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Transcript
WIRELESS PROTOCOLS The ISO OSI Reference Model • A Network is a complex piece of hardware and Software. • Therefore, International Standard Organization (ISO) adopted a layered approach for networks. • The network is broken into seven layers, each of them has well-defined function such that each layer offers a service to the higher layer. • The model is called ISO OSI (Open Systems Interconnect) Reference Model. • IEEE defined a four layers model for LAN The OSI Reference Model Layers Host A Host B Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Physical Medium IEEE 802 Model Layers Protocol Definision • Protocol: is a set of rules and conventions used by the layer to communicate with similar peer layer in another (remote) system. • Peer: The similar layer on a different machine. – The peer processes communicate with each other using a protocol. • A set of layers and protocols is called a network architecture. Key Features of a Protocol • Syntax – Concerns the format of the data blocks • Semantics – Includes control information for coordination and error handling • Timing – Includes speed matching and sequencing Agents Involved in Protocol Communication • Applications – Exchange data between computers (e.g., electronic mail) • Computers – Connected to networks • Networks – Transfers data from one computer to another Protocol Architecture • Functions of physical layer: – – – – Encoding/decoding of signals Preamble generation/removal (for synchronization) Bit transmission/reception Includes specification of the transmission medium Protocol Architecture • Functions of medium access control (MAC) Sub layer: – On transmission, assemble data into a frame with address and error detection fields – On reception, disassemble frame and perform address recognition and error detection – Govern access to the LAN transmission medium • Functions of logical link control (LLC) Sub Layer: – Provide an interface to higher layers and perform flow and error control Separation of LLC and MAC • The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control • For the same LLC, several MAC options may be provided MAC Frame Format • MAC control – Contains Mac protocol information • Destination MAC address – Destination physical attachment point • Source MAC address – Source physical attachment point • CRC – Cyclic redundancy check Logical Link Control • Characteristics of LLC not shared by other control protocols: – Must support multiaccess, shared-medium nature of the link – Relieved of some details of link access by MAC layer LLC Services • Unacknowledged connectionless service – No flow- and error-control mechanisms – Data delivery not guaranteed • Connection-mode service – Logical connection set up between two users – Flow- and error-control provided • Acknowledged connectionless service – Cross between previous two – Datagrams acknowledged – No prior logical setup Differences between LLC and HDLC • LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation) • LLC supports unacknowledged connectionless service (type 1 operation) • LLC supports acknowledged connectionless service (type 3 operation) • LLC permits multiplexing by the use of LLC service access points (LSAPs) IEEE 802.11 Services Mobile IP Uses • Enable computers to maintain Internet connectivity while moving from one Internet attachment point to another • Mobile – user's point of attachment changes dynamically and all connections are automatically maintained despite the change • Nomadic - user's Internet connection is terminated each time the user moves and a new connection is initiated when the user dials back in – New, temporary IP address is assigned Operation of Mobile IP • Mobil node is assigned to a particular network – home network • IP address on home network is static – home address • Mobile node can move to another network – foreign network • Mobile node registers with network node on foreign network – foreign agent • Mobile node gives care-of address to agent on home network – home agent Capabilities of Mobile IP • Discovery – mobile node uses discovery procedure to identify prospective home and foreign agents • Registration – mobile node uses an authenticated registration procedure to inform home agent of its care-of address • Tunneling – used to forward IP datagrams from a home address to a care-of address Discovery • Mobile node is responsible for ongoing discovery process – Must determine if it is attached to its home network or a foreign network • Transition from home network to foreign network can occur at any time without notification to the network layer • Mobile node listens for agent advertisement messages – Compares network portion of the router's IP address with the network portion of home address Move Detection • Mobile node may move from one network to another due to some handoff mechanism without IP level being aware – Agent discovery process is intended to enable the agent to detect such a move • Algorithms to detect move: – Use of lifetime field – mobile node uses lifetime field as a timer for agent advertisements – Use of network prefix – mobile node checks if any newly received agent advertisement messages are on the same network as the node's current care-of address Co-Located Addresses • If mobile node moves to a network that has no foreign agents, or all foreign agents are busy, it can act as its own foreign agent • Mobile agent uses co-located care-of address – IP address obtained by mobile node associated with mobile node's current network interface • Means to acquire co-located address: – Temporary IP address through an Internet service, such as DHCP – May be owned by the mobile node as a long-term address for use while visiting a given foreign network Registration Process • Mobile node sends registration request to foreign agent requesting forwarding service • Foreign agent relays request to home agent • Home agent accepts or denies request and sends registration reply to foreign agent • Foreign agent relays reply to mobile node Wireless Application Protocol (WAP) • Open standard providing mobile users of wireless terminals access to telephony and information services – Wireless terminals include wireless phones, pagers and personal digital assistants (PDAs) – Designed to work with all wireless network technologies such as GSM, CDMA, and TDMA – Based on existing Internet standards such as IP, XML, HTML, and HTTP – Includes security facilities WAP Protocol Stack WAP Programming Model Wireless Datagram Protocol (WDP) • Used to adapt higher-layer WAP protocol to the communication mechanism used between mobile node and WAP gateway • WDP hides details of the various bearer networks from the other layers of WAP • Adaptation may include: – Partitioning data into segments of appropriate size for the bearer – Interfacing with the bearer network Wireless Control Message Protocol (WCMP) • Performs the same support function for WDP as ICMP does for IP • Used in environments that don’t provide IP bearer and don’t lend themselves to the use of ICMP • Used by wireless nodes and WAP gateways to report errors encountered in processing WDP datagrams • Can also be used for informational and diagnostic purposes