Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Computer network wikipedia , lookup
Zero-configuration networking wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Airborne Networking wikipedia , lookup
Policies promoting wireless broadband in the United States wikipedia , lookup
Wireless security wikipedia , lookup
List of wireless community networks by region wikipedia , lookup
Cellular network wikipedia , lookup
INFO 331 Computer Networking Technology II Chapter 6 Wireless Networking Dr. Jennifer Booker INFO 331 chapter 6 1 www.ischool.drexel.edu 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 – 5.5 billion by 2011 • Distinguish between wireless connectivity and the mobility that affords – Some wireless devices are stationary INFO 331 chapter 6 2 www.ischool.drexel.edu 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 communication links are analogous to the wired variety INFO 331 chapter 6 3 www.ischool.drexel.edu 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 4 www.ischool.drexel.edu 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 5 www.ischool.drexel.edu 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 6 www.ischool.drexel.edu 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 7 www.ischool.drexel.edu 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 8 www.ischool.drexel.edu Wireless Links Problems • The bit error rates (BER) and signal-tonoise ratio (SNR) are inversely related – A high SNR means a lower BER – The BER is expressed as a number, e.g. 10-5 – SNR is given in decibels (dB) • Can increase SNR somewhat by using more transmission power – Higher transmission rates have higher BER – Can modulate transmission to adapt INFO 331 chapter 6 9 www.ischool.drexel.edu Wireless Links Problems B A C C’s signal strength A’s signal strength space Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interfering at B Images from Kurose’s slides INFO 331 chapter 6 10 www.ischool.drexel.edu Some Wireless Protocols From the author’s PPT slides INFO 331 chapter 6 11 www.ischool.drexel.edu 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 12 www.ischool.drexel.edu CDMA • In CDMA, the original data stream is multiplied by a code which changes much faster than the data, the chipping rate – In the example on page 523, 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 13 www.ischool.drexel.edu CDMA Example sender d0 = 1 data bits code Zi,m= di.cm -1 --1 -1 -1 1 -1 1 --1 1 1 1 1 -1 --1 -1 -1 1 -1 slot 1 -1 -1 slot 1 channel output 1 --1 1 1 1 1 1 1 1 1 d1 = -1 1 1 1 channel output Zi,m -1 --1 -1 -1 1 -1 slot 0 1 --1 1 -1 --1 -1 -1 1 -1 slot 0 channel output M Di = Zi,m.cm m=1 received input code receiver 1 1 1 1 1 1 1 -1 --1 -1 -1 1 -1 -1 -1 1 1 1 1 --1 1 -1 --1 -1 -1 1 -1 --1 1 1 1 1 -1 --1 -1 -1 1 -1 slot 1 M 1 1 --1 1 -1 --1 -1 -1 1 -1 slot 0 INFO 331 chapter 6 d0 = 1 d1 = -1 slot 1 channel output slot 0 channel output 14 www.ischool.drexel.edu 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 15 www.ischool.drexel.edu 802.11 LAN Protocols • The WiFi or 802.11 protocols are used for local wireless networks • 802.11g and 802.11n 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 – 802.11n uses multiple antennae at 2.4 GHz • All use CSMA/CA as medium access protocol, have the same frame structure INFO 331 chapter 6 16 www.ischool.drexel.edu 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 – Notice each band is a range of frequencies (2.4 – 2.485 or 5.1 – 5.8 GHz); and they typically have 11 channels in that range INFO 331 chapter 6 17 www.ischool.drexel.edu 802.11 LAN Protocols • Both 2.4 (for .11b, g, and n) and 5.8 GHz (for .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 www.ischool.drexel.edu 802.11 LAN Protocols • 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 or about 5” • For 5.8 GHz, l = 3E10 cm/s / 5.8E9 s-1 = 5.2 cm INFO 331 chapter 6 19 www.ischool.drexel.edu 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 20 www.ischool.drexel.edu 802.11 LAN Architecture INFO 331 chapter 6 21 www.ischool.drexel.edu 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 22 www.ischool.drexel.edu 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 23 www.ischool.drexel.edu 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 24 www.ischool.drexel.edu 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 25 www.ischool.drexel.edu 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 26 www.ischool.drexel.edu 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 27 www.ischool.drexel.edu 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 midtransmission INFO 331 chapter 6 28 www.ischool.drexel.edu 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 29 www.ischool.drexel.edu 802.11 ARQ sender receiver DIFS data SIFS ACK INFO 331 chapter 6 30 www.ischool.drexel.edu 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 31 www.ischool.drexel.edu 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 32 www.ischool.drexel.edu 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 33 www.ischool.drexel.edu 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 34 www.ischool.drexel.edu 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 in 2004, for example INFO 331 chapter 6 35 www.ischool.drexel.edu 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 – Hence the 802.11 protocols are both link and physical layer protocols INFO 331 chapter 6 36 www.ischool.drexel.edu 802.11 Frame Fields – – – – – – – – – Frame control (2 B, expanded 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 37 www.ischool.drexel.edu 802.11 Frames frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 frame address address address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 Retry 1 0 - 2312 4 payload CRC 1 Power More mgt data bytes 1 1 WEP Rsvd bits frame type (RTS, CTS, ACK, data) INFO 331 chapter 6 38 www.ischool.drexel.edu 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 39 www.ischool.drexel.edu 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) is particularly important, since there is a large chance of bit errors • We’ll ignore the other fields for now INFO 331 chapter 6 40 www.ischool.drexel.edu 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 41 www.ischool.drexel.edu 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 42 www.ischool.drexel.edu 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 43 www.ischool.drexel.edu 802.15 WPAN • The 802.11 standards are designed for wireless communication up to 100 meters • The 802.15.1 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 44 www.ischool.drexel.edu 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 (frequencyhopping spread spectrum, or FHSS) – Data rates up to 4 Mbps INFO 331 chapter 6 45 www.ischool.drexel.edu Zigbee • The 802.15.4 standard defines Zigbee, which is targeted at low power, low data rate, infrequent short range communications – Home temperature and security sensors, for examples – from the Internet of Things – Channel data rates from 20-250 kbps – There are ‘full function’ devices which can act as a master controller for ‘reduced function’ devices INFO 331 chapter 6 46 www.ischool.drexel.edu 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 47 www.ischool.drexel.edu 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 48 www.ischool.drexel.edu 2G Cellular Architecture INFO 331 chapter 6 49 www.ischool.drexel.edu Sharing Frequencies • Each cell tower handles many calls simultaneously, so multiple access protocols are needed – Combined FDM and TDM – CDMA (code division, not carrier sense) INFO 331 chapter 6 50 www.ischool.drexel.edu Cell Technology Generations • The standards used for communication between cell phones and cell towers are grouped by the generation of technology involved • First Generation (G1) was the analog FDMA phone, now obsolete in the US • Second Generation (G2) was the start of digital phone service (no data) INFO 331 chapter 6 51 www.ischool.drexel.edu Second Generation • Second generation cell phones used – IS-136, a combined FDM/TDM derived from FDMA – GSM, a European-initiated FDM/TDM, now widely used in North America – IS-95, a CDMA-based approach from Qualcomm • To bridge the gap to third generation, generation 2.5 was developed INFO 331 chapter 6 52 www.ischool.drexel.edu Generation 2.5 • Generation 2.5 includes – GPRS, an upgrade from GSM which uses circuit switching (slow and inefficient for Internet); max data rate only 9.6 kbps – EDGE, was to replace GSM/GPRS and crank data rate up to 384 kbps – CDMA2000, an upgrade of IS-95 to get up to 144.4 kbps, also called 1xRTT INFO 331 chapter 6 53 www.ischool.drexel.edu 3G • UMTS is a typical third generation cellular technology – It’s built on top of GSM technology for backward compatibility – SGSN nodes act like switches to communicate with mobile nodes – GGSN nodes act like gateway routers to connect SGSNs to each other and the Internet INFO 331 chapter 6 54 www.ischool.drexel.edu 3G Architecture INFO 331 chapter 6 55 www.ischool.drexel.edu 3G Edge Technology • Third generation cellular technology uses a Radio Network Controller (RNC) to connect to mobile users – An RNC may be connected to several cell transceivers but then has to connect to both the 2G phone technology (an MSC) as well as 3G Internet technology (an SGSN) • UMTS uses multiple frequencies at once via Direct Sequence Wideband CDMA (DS-WCDMA), also known as HSPA INFO 331 chapter 6 56 www.ischool.drexel.edu 4G LTE • 4G Long Term Evolution (LTE) came from the 3rd Generation Partnership (3GPP) – Features Evolved Packet Core (EPC), an all-IP approach with good resource management to ensure high quality of service (QoS) – Uses both FDM and TDM to create orthogonal frequency division multiplexing (OFDM) and reallocating slots as often as every millisecond – Uses multiple input multiple output (MIMO) antennae (like 802.11n) to get up to 100 Mbps downstream and 50 Mbps upstream INFO 331 chapter 6 57 www.ischool.drexel.edu WiMAX • WiMAX was the World Interoperability for Microwave Access, IEEE 802.16 • Created by Sprint in 2006, it was the first 4G cell technology • Based more on wireless networking technology more than cellular • It is being replaced by LTE, which came out in 2010 http://www.pcmag.com/article2/0,2817,2403490,00.asp INFO 331 chapter 6 58 www.ischool.drexel.edu Mobility Management • That concludes addressing the wireless aspect of networking • Now, how do we handle a host moving from one part of the network to another? – From the network layer, a laptop that moves around in one subnet isn’t mobile – From the link layer, if they stay keep using one access point, they aren’t mobile INFO 331 chapter 6 59 www.ischool.drexel.edu What is mobile? • Does a user connect separately at different parts of the network, or need to maintain a connection while moving? • Does their IP address need to be the same? • What wired infrastructure is available? INFO 331 chapter 6 60 www.ischool.drexel.edu Mobility Terms • Your home network is the network you started in – Your first hop router is a home agent • While moving, you are in a foreign or visited network – Your first hop router is a foreign agent • You want to communicate with a correspondent INFO 331 chapter 6 61 www.ischool.drexel.edu Mobility Terms Home agent in home network INFO 331 chapter 6 62 www.ischool.drexel.edu Addressing • As hinted in the previous slide, addressing is a key concern • How does the visited network indicate the home host is there? – Could update routing tables to indicate that particular address is in the visited network – But what about when 1000’s of users are mobile? Routing tables would get huge & hard to maintain INFO 331 chapter 6 63 www.ischool.drexel.edu Addressing • Instead, push mobility concerns to the edge of the network – the edge routers – Let the home agent keep track of the permanent (home) address, and the foreign address – A care-of-address (COA) is the address of the foreign agent of the host – The COA is used to re-route datagrams to the foreign agent, who then passes them to the host • Use this via indirect or direct routing INFO 331 chapter 6 64 www.ischool.drexel.edu Indirect Routing • We could blindly forward datagrams to the home agent – Let it change the address to the COA/foreign agent – The foreign agent sends them to the host • It works, but it’ll take a while • The home agent needs to encapsulate the datagram to get to the COA, who unwraps it – This is like tunneling for IPv6 INFO 331 chapter 6 65 www.ischool.drexel.edu Indirect Routing INFO 331 chapter 6 66 www.ischool.drexel.edu Indirect Routing • So for indirect routing, we need – – – – A mobile node to foreign agent protocol A foreign agent to home agent protocol A home agent encapsulation protocol A foreign agent de-encapsulation protocol • Every time the node moves to a new foreign agent, it has to register its presence (association) and update its home agent • Is used in the mobile IP standard (RFC 5944) INFO 331 chapter 6 67 www.ischool.drexel.edu Direct Routing • Direct routing avoids the inefficiency inherent in indirect routing – The correspondent goes through a corresponding agent (router), who learns the COA of the node – Then the corresponding agent sends data directly to the COA • Need a mobile-user location protocol, to get the COA from the home agent INFO 331 chapter 6 68 www.ischool.drexel.edu Direct Routing INFO 331 chapter 6 69 www.ischool.drexel.edu Direct Routing • But how update the corresponding agent if the node’s COA changes during a session? – Use an anchor foreign agent (first foreign agent used) to keep track of the current COA – Then if the node is out of the anchor’s network, encapsulate it and forward to the current foreign agent • A little tedious, but probably more efficient than indirect routing INFO 331 chapter 6 70 www.ischool.drexel.edu Mobile IP • How mobile IP addresses can be handled is a huge topic • RFC 5944, hinted earlier, defines many allowable approaches – With or without foreign agents – How agents and nodes can discover each other – Single or multiple COAs – Many forms of encapsulation INFO 331 chapter 6 71 www.ischool.drexel.edu Mobile IP • The three key functions of mobile IP are – Discovery - how agents and nodes advertise their presence to each other – Registration – how nodes and agents register and deregister COAs with one’s home agent – Indirect routing – how home agents can reroute datagrams, with forwarding rules, error handling, and different forms of encapsulation INFO 331 chapter 6 72 www.ischool.drexel.edu Agent Discovery • A node arriving at a new network needs to identify the network – This is called agent discovery • Two ways to do this are agent advertisement or agent solicitation • Agent advertisement is when the agent broadcasts its services over ICMP (type 9, router discovery) INFO 331 chapter 6 73 www.ischool.drexel.edu Agent Advertisement • The broadcast gives the IP address of the router (agent) and: – Whether the agent is willing to act as a home and/or foreign agent (H or F bits) – If registration is needed before you can get a COA in a foreign network (R bit) – If other forms of encapsulation is needed (M or G bits) – COA data (one or more COA addresses) INFO 331 chapter 6 74 www.ischool.drexel.edu ICMP Agent Advertisement 0 16 8 type = 9 24 checksum =9 code = 0 =9 standard ICMP fields router address type = 16 length registration lifetime sequence # RBHFMGV bits reserved 0 or more care-ofaddresses INFO 331 chapter 6 mobility agent advertisement extension 75 www.ischool.drexel.edu Agent Solicitation • Agent solicitation is used when a node wants to find agents without waiting for advertisements – Solicitations are ICMP messages with type = 10 • When an agent gets a solicitation, it responds directly to the node, and registration proceeds normally from there INFO 331 chapter 6 76 www.ischool.drexel.edu Registration with home agent • When a mobile node gets a COA, that address must be registered with its home agent (router) • This could be done by the foreign agent, or by the node • In the former case, there are four steps INFO 331 chapter 6 77 www.ischool.drexel.edu Registration with home agent 1. Node sends registration message to foreign agent (over UDP, port 434) 2. Foreign agent gets message, records node’s permanent IP address, and sends registration message (UDP:434) to home agent 3. Home agent verifies the message, and connects node’s permanent IP to the COA 4. Foreign agent gets registration reply, and forwards it to the mobile node INFO 331 chapter 6 78 www.ischool.drexel.edu Registration with home agent home agent HA: 128.119.40.7 foreign agent COA: 79.129.13.2 visited network: 79.129.13/24 ICMP agent adv. COA: 79.129.13.2 …. registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format …. Mobile agent MA: 128.119.40.186 registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 …. registration reply time HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format …. registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 …. INFO 331 chapter 6 79 www.ischool.drexel.edu Registration with home agent • When registration is complete, the node can get data sent to its permanent address via the new COA – The actual registration lifetime granted (in seconds) is less than that requested – The identification number acts like a sequence number, to match reply with its request • Deregistering a COA isn’t needed, since it will be overwritten by a new COA INFO 331 chapter 6 80 www.ischool.drexel.edu Managing Cellular Mobility • For contrast to IP networks, let’s peek at how cellular networks manage handing off a connection • Look at the GSM architecture, since it’s a mature example – It follows an indirect approach – The home network is officially called the home public land mobile network (PLMN) – The foreign network is here a visited network INFO 331 chapter 6 81 www.ischool.drexel.edu Managing Cellular Mobility • The home network maintains a home location register (HLR) with your cell phone number subscriber information, and current location information • 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 – Here call this switch the home MSC INFO 331 chapter 6 82 www.ischool.drexel.edu Managing Cellular Mobility • The visited network maintains the visitor location register (VLR), with an entry for each mobile user currently in the network – The VLR and the MSC are generally colocated • So a given cellular network is the home network for its subscribers, and a visited network for phones from other providers INFO 331 chapter 6 83 www.ischool.drexel.edu Routing Calls to Cellular User • For a call to get to a cellular user: – A correspondent places the call – The call is routed to the MSC in the home network – The home MSC checks the HLR to see where the user is located • 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 INFO 331 chapter 6 84 www.ischool.drexel.edu Routing Calls to Cellular User • Or it will return the VLR of the visited network; the MSC will ask the VLR for the roaming number – Given the roaming number, the MSC can now route the call to the VLR and get to the user • For this to work, the user must exchange signaling messages with the VLR, who then passes that information to the HLR INFO 331 chapter 6 85 www.ischool.drexel.edu Routing Calls to Cellular User INFO 331 chapter 6 86 www.ischool.drexel.edu Handoffs in GSM • Handoff is when a user changes association during a call – Here from the old base station to the new base station • If both base stations share the same MSC, life is easier – Might need to handoff due to weak signal, or high traffic load on the old base station INFO 331 chapter 6 87 www.ischool.drexel.edu Handoffs in GSM • The handoff process includes – Old base station (BS) informs MSC that handoff is needed – MSC sets up path for new BS and opens channel – New BS allocates resources and new channel – New BS tells MSC and old BS that user should be told what’s going on INFO 331 chapter 6 88 www.ischool.drexel.edu Handoffs in GSM – Mobile user is told it should handoff – Mobile and new BS exchange messages to activate new channel – Mobile user sends handoff complete message to new BS – Old BS de-allocates resources • So how does this process change when a different MSC is involved? INFO 331 chapter 6 89 www.ischool.drexel.edu Handoffs in GSM • For handoff between MSCs, the first one is the anchor MSC • The anchor MSC stays the same regardless of where the user goes • The current user location is the visited MSC • Hence the home MSC, anchor MSC, and visited MSC are tracked throughout the call – IS-41 networks maintain chains of MSCs INFO 331 chapter 6 90 www.ischool.drexel.edu GSM versus IP networks GSM element Comment on GSM element Mobile IP element Home system Network to which the mobile user’s permanent phone number belongs Home network Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR) Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information Home agent Visited System Network other than home system where mobile user is currently residing Visited network Visited Mobile services Switching Center. Visitor Location Record (VLR) Visited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user Foreign agent Mobile Station Roaming Number (MSRN), or “roaming number” Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent. Care-ofaddress INFO 331 chapter 6 91 www.ischool.drexel.edu Mobile effect on higher layers • Mobile protocols clearly affect the physical, link, and often the network layers • Are transport and application layers affected? – Mostly performance is affected – Since TCP retransmits lost segments, much worse performance can be seen under wireless • TCP can’t tell if packet was lost, or had bit errors, or during handoff • The congestion window size (CongWin) is reduced frequently, reducing efficiency, even though there may be little actual congestion INFO 331 chapter 6 92 www.ischool.drexel.edu Mobile effect on higher layers • Ways around this have been proposed – Use ARQ and/or FEC to detect and repair bit errors – Split TCP into two segments; one wired and one wireless – Use TCP-aware link protocols – Change TCP so it handles wireless losses differently than wired losses • Applications need to consider low bandwidth, e.g. from 3G phone, and small image sizes INFO 331 chapter 6 93 www.ischool.drexel.edu Summary • Wireless and mobile computing has revolutionized telephony – Computers and phones used to be completely separate devices! – Huge contribution to ubiquitous computing • We examined traits of wireless connections, the 802.11 family of protocols, two 802.15 protocols, and how 3G and 4G cellular networks provide Internet access – Concluded looking at wireless mobility issues INFO 331 chapter 6 94 www.ischool.drexel.edu