ul><ul><li>Duration (2 B) for timeout or CTS period </li></ul></ul><ul><ul><li>Address 1 (6 B) MAC of destination node </li></ul></ul><ul><ul><li>Address 2 (6 B) MAC of transmitting node </li></ul></ul><ul><ul><li>Address 3 (6 B) MAC of router leaving this BSS </li></ul></ul><ul><ul><li>Sequence control (2 B) just like in TCP </li></ul></ul><ul><ul><li>Address 4 (6 B) used only for ad hoc networks </li></ul></ul><ul><ul><li>Payload (data) (0-2132 B) </li></ul></ul><ul><ul><li>CRC code (4 B) [size verified here ] </li></ul></ul>INFO 331 chapter 6
  38. 38. 802.11 Frames INFO 331 chapter 6 bits bytes
  39. 39. 802.11 Frame Fields <ul><li>The sizes for frame control parts are in bits (total 16 bits = 2 bytes) </li></ul><ul><ul><li>The Type field also distinguishes association frames from normal data frames </li></ul></ul><ul><ul><li>WEP is an encryption mode </li></ul></ul><ul><li>The duration field can be the timeout interval, or time for a clear to send (CTS) </li></ul><ul><li>Address 3 is critical for communicating across wireless networks </li></ul>INFO 331 chapter 6
  40. 40. 802.11 Frame Fields <ul><li>Sequence numbers are also used to tell multipath echoes apart, in addition to detecting retransmissions </li></ul><ul><li>Address 4 is only used for ad hoc networks </li></ul><ul><li>The CRC field (4 B, not 2) is particularly important, since there is a large chance of bit errors </li></ul><ul><li>We’ll ignore the other fields for now </li></ul>INFO 331 chapter 6
  41. 41. Mobility within IP subnet <ul><li>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 </li></ul><ul><li>If the BSS’ are connected by a hub, there’s no problem – the host disassociates from one AP and associates with another </li></ul>INFO 331 chapter 6
  42. 42. Mobility within subnet <ul><li>If the BSS’ are connected by a switch, the self-learning features of switches is too slow to keep up well </li></ul><ul><ul><li>The new AP has to send a broadcast Ethernet message to update the switch with the new association </li></ul></ul><ul><li>An 802.11f standards group was working on this issue – standard was withdrawn 2/06 </li></ul>INFO 331 chapter 6
  43. 43. Advanced 802.11 Features <ul><li>802.11 hints at supporting added features </li></ul><ul><ul><li>Adapt transmission rate, depending on the SNR (signal to noise ratio) and other channel characteristics (e.g. lost frames) </li></ul></ul><ul><ul><li>Power management, by limiting the time various functions are on; done by putting itself to sleep </li></ul></ul><ul><ul><ul><li>It can tell its access point it’s asleep, so frames aren’t sent to it until it wakes up! </li></ul></ul></ul>INFO 331 chapter 6
  44. 44. 802.15 WPAN <ul><li>The 802.11 standards are designed for wireless communication up to 100 meters </li></ul><ul><li>The 802.15 wireless personal area network ( WPAN ) is for ad hoc wireless networking with a range of about 10 meters </li></ul><ul><li>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 </li></ul>INFO 331 chapter 6
  45. 45. 802.15 WPAN <ul><li>The master node decides which devices are active or parked </li></ul><ul><ul><li>Can have up to 255 parked devices </li></ul></ul><ul><li>Operates at 2.4 GHz using TDM with slot of 625  s, and 79 channels </li></ul><ul><li>Hops randomly across channels (frequency-hopping spread spectrum, or FHSS) </li></ul><ul><li>Data rates up to 4 Mbps </li></ul>INFO 331 chapter 6
  46. 46. WiMAX <ul><li>WiMAX is World Interoperability for Microwave Access , IEEE 802.16 </li></ul><ul><li>It uses a base station to coordinate sending and receiving packets, similar to 802.11 infrastructure mode, using TDM </li></ul><ul><li>Each frame defines the physical layer properties for later packets; hence the transmission approach can change to get the best reception possible </li></ul>INFO 331 chapter 6
  47. 47. WiMAX <ul><li>The transmission time allocated to each subscriber can be controlled </li></ul><ul><li>WiMAX uses a connection identifier in the packet to allow quality of service (QoS) to be customized </li></ul><ul><ul><li>MAC addresses are mapped to the connection identifiers </li></ul></ul><ul><li>WiMAX is a complex beast, and is changing rapidly </li></ul>INFO 331 chapter 6
  48. 48. Cellular Internet Access <ul><li>Since Wi-Fi is limited to about 100 meters, how do we connect to the Internet when far from an access point? </li></ul><ul><ul><li>Use your cell phone! </li></ul></ul><ul><li>Key concerns are: </li></ul><ul><ul><li>Is it fast? </li></ul></ul><ul><ul><li>Is it reliable? </li></ul></ul><ul><ul><li>Is it going to be better than a long distance wireless LAN? </li></ul></ul>INFO 331 chapter 6
  49. 49. GSM Cellular Architecture <ul><li>Cellular architecture is broken into … cells </li></ul><ul><li>Each cell is a geographic area served by a cell tower, which routes through a mobile switching center (MSC) </li></ul><ul><ul><li>Acts like a switching center or central office </li></ul></ul><ul><li>The center is connected to the Internet directly, and/or the phone system (Public Switched Telephone Network) </li></ul>INFO 331 chapter 6
  50. 50. Cellular Architecture INFO 331 chapter 6
  51. 51. Sharing Frequencies <ul><li>Each cell tower handles many calls simultaneously, so multiple access protocols are needed </li></ul><ul><ul><li>Combined FDM and TDM </li></ul></ul><ul><ul><li>CDMA (code division, not carrier sense) </li></ul></ul>INFO 331 chapter 6
  52. 52. Cell Technology Generations <ul><li>The standards used for communication between cell phones and cell towers are grouped by the generation of technology involved </li></ul><ul><li>First Generation (G1) was the analog FDMA phone, now nearly dead in the US </li></ul><ul><li>Second Generation (G2) was the start of digital phone service </li></ul>INFO 331 chapter 6
  53. 53. Second Generation <ul><li>Second generation cell phones used </li></ul><ul><ul><li>IS-136 , a combined FDM/TDM derived from FDMA </li></ul></ul><ul><ul><li>GSM , a European-initiated FDM/TDM, now widely used in North America </li></ul></ul><ul><ul><li>IS-95 , a CDMA-based approach from Qualcomm </li></ul></ul><ul><li>To bridge the gap to third generation, generation 2.5 was developed </li></ul>INFO 331 chapter 6
  54. 54. Generation 2.5 <ul><li>Generation 2.5 includes </li></ul><ul><ul><li>GPRS, an upgrade from GSM which uses circuit switching (slow and inefficient for Internet); max data rate only 9.6 kbps </li></ul></ul><ul><ul><li>EDGE, was to replace GSM/GPRS and crank data rate up to 384 kbps </li></ul></ul><ul><ul><li>CDMA2000, an upgrade of IS-95 to get up to 144.4 kbps, also called 1xRTT </li></ul></ul>INFO 331 chapter 6
  55. 55. 3G <ul><li>3G cell technology claims at least 2 Mbps indoors, and 384 kbps outdoors </li></ul><ul><li>Is really UMTS/HSDPA, but that’s too long! </li></ul><ul><li>Runs on multiple frequencies: 850, 1900, and 2100 MHz* </li></ul>* and INFO 331 chapter 6
  56. 56. Generations 3 and 4 <ul><li>Third generation cellular technology includes </li></ul><ul><ul><li>UMTS, a GSM upgrade by Cingular and T-mobile to get realistic speeds of 300-400 kbps </li></ul></ul><ul><ul><li>More CDMA2000 variations, such as EV-DO and EV-DV, aiming for peak speeds of 2.4 Mbps </li></ul></ul><ul><li>Generation 4 might see WiMax take over the cell phone wars, possibly in conjunction with the 3G cell standards </li></ul><ul><li>Next slide is c*net’s view of cell technology </li></ul>INFO 331 chapter 6
  57. 57. Cellular Internet Technologies From cnet . INFO 331 chapter 6
  58. 58. 4G and Beyond <ul><li>We’d like to see cell and wireless IP technologies merge so we can </li></ul><ul><ul><li>take the best connection speed available, </li></ul></ul><ul><ul><li>keep a TCP connection when we move around, </li></ul></ul><ul><ul><li>support real time voice and video over IP, </li></ul></ul><ul><ul><li>and be available anywhere </li></ul></ul><ul><li>Oddly enough, it isn’t that far away… </li></ul>INFO 331 chapter 6
  59. 59. Mobility Management <ul><li>That concludes addressing the wireless aspect of networking </li></ul><ul><li>Now, how do we handle a host moving from one part of the network to another? </li></ul><ul><ul><li>From the network layer, a laptop that moves around in one subnet isn’t mobile </li></ul></ul><ul><ul><li>From the link layer, if they stay keep using one access point, they aren’t mobile </li></ul></ul>INFO 331 chapter 6
  60. 60. What is mobile? <ul><li>Does a user connect separately at different parts of the network, or need to maintain a connection while moving? </li></ul><ul><li>Does their IP address need to be the same? </li></ul><ul><li>What wired infrastructure is available? </li></ul>INFO 331 chapter 6
  61. 61. Mobility Terms <ul><li>Your home network is the network you started in </li></ul><ul><ul><li>Your first hop router is a home agent </li></ul></ul><ul><li>While moving, you are in a foreign or visited network </li></ul><ul><ul><li>Your first hop router is a foreign agent </li></ul></ul><ul><li>You want to communicate with a correspondent </li></ul>INFO 331 chapter 6
  62. 62. Mobility Terms Home agent in home network INFO 331 chapter 6
  63. 63. Addressing <ul><li>As hinted in the previous slide, addressing is a key concern </li></ul><ul><li>How does the visited network indicate the home host is there? </li></ul><ul><ul><li>Could update routing tables to indicate that particular address is in the visited network </li></ul></ul><ul><ul><li>But what about when 1000’s of users are mobile? Routing tables would get huge & hard to maintain </li></ul></ul>INFO 331 chapter 6
  64. 64. Addressing <ul><li>Instead, push mobility concerns to the edge of the network – the edge routers </li></ul><ul><ul><li>Let the home agent keep track of the permanent (home) address, and the foreign address </li></ul></ul><ul><ul><li>A care-of-address (COA) is the address of the foreign agent of the host </li></ul></ul><ul><ul><li>The COA is used to re-route datagrams to the foreign agent, who then passes them to the host </li></ul></ul><ul><li>Use this via indirect or direct routing </li></ul>INFO 331 chapter 6
  65. 65. Indirect Routing <ul><li>We could blindly forward datagrams to the home agent </li></ul><ul><ul><li>Let it change the address to the COA/foreign agent </li></ul></ul><ul><ul><li>The foreign agent sends them to the host </li></ul></ul><ul><li>It works, but it’ll take a while </li></ul><ul><li>The home agent needs to encapsulate the datagram to get to the COA, who unwraps it </li></ul><ul><ul><li>This is like tunneling for IPv6 </li></ul></ul>INFO 331 chapter 6
  66. 66. Indirect Routing INFO 331 chapter 6
  67. 67. Indirect Routing <ul><li>So for indirect routing, we need </li></ul><ul><ul><li>A mobile node to foreign agent protocol </li></ul></ul><ul><ul><li>A foreign agent to home agent protocol </li></ul></ul><ul><ul><li>A home agent encapsulation protocol </li></ul></ul><ul><ul><li>A foreign agent de-encapsulation protocol </li></ul></ul><ul><li>Every time the node moves to a new foreign agent, it has to register its presence (association) and update its home agent </li></ul><ul><li>Is used in the mobile IP standard (RFC 3344) </li></ul>INFO 331 chapter 6
  68. 68. Direct Routing <ul><li>Direct routing avoids the inefficiency inherent in indirect routing </li></ul><ul><ul><li>The correspondent goes through a corresponding agent (router), who learns the COA of the node </li></ul></ul><ul><ul><li>Then the corresponding agent sends data directly to the COA </li></ul></ul><ul><li>Need a mobile-user location protocol , to get the COA from the home agent </li></ul>INFO 331 chapter 6
  69. 69. Direct Routing INFO 331 chapter 6
  70. 70. Direct Routing <ul><li>But how update the corresponding agent if the node’s COA changes during a session? </li></ul><ul><ul><li>Use an anchor foreign agent (the first foreign agent used) to keep track of the current COA </li></ul></ul><ul><ul><li>Then if the node is out of the anchor’s network, encapsulate it and forward to the current foreign agent </li></ul></ul><ul><li>A little tedious, but probably more efficient than indirect routing  </li></ul>INFO 331 chapter 6
  71. 71. Mobile IP <ul><li>How mobile IP addresses can be handled is a huge topic </li></ul><ul><li>RFC 3344, hinted earlier, defines many allowable approaches </li></ul><ul><ul><li>With or without foreign agents </li></ul></ul><ul><ul><li>How agents and nodes can discover each other </li></ul></ul><ul><ul><li>Single or multiple COAs </li></ul></ul><ul><ul><li>Many forms of encapsulation </li></ul></ul>INFO 331 chapter 6
  72. 72. Mobile IP <ul><li>The three key functions of mobile IP are </li></ul><ul><ul><li>Discovery - how agents and nodes advertise their presence to each other </li></ul></ul><ul><ul><li>Registration – how nodes and agents register and deregister COAs with one’s home agent </li></ul></ul><ul><ul><li>Indirect routing – how home agents can reroute datagrams, with forwarding rules, error handling, and different forms of encapsulation </li></ul></ul>INFO 331 chapter 6
  73. 73. Agent Discovery <ul><li>A node arriving at a new network needs to identify the network </li></ul><ul><ul><li>This is called agent discovery </li></ul></ul><ul><li>Two ways to do this are agent advertisement or agent solicitation </li></ul><ul><li>Agent advertisement is when the agent broadcasts its services over ICMP (type 9, router discovery) </li></ul>INFO 331 chapter 6
  74. 74. Agent Advertisement <ul><li>The broadcast gives the IP address of the router (agent) and: </li></ul><ul><ul><li>Whether the agent is willing to act as a home and/or foreign agent (H or F bits) </li></ul></ul><ul><ul><li>If registration is needed before you can get a COA in a foreign network (R bit) </li></ul></ul><ul><ul><li>If other forms of encapsulation is needed (M or G bits) </li></ul></ul><ul><ul><li>COA data (one or more COA addresses) </li></ul></ul>INFO 331 chapter 6
  75. 75. Agent Advertisement INFO 331 chapter 6
  76. 76. Agent Solicitation <ul><li>Agent solicitation is used when a node wants to find agents without waiting for advertisements </li></ul><ul><ul><li>Solicitations are ICMP messages with type = 10 </li></ul></ul><ul><li>When an agent gets a solicitation, it responds directly to the node, and registration proceeds normally from there </li></ul>INFO 331 chapter 6
  77. 77. Registration with home agent <ul><li>When a mobile node gets a COA, that address must be registered with its home agent (router) </li></ul><ul><li>This could be done by the foreign agent, or by the node </li></ul><ul><li>In the former case, there are four steps </li></ul>INFO 331 chapter 6
  78. 78. Registration with home agent <ul><ul><li>Node sends registration message to foreign agent (over UDP, port 434) </li></ul></ul><ul><ul><li>Foreign agent gets message, records node’s permanent IP address, and sends registration message (UDP/434) to home agent </li></ul></ul><ul><ul><li>Home agent verifies the message, and connects node’s permanent IP to the COA </li></ul></ul><ul><ul><li>Foreign agent gets registration reply, and forwards it to the mobile node </li></ul></ul>INFO 331 chapter 6
  79. 79. Registration with home agent INFO 331 chapter 6
  80. 80. Registration with home agent <ul><li>When registration is complete, the node can get data sent to its permanent address via the new COA </li></ul><ul><ul><li>The actual registration lifetime granted (in seconds) is less than that requested </li></ul></ul><ul><ul><li>The identification number acts like a sequence number, to match reply with its request </li></ul></ul><ul><li>Deregistering a COA isn’t needed, since it will be overwritten by a new COA </li></ul>INFO 331 chapter 6
  81. 81. Managing Cellular Mobility <ul><li>For contrast to IP networks, let’s peek at how cellular networks manage handing off a connection </li></ul><ul><li>Look at the GSM architecture, since it’s a mature example </li></ul><ul><ul><li>It follows an indirect approach </li></ul></ul><ul><ul><li>The home network is officially called the home public land mobile network (PLMN) </li></ul></ul><ul><ul><li>The foreign network is here a visited network </li></ul></ul>INFO 331 chapter 6
  82. 82. Managing Cellular Mobility <ul><li>The home network maintains a home location register ( HLR ) with your cell phone number subscriber information, and current location information </li></ul><ul><li>A switch in the home network, the gateway mobile services switching center (GMSC), is contacted when an outside call is placed to the cell phone </li></ul><ul><ul><li>Here call this switch the home MSC </li></ul></ul>INFO 331 chapter 6
  83. 83. Managing Cellular Mobility <ul><li>The visited network maintains the visitor location register (VLR), with an entry for each mobile user currently in the network </li></ul><ul><ul><li>The VLR and the MSC are generally colocated </li></ul></ul><ul><li>So a given cellular network is the home network for its subscribers, and a visited network for phones from other providers </li></ul>INFO 331 chapter 6
  84. 84. Routing Calls to Cellular User <ul><li>For a call to get to a cellular user: </li></ul><ul><ul><li>A correspondent places the call </li></ul></ul><ul><ul><li>The call is routed to the MSC in the home network </li></ul></ul><ul><ul><li>The home MSC checks the HLR to see where the user is located </li></ul></ul><ul><ul><ul><li>It might return the mobile station roaming number (MSRN, here just roaming number ), a fake phone number which points to the user when in the network </li></ul></ul></ul>INFO 331 chapter 6
  85. 85. Routing Calls to Cellular User <ul><ul><ul><li>Or it will return the VLR of the visited network; the MSC will ask the VLR for the roaming number </li></ul></ul></ul><ul><ul><li>Given the roaming number, the MSC can now route the call to the VLR and get to the user </li></ul></ul><ul><li>For this to work, the user must exchange signaling messages with the VLR, who then passes that information to the HLR </li></ul>INFO 331 chapter 6
  86. 86. Routing Calls to Cellular User INFO 331 chapter 6
  87. 87. Handoffs in GSM <ul><li>Handoff is when a user changes association during a call </li></ul><ul><ul><li>Here from the old base station to the new base station </li></ul></ul><ul><li>If both base stations share the same MSC, life is easier </li></ul><ul><ul><li>Might need to handoff due to weak signal, or high traffic load on the old base station </li></ul></ul>INFO 331 chapter 6
  88. 88. Handoffs in GSM <ul><li>The handoff process includes </li></ul><ul><ul><li>Old base station (BS) informs MSC that handoff is needed </li></ul></ul><ul><ul><li>MSC sets up path for new BS and opens channel </li></ul></ul><ul><ul><li>New BS allocates resources and new channel </li></ul></ul><ul><ul><li>New BS tells MSC and old BS that user should be told what’s going on </li></ul></ul>INFO 331 chapter 6
  89. 89. Handoffs in GSM <ul><ul><li>Mobile user is told it should handoff </li></ul></ul><ul><ul><li>Mobile and new BS exchange messages to activate new channel </li></ul></ul><ul><ul><li>Mobile user sends handoff complete message to new BS </li></ul></ul><ul><ul><li>Old BS de-allocates resources </li></ul></ul><ul><li>So how does this process change when a different MSC is involved? </li></ul>INFO 331 chapter 6
  90. 90. Handoffs in GSM <ul><li>For handoff between MSCs, the first one is the anchor MSC </li></ul><ul><li>The anchor MSC stays the same regardless of where the user goes </li></ul><ul><li>The current user location is the visited MSC </li></ul><ul><li>Hence the home MSC, anchor MSC, and visited MSC are tracked throughout the call </li></ul><ul><ul><li>IS-41 networks maintain chains of MSCs </li></ul></ul>INFO 331 chapter 6
  91. 91. GSM versus IP networks INFO 331 chapter 6
  92. 92. Mobile effect on higher layers <ul><li>Mobile protocols clearly affect the physical, link, and often the network layers </li></ul><ul><li>Are transport and application layers affected? </li></ul><ul><ul><li>Mostly performance is affected </li></ul></ul><ul><ul><li>Since TCP retransmits lost segments, much worse performance can be seen under wireless </li></ul></ul><ul><ul><ul><li>The congestion window size (CongWin) is reduced frequently, reducing efficiency, even though there may be little actual congestion </li></ul></ul></ul>INFO 331 chapter 6
  93. 93. Mobile effect on higher layers <ul><li>Ways around this have been proposed </li></ul><ul><ul><li>Use ARQ methods to detect and repair bit errors </li></ul></ul><ul><ul><li>Split TCP into two segments; one wired and one wireless </li></ul></ul><ul><ul><li>TCP-aware link protocols </li></ul></ul><ul><ul><li>Change TCP so it handles wireless losses differently than wired losses </li></ul></ul><ul><li>Applications need to consider low bandwidth, e.g. from 3G phone, and small image sizes </li></ul>INFO 331 chapter 6
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