Transcript Wireless LAN and IEEE 802.11

Wireless LAN and IEEE
802.11
By harit desai
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.
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.

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
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.

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.
IEEE 802.11 layers







Physical layer actually handles the transmission between the nodes.
Transmission can be direct sequence spread spectrum ,frequencyhopping 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.








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.
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.
– 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.
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.
– 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.

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 presence and 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.

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.