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    Chapter 6 Chapter 6 Presentation Transcript

    • 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
        • Over 2 billion cell phones worldwide [ ITU ] as of 2005, many now Internet-aware
        • Increasing numbers of laptops, palmtops, PDAs, and other mobile networked devices
      • Distinguish between wireless connectivity and the mobility that affords
        • Some wireless devices are stationary
    • 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
    • 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
    • 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
    • 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
    • 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?
    • 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 for wireless links
    • Wireless Links Problems Images from Kurose’s slides
    • CDMA
      • Last term we covered three approaches to sharing links (multiple access)
        • Channel partitioning (TDM and FDM)
        • Random access protocols (ALOHA & C S MA)
        • Taking turns protocols (polling or token ring)
      • Here we need another type of multiple access protocol – Code Division Multiple Access (CDMA)
    • 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
    • CDMA Example
    • 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
    • 802.11 LAN Protocols
      • The WiFi or 802.11 protocols are used for local wireless networks
      • 802.11a and 802.11g are most common currently
        • Both provide service at up to 54 Mbps
        • 802.11a operates at 5.8 GHz, 802.11g at 2.4 GHz
      • All use CSMA/CA as their medium access protocol, and have the same frame structure
    • 802.11 LAN Protocols
    • 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
    • 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
      • 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
    • 802.11 LAN Protocols
      • 802.11n is being standardized
        • Uses two or more antennae to send and receive, and should be over 100 Mbps
      • What wavelength are the 802.11 bands?
        •  = c = 3E10 cm/s
        •  = c/ 
          • For 2.4 GHz,  = 3E10 cm/s / 2.4E9 s -1 = 12.5 cm
          • For 5.8 GHz,  = 3E10 cm/s / 5.8E9 s -1 = 5.2 cm
    • 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
    • 802.11 Architecture
    • 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
    • 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-router’
        • Channels 1-11 are available, but only channels 1, 6, and 11 are non-overlapping
    • 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 out beacon frames periodically, with the AP’s SSID and MAC address
        • You choose which AP with which to associate
    • 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, which might verify identity with a Radius or Diameter server
    • 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)
    • 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
    • 802.11 ARQ
      • To transmit data from a sender to a 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
    • 802.11 ARQ
    • 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
    • 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
    • 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
    • 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 a threshold, so that only files larger than threshold are allowed to use RTS/CTS
    • 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
    • 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
    • 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 ]
    • 802.11 Frames bits bytes
    • 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 the time for a clear to send (CTS)
      • Address 3 is critical for communicating across wireless networks
    • 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
    • Mobility within 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
    • 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
    • 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!
    • 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
    • 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  s, and 79 channels
      • Hops randomly across channels (frequency-hopping spread spectrum, or FHSS)
      • Data rates up to 721 kbps
    • 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
    • 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
    • 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?
    • 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)
    • Cellular Architecture
    • 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)
    • 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 essentially dead in the US
      • Second Generation (G2) was the start of digital phone service
    • 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
    • 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
    • 3G
      • 3G cell technology claims at least 2 Mbps indoors, and 384 kbps outdoors
      • Is really UMTS/HSDPA, but that’s too long!
      • Runs on multiple frequencies: 850, 1900, and 2100 MHz*
      * http://hspa.gsmworld.com/ and http://www.apple.com/iphone/specs.html
    • Generations 3 and 4
      • Third generation cellular technology includes
        • UMTS, a GSM upgrade by Cingular and T-mobile to get realistic speeds of 300-400 kbps
        • More CDMA2000 variations, such as EV-DO and EV-DV, aiming for peak speeds of 2.4 Mbps
      • Generation 4 might see WiMax take over the cell phone wars, possibly in conjunction with the 3G cell standards
      • Next slide is c*net’s view of cell technology
    • Cellular Internet Technologies From cnet . See handout for definitions.
    • 4G and Beyond
      • We’d like to see cell and wireless IP technologies merge so we can
        • take the best connection speed available,
        • keep a TCP connection when we move around,
        • support real time voice and video over IP,
        • and be available anywhere
      • Oddly enough, it isn’t that far away…
    • 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
    • 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?
    • 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
    • Mobility Terms Home agent in home network
    • 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
    • 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
    • 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 then unwraps it
        • This is like tunneling for IPv6
    • Indirect Routing
    • 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 3344)
    • 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
    • Direct Routing
    • Direct Routing
      • But how update the corresponding agent if the node’s COA changes during a session?
        • Use an anchor foreign agent (the 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 
    • Mobile IP
      • How mobile IP addresses can be handled is a huge topic
      • RFC 3344, 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
    • 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
    • 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)
    • 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)
    • Agent Advertisement
    • 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
    • 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
    • Registration with home agent
        • Node sends registration message to foreign agent (over UDP, port 434)
        • Foreign agent gets message, records node’s permanent IP address, and sends registration message (UDP/434) to home agent
        • Home agent verifies the message, and connects node’s permanent IP to the COA
        • Foreign agent gets registration reply, and forwards it to the mobile node
    • Registration with home agent
    • 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
    • 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
    • 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
    • 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
    • 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
          • Or it will return the VLR of the visited network; the MSC will ask the VLR for the roaming number
    • Routing Calls to Cellular User
        • 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
    • Routing Calls to Cellular User
    • 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
    • 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
        • Mobile user is told it should handoff
    • Handoffs in GSM
        • 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?
    • 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
    • GSM versus IP networks
    • Mobile effect on higher layers
      • Mobile protocols clearly affect the physical, link, and often the network layers
      • Are the transport and application layers affected too?
        • Mostly performance is affected
        • Since TCP retransmits lost segments, much worse performance can be seen under wireless
          • The congestion window size (CongWin) is reduced frequently, reducing efficiency, even though there may be little actual congestion
    • Mobile effect on higher layers
      • Ways around this have been proposed
        • Use ARQ methods to detect and repair bit errors
        • Split TCP into two segments; one wired and one wireless
        • 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