Wireless LAN and IEEE 802.11

1. Wireless LAN and IEEE 802.11 By harit desai

2. Introduction • LANs were limited to the physical, hardwired infrastructure. • Major benefit from wireless LANs is the increased mobility and flexibility • W LANs offer connectivity and convenience of wired LANs.

3. Mobile IP • Here, focus is on network layer. • IP address of the mobile machine does not change when it moves from home n/w to foreign n/w. • To maintain connection between mobile node and the network ,a forwarding routine is implemented. • When a mobile agent moves from home n/w to foreign n/w, mobile agent tells the home agent to which foreign agent their packets should be forwarded. • Also the mobile agent registers itself with that foreign agent.

4. Thus , all packets intended for the mobile agent are forwarded by home agent to the foreign agent which sends them to mobile agent. • When mobile agent returns to its original network ,it informs both agents that the original configuration is restored. • Drawbacks:- there is a need for store and forwarding of packets while the mobile agent is neither on home nor foreign n/w. • It works only for IPv4 and does not take advantage of the features of the newer IPv6

5. IEEE 802.11 Architectures • Two ways: Ad-hoc and infrastructure. • Ad-hoc: computers are brought together to form a network “on the fly”. • There is no structure to the n/w. • There are no fixed points. • Usually every node is able to communicate with every other node . • Election algorithm is used to elect one machine as the base station in a n/w . • Uses the broadcast and flooding method to establish who’s who in the n/w.

6. Infrastructure: this architecture uses fixed network access points with which mobile nodes can communicate. • Similar to cellular networks. • Network access points may be connected to landlines to enhance the LAN’s capability by bridging wireless nodes to other wired nodes.

7. IEEE 802.11 layers • Physical layer actually handles the transmission between the nodes. • Transmission can be direct sequence spread spectrum ,frequency-hopping spread spectrum or infrared pulse position modulation. • Data rates of 1Mbps or 2Mbps. • Infrared is consider more secure to eavesdropping, because IR transmission require absolute line of sight links. • MAC layer is responsible for maintaining order in the use of shared medium. • 802.11 standard specifies a CSMA/CA protocol. • When a node receives a packet to be transmitted ,it first listens to ensure no other node is transmitting.if the channel is clear , it then transmits the packet.

8. Otherwise , it chooses a random “back off factor” which determines the amount of time the node must wait before transmitting. • During periods when the channel is clear, the node decrements its backoff counter. • When backoff counter reaches zero, the node transmits the packet. • Since the probability that two nodes will choose the same back off factor is small, collision between packets are minimized. • Collision detection cannot be used because when a node is transmitting it cannot hear any other node in the system. • So , transmitting node first sends out a short ready-to-send message containing the information on the length of the message. • If receiving node hears RTS, it responds with a short clear-to-send packet. • When a packet is received successfully(CRC), the receiving node sends an acknowledgment packet.This back-and-forth exchange is necessary to avoid the “hidden node” problem.

9. WirelessMAN and IEEE 802.16 • WirelessMAN provides network access to buildings through exterior antennas communicating with central radio base stations(BSs). • Physical layer : 10 – 66 GHz specification. • Line of sight propagation is a practical necessity. • BS transmits a TDM signal with individual subscriber stations allocated time slots serially. • Access in the uplink direction is by TDMA. • TDD : time division duplexing in which the uplink and downlink share a same channel but do not transmit simultaneously.

10. FDD : Frequency division duplexing in which uplink and downlink operate on separate channels. • 2 – 11 GHz : Here both licensed and license exempted are addressed in IEEE 802.16a. • It is driven by the need for non-line of sight operation because rooftops may be too low and there may be an obstruction. • Is uses single carrier modulation format.

11. MAC layer • Two general service-specific convergence sublayers for mapping services to and from 802.16 MAC connections. • ATM convergence sublayer is defined for ATM services and packet convergence sublayer is defined for mapping packet services such as IPv4,IPv6 , ethernet and virtual local area network. • Primary task of the sublayer is to classify service data units(SDUs) to proper MAC connection and enable bandwidth allocation. • Common part sublayer : MAC 802.16 is design to support a point to multipoint architecture with central BS. • On downlink , data to SS are multiplexed in TDM fashion and the uplink is shared between SS in TDMA fashion. • 802.16 MAC is a connection oriented service. Also connectionless service are mapped to a connection.

12. Connection are referenced with 16 bit connection identifiers (CIDs) and may require continuously granted bandwidth or bandwidth on demand. • Each access has 48 bit MAC address but this serves as equipment identifier , since the primary address is used during the operations are CIDs. • Upon entering the network, the access is assigned 3 management connections from each directions. • 3 connections reflect 3 different QOS requirements used by different levels. • First is used for the transfer of short , time-critical MAC and radio link control messages ( RLC ). • Primary management connection is used to transfer longer more delay tolerant messages, such as authentication and connection setup. • Secondary management connection is used for transfer of standards-based management messages such as DHCP , TFTP , and SNMP.

13. MAC PDU Formats : It is the data unit exchanged between the MAC layers of the BS and its SSs. • A MAC PDU consists of fixed-length MAC header, variable-length payload, and an optional cyclic redundancy check. • Two header formats: the generic header and the bandwidth request header. bandwidth request MAC PDUs contains no payload and MAC PDUs contain either MAC management messages or convergence sublayer data. • Three types of MAC subheader may be present • The grant management subheader is used by an SS to convey bandwidth management needs to its BS. • The fragmentation subheader contains information that indicates the presenceand orientation in the payload of any fragments of SDUs. • The packing subheader is used to indicate the packing of multiple SDUs into a single PDU.

14. SS Authentication and Registration — • Each SS contains both a manufacturer-issued X.509 digital certificate and the certificate of the manufacturer. • These certificates establish a link between the 48-bit MAC address of the SS and its public RSA key. • These are sent to the BS by the SS in the Authorization Request and Authentication Information messages. • If the SS is authorized, the BS will respond to its request with an Authorization Reply containing an Authorization Key (AK) encrypted with the SS’s public key and used to secure further transactions.  • Upon successful authorization, the SS will register with the network. • IP Connectivity — • After registration, the SS attains an IP address via DHCP and establishes the time of day via the Internet Time Protocol. • The DHCP server also provides the address of the TFTP server from which the SS can request configuration file. • This file provides a standard interface for providing vendor-specific configuration information.