WIRELESS COMMUNICATION Husnain Sherazi Lecture 1

Wireless Communication Networks It is a type of computer network which does not require wires between the network devices. It uses radio waves to connect different devices. Wireless networks are convenient. Mobility. Expandable. Easy connectivity. Normally less costly.

Wireless Networks

Elements of Wireless Networks

Link Characteristics

Wired vs. Wireless Networks Wired NetworksWireless Networks Based on CablesBased on radio frequencies Communication within metersCommunication over kilometers SecureLess secure Not MobileMobile Less interferenceMore Interference Cable expensesNo cable expenses Difficult to setupEasy to setup Difficult to manageEasy to manage Nodes are visibleNodes are not always visible

Wireless Hopping Wireless hopping means whether the data is transferred directly to the end device or by jumping from one device to other and then the end device. Wireless Hopping can be Single Hop Infrastructure based Single Hop Infrastructure less Multi Hops Infrastructure based Multi Hops Infrastructure less

Single Hop Infrastructure based These networks have base stations. These base stations are connected with wired networks All the communication is done between the base stations and the end devices. All this communication is done directly on a single hop Networks fall in this category. Wi-Fi networks used in classroom and café etc. are all part of Single Hop Infrastructure based.

Single Hop Infrastructure based

Single Hop Infrastructure less These networks have no base stations. No base stations are connected with wired networks. Communication is done by one coordinator (node). This coordinator node transmits for others as well. A node connects with other node to be a part of that network. All this communication is done directly on a single hop. Bluetooth networks fall in this category. Ad-hoc networks are also example of Single Hop Infrastructure less.

Single Hop Infrastructure less

Multi Hop Infrastructure based These networks have base stations. These base stations are connected with wired networks. Nodes can communicate with the base station directly and sometimes by using other nodes for communication. This communication of nodes with base station using other nodes is done through multi hops. Some wireless sensor networks fall in this category.

Multi Hop Infrastructure based Base Station Node

Multi Hop Infrastructure less These networks have no base stations. No base stations are connected with wired networks. Nodes have to transmit data to the destination by relaying on other nodes. Such communication is done using Multi Hop but with no base station. Mobile Ad-hoc Networks and Vehicular Ad-hoc Networks are part of this network

Multi Hop Infrastructure less

and Wireless LANs The most commonly used Wireless LAN is IEEE It is also called Wi-Fi ( Wi reless Fi delity). Several standards are available a b g n Most popular these days are combination of these standards (802.11a/b/g) or (802.11b/g).

and Wireless LANs These standards share some same attributes CSMA/CA Support for Infrastructure and Ad-hoc mode Ability to reduce transmission rate to reach out more distance A new standard n is used for multiple inputs and outputs.

Architecture The most important building block of a Architecture is a Basic Service Set (BSS). A BSS can have one or more nodes. BSS can also have a central device; Access Point (AP). These APs can connect to other devices like Router and Switches. In a Home network, there is only one Router and one Access Point which are sometimes combined into a Wireless Router.

Architecture

This network can be Ad-Hoc. In an Ad-Hoc network, nodes can group themselves without any need of a central device (AP). In Ad-Hoc network, connection is made by the devices in near proximity. Normally this type of network is made when nodes want to transfer data to each other and there is no central device available.

Architecture Ad-Hoc Network

Channel and Associations When AP is installed, it is provided with Service Set Identifier (SSID) Channel Number When a node enters the range of an AP, it must associate with AP to use the internet. Node can associate with exactly one AP. The associated AP can only send and receive the frames to and from the node. To associate with an AP, the node should accept the beacon frames sent by an AP.

Channel and Associations Beacon Frame SSID of AP MAC address of AP AP sends beacon frames periodically and nodes can listen to these beacon frames.

Active and Passive Scanning Nodes can scan different channels for beacon frames from APs. This scanning is called Passive Scanning. Nodes can broadcast a probe frame to all the APs in range. APs respond to this request from Nodes. This scanning is called Active Scanning. Association Request and Response are used in Passive Scanning. Probe Request, Probe Response and Association Request and Association Response are used in Active Scanning.

Active and Passive Scanning

MAC Protocol In Wireless communication, there are chances that data can be lost because of several reasons. To avoid data loss, we can use acknowledgment called Link Layer Acknowledgment Scheme. In this scheme, when receiver receives the frame, it wait for some time and then sends the acknowledgment. This wait time is called Short Inter-Frame Spacing (SIFS). If acknowledgment is not received by sender, it will retransmit. If retransmit acknowledgments are received for fixed time, the frames are discarded.

Distributed Inter-Frame Space Short Inter-Frame Space

CSMA/CA To overcome the problem of collision, Ethernet uses a technique called CSMA/CD. However in Wireless communication, a different technique is used to overcome collision. In Wireless communication, it is called Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA)

CSMA/CA Node checks the channel Channel Idle? Node waits for some time called DIFS and then transmits the frames Node choses a backoff value and counts it down till 0. Transmits the frames if value reaches 0. Waits for acknowledgment NoYes

CSMA/CA In Wireless transmission, the target is to avoid collision instead of detecting collision. So a backoff value is used to count down till 0, before the frame is sent to destination. This wait time is called Distributed Inter-Frame Space. The value of backoff will be reduced only when the channel is idle. If the channel is busy, the value of backoff will be frozen.

Hidden Terminals There are scenarios in which nodes are hidden from each other. If two nodes are associating with an AP. Nodes might be very far from each other so that they cannot listen to each other. However AP can listen to both the nodes. In this case, there are chances of collision, since both the nodes do not see each other.

Hidden Terminals

Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C senses carrier, finds medium in use and has to wait A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B Hidden and Exposed Terminals BAC

CSMA/CA To avoid collision, two concepts are used RTS (Request to Send) CTS (Clear to Send) Whenever a node wants to send some data, it will send a RTS to AP. AP after receiving the request will broadcast a CTS to make sure no other node uses the channel. Node can now send the data to AP. AP after receiving the data will send an acknowledgment.