UNIT II NETWORKING

Transcription

1 UNIT II INTRODUCTION TO WIRELESS NETWORKING

2 Wireless Network The cellular telephone system is responsible for providing coverage throughout a particular area known as coverage region or market The interconnection of many such markets defines a wireless network capable of providing service to mobile users throughout country or continent

3 Common air interface Handshake communication protocol

4 wired 1. Transfer of information takes place over landline trunked lines (trunks) made of OF cables, copper cables, microwave links and satellite links. 2. Network configurations are virtually static. Distance is a constraint for the subscribers 3. Subscriber s change of residence needs reprogramming at the local CO of subscriber. 4. The available bandwidth can be increased by installing high capacity cables. wireless 1. Transfer of information takes place with the help of signals which travel in air as ElectroMagnetic waves. 2. Network configurations are highly dynamic. The network configuration gets rearranged every time a subscriber moves into a different coverage region. 3. Wireless networks must reconfigure themselves for users within small intervals of time to provide roaming and handoff facility. 4. W/L networks are constrained by the little RF cellular B/W provided for each user.

5 Local Exchange

6 PSTN-Public switched telephone N/W 1. LATA-Local access and transport area. Geographical grouping of towns is taken care by LATA. 2. LEC- Local exchange carrier. IntraLATA telephone service. LATA LEC Surrounding LATA 3. CO-Central Office-Provides PSTN connection to CPE. 4. CPE-Customer Premises Equipment-PBX or Residence telephone.

7 PSTN 5. PBX-Private Branch Exchange-Office connections are dealt by PBX. --Intra PBX No LEC involved. --Connection of PBX to CO is maintained by LEC. 6. IXC-Inter Exchange Carriers are used to connect different LEC s throughout the country or continent. LEC IXC Long Distance LEC

8 Limitations of Wireless Networks

9 Limitations: 1. Extreme complex communication system. 2. Radio channel is extremely hostile and random in nature. 3. Growth in wireless systems leads to increase in BS s which increases the switching burden of MSC 4. Extra overhead is required for MSC as the geographical location of MU changes constantly.

10 Wireless Links: High Bit Error Rate Decreasing signal strength Disperses as it travels greater distance Attenuates as it passes through matter 10

11 Wireless Links: High Bit Error Rate Interference from other sources Radio sources in same frequency band E.g., 2.4 GHz wireless phone interferes with b wireless LAN Electromagnetic noise (e.g., microwave oven) 11

12 Wireless Links: High Bit Error Rate Multi-path propagation Electromagnetic waves reflect off objects Taking many paths of different lengths Causing blurring of signal at the receiver receiver transmitter 12

13 Dealing With Bit Errors Wireless vs. wired links Wired: most loss is due to congestion Wireless: higher, time-varying bit-error ate Dealing with high bit-error rates Sender could increase transmission power Requires more energy (bad for battery-powered hosts) Creates more interference with other senders Stronger error detection and recovery More powerful error detection codes Link-layer retransmission of corrupted frames 13

14 Wireless Links: Broadcast Limitations Wired broadcast links E.g., Ethernet bridging, in wired LANs All nodes receive transmissions from all other nodes Wireless broadcast: hidden terminal problem C A and B hear each other B and C hear each other But, A and C do not A B So, A and C are unaware of their interference at B. 14

15 Wireless Links: Broadcast Limitations Wired broadcast links E.g., Ethernet bridging, in wired LANs All nodes receive transmissions from all other nodes Wireless broadcast: fading over distance A B C A s signal strength C s signal strength A and B hear each other B and C hear each other But, A and C do not So, A and C are unaware of their interference at B. space 15

16 Merging Wireless Networks and PSTN 1. Signaling and Voice traffic were sent on the same trunked lines i.e., a single physical connection was used to handle both of them. 2. In mid 1980 s when W/L evolved, the PSTN was transformed into two parallel networksone for voice and other for setup, which is known as Common Channel Signaling(CCS). 3. Access to signaling N/W is provided by IXC s. 4. North America- Signaling-SS7.

17 Development of Wireless Networks First generation wireless Networks: 1. Based on analog technology. All cellular systems used FM, and cordless telephones used a single BS to communicate with single portable terminal. 2. Did not have CCS. Eg: AMPS. Forward Voice channel Voice circuits Reverse Voice channel Mobile User Base Station MSC Reverse Setup channel Forward setup channel 9600bps Data Link

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19 The system control for each market resides in MSC, which maintains all mobile related information and controls each mobile handoff. Autonomous Registration-The MU notifies a serving MSC of its presence and location. IS-41 relies on the concept of Autonomous Registration. MSC also performs network management functions such as call handling and processing, billing, fraud detection within the market. MSC is interconnected with the PSTN via trunks and a tandem switch. MSC s are connected to other MSC s via dedicated signaling channels for exchange of location, validation and call signaling information.

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21 First Generation Wireless N/W s 1. Provide analog speech and inefficient, low-rate data transmission between the MU and BS. 2. Until early 1990 s the user had to register manually each time he/she entered any market. 3. Evolution of IS-41 standard allowed different cellular systems to automatically accommodate users who roam into new coverage region.

22 Second Generation(2G) Employ digital modulation and advance call processing capabilities. Eg:GSM,lS-95, IS-136 standards of US,Personal Access Comm. Systems(PACS), Digital European Cordless Telephone(DECT-Europe). MSC s burden is downsized by the deployment of BSC s(base Station Controller), which is inserted b/w several BS s and MSC. Use digital voice coding and digital modulation. CCS is introduced.

23 Second Generation(2G) Unlike First generation which is only developed for voice, the 2G has been specifically designed to provide paging, and other data services such as facsimile and high data rate network access. The handoff process involved is Mobile assisted i.e., MAHO(Mobile Assisted Handoff). The mobile units also perform several other functions such as received power reporting, adjacent BS scanning, data encoding and encryption, which is not seen in First generation.

24 Third generation(3g) Mainly developed to provide single set of standards that can meet wide range of wireless applications and provide universal access throughout the world. A universal personal communicator will provide access to a variety of voice, data and video communication services. Eg: B-ISDN, 3G PCS,3G PCN,UMTS(Europe) Packet Radio communication is used providing high speed and reliable information transfer.

25 Traffic Routing in Wireless Networks The voice call should always be continuous in nature, whereas the control and signaling information can be bursty and can share the network with other bursty users. Routing services determine the priority and the type of service to be given to all the users. Two Types: 1. Connection-oriented (Virtual Circuit Routing) 2. Connectionless(Datagram Services)

26 Connection Oriented Service Consider the case when you make a telephone call. Pick up your telephone Dial the destination number which is unique Say what you want to convey Hang up your phone A similar mode of operation is followed in the case of connection oriented services Establish a connection between the sender and the receiver Make use of the connection by sending and receiving information When done, terminate the existing connection

27 Connectionless Service A connectionless services, as it s name suggests, is just a service with no pre established connection. It is more like a postal system wherein you send and receive mails with the help of destination address and several offices that route your mails to the correct destination. The packets send need not arrive in the same order in which they are sent. This is basically because all packets might not take the same route since there is no preset connection in such services. The routers route the packets according to their built in algorithms and the receiver might need to re order the packet.