A.Sanyasi RaoAMIE; M.Tech; MISTE; MIETE

Assoc. Prof, Dept. of ECEBalaji Institute of Engineering & Sciences

Communication Channel

Transmitter Receiver

The medium used to transmit the signal from the transmitter to the receiver

Wireline / Wireless channel

Channel

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Wireline Channel

Transmitter Receiver

Wireline Channel, e.g. copper wire

•Too many noises? Shielded against electromagnetic noise

•Large signal attenuation? Use repeaters

•Data speed too low? Upgrade to coaxial cable

•Data speed still too low? Upgrade to optical fiber

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Fading Effect

• Typical Indoor Wireless Environment– Signal strength fluctuates significantly

• Wireless channel cannot be engineered.– You can only improve your transmission and reception techniques.

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Bit Error Rate

• Optical fiber:

10-11 or 10-12

• Mobile channel: • Good quality: 10-6

• Actual condition: 10-2 or worse

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Cellular System

The mobile subscribers must be connected to base stations inorder to perform communications. This is done through a protocolcalled Common Air Interface(CAI), which establishes radio linksbetween base station and subscribers.

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The CAI specifies exactly how mobile subscribers and base stationscommunicate over radio frequencies and also defines the control channel signalingmethods.

The CAI must provide a great deal of channel reliability to ensure thatdata is properly sent and received between the mobile and the base station, and assuch specifies speech and channel coding.

At the base station, the air interface portion (i.e., signaling andsynchronization data) of the mobile transmission is discarded, and the remainingvoice traffic is passed along to the MSC on fixed networks.

While each base station may handle on the order of 50 simultaneous calls,a typical MSC is responsible for connecting as many as 100 base stations to thePSTN(as many as 5000 calls at one time), so the connection between the MSC andthe PSTN requires substantial capacity at any instant of time.

The basic concepts and standards used in today’s wireless networks arecovered in a manner which first addresses the mobile-to-base link, followed by theconnection of the base station to the MSC, the connection of the MSC to the PSTN,and the interconnection of MSCs throughout the world.

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Difference b/w Wireless and Fixed Telephone Networks

Fixed Telephone Network Wireless Network

•These are virtually static.•In this, network connections aredifficult to change.•Channel bandwidth can be increasedby installing high capacity cables.•In this, the efficient transmission ispossible to all the connectedsubscribers.•These networks are stable.•Transmission is fast.

•These are secure.

•These are highly dynamic.•They can reconfigure themselves.

•They are forced to use small RF cellularbandwidths.•It is not available across the entirecampus. It is likely to receive fluctuatingsignals.•These are unstable.•Transmission is very slow, it is up to 1-108 Mbps. In slowest wired networks,the transmission rate is around 100Mbps.Hence, it is undesirable in terms of datarate when compared to even slowest ofwired networks.•These are not secure.

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The Public Switched Telephone network (PSTN)

•Simplified illustration of a local telephone network, called a localexchange. Each local exchange consists of a central office (CO) whichprovides PSTN connection to the customer premises equivalent (CPE)which may be an individual phone at a residence or a private branchexchange (PBX) at a place of business.

•The CO may handle as many as a million telephone connections. TheCO is connected to a tandem switch which in turn connects the localexchange of the PSTN.

•Sometimes IXCs connect to the CO switch to avoid local transportcharges levied by the LEC.

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•A PBX allows an organization to provide internal calling and otherin-building services (which do not involve the LEC), as well asprivate networking between organizational sites (through leasedline from LEC and IXC providers), in addition to conventional localand long distance services which pass through the CO.

•Telephone connections within a PBX are maintained by theprivate owner, whereas connection of the PBX to the privateowner, whereas connection of the PBX to the CO is provided andmaintained by the LEC.

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•Each country is responsible for the regulation of the PSTN within itsborders. In the PSTN, each city or geographic grouping of towns iscalled local access and transport area (LATA).

•Surrounding LATAs are connected by a company called exchangecarrier (LEC).

•A long distance telephone company collects toll fees to provideconnection between different LATAs over its long distance network.

•These companies are referred to as interexchange carriers (IXCs),and own and operate large fiber optic and microwave radio networkswhich are connected to LECs throughout a country or continent.

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Challenges in Wireless Communications

• Three major challenges:

– Wireless Channel

– Mobility

– Device Limitation

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Wireless: Limited bandwidth Broadcast medium: requires multiple access schemes Variable link quality (noise, interference) High latency, higher jitter Heterogeneous air interfaces Security: easier snooping

Mobility: User location may change with time Speed of mobile impacts wireless bandwidth Need mechanism for handoff Security: easier spoofing

Portability Limited battery, storage, computing, and UI

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• For wireline systems, it is assumed that the channel is error free

• Many protocols are designed with this assumption

• These protocols do not work well in a wireless environment

What if more than 1 transmitter?

Switching Center

or

Network Access Point

Every user accesses the network by means of a dedicated channel

New user is served by a new wire-line circuit

Access capacity is “unlimited”.

Dedicated Channel

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How about Wireless networks?

Base Station

Wireless users access the network by means of a shared channel

Access capacity is inherently limited.

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Limitations of Wireless Networking

1. It has extremely hostile and random nature of the radio channel.2. Large number of base stations are required and all of these must

be connected to mobile switching centers.3. In order to provide service to the subscribers travelling over a

wide range of velocities, the mobile switching center must switchthe calls rapidly between base stations.

4. As the number of base stations increases, the load on mobileswitching center increases.

5. This system is limited to operate in a limited bandwidth.6. Every mobile subscriber is connected to PSTN by using mobile

switching center, hence this requires equal number of connectionsto local exchange carrier.

7. The interface is required between base station and subscribers.8. Spectrally efficient modulation techniques, frequency reuse

techniques and geographically distributed radio access points arevital components of wireless networks.

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Merging wireless networks and the PSTN

•Throughout the world, first generation wireless systems (analogcellular and cordless telephones) were developed in the early andmid 1980s.

•Most analog landline telephone links throughout the world sentsignaling information along the same trunked lines as voice traffic.

•A single physical connection was used to handle both signalingtraffic and voice traffic for each user.

•The overhead required in the PSTN to handle signaling data onthe same trunks as voice traffic was inefficient, since this requireda voice trunk to be dedicated during periods of time when novoice traffic was actually being carried.

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•The advantage of separate but parallel signaling channel allows the voice trunks to be used strictly for revenue-generating voice traffic, and supports many users on each trunked line.

•Thus, during the mid 1980s, the PSTN was transformed into two parallel networks- one dedicated to user traffic, and one dedicated to call signaling traffic. This technique is called common channel signaling.

•In many of today’s cellular telephone systems, voice traffic is carried on the PSTN while signaling information for each call is carried on a separate signaling channel.

•Access to the signaling network is usually provided by IXCs for a negotiated fee.

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Deployment of wireless networks

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First Generation Wireless Networks

•First generation cellular and cordless telephone networks are based on analog technology.

•All first generation cellular systems use FM modulation, and cordless telephones use a single base station to communicate with a single portable terminal.

•A typical example of a first generation cellular telephone system is the AMPS system used in the US.

•First generation cellular radio network, which includes the mobile terminals, the base stations, and MSCs.

•In first generation cellular networks, the system resides in the MSC, which maintains all mobile related information and controls each mobile handoff.

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•The MSC also performs all of the network management functions,such as call handling and processing, billing and fraud detection withinthe market.

•The MSC is interconnected with the PSTN via landline trunked lines(trunks) and a tandem switch.

•First generation wireless systems provide analog speech andinefficient low rate date transmission between the base station andthe mobile user.

•However, the speech signal are usually digitized using a standard,TDM format for transmission between the base station and the MSCand are always digitized for distribution from the MSC to the PSTN.

•The global cellular network is required to keep track of all mobileusers that are registered in all markets throughout the network, sothat it is possible to forward incoming calls to roaming users at anylocation throughout the world.

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•When the user roams into a new market covered by a differentservice provider, the wireless network must register the user in thenew area and cancel its registration with the previous serviceprovider so that calls may be routed to the roamer as it movesthrough the coverage areas of different MSCs.

•Until the early 1990s, US cellular customers that roamed betweendifferent cellular systems has to register manually each time theyentered a new market during long distance travel.

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•This required the user to call an operator to request registration.In the early 1990s, US cellular carriers implemented the networkprotocol standard IS-41 to allow different cellular systems toautomatically accommodate subscribers who roam in to theircoverage region. This is called inter operator roaming. IS-41 allowsMSCs of different service providers to pass information about theirsubscribers to other MSCs on demand.

•IS-41 relies on a feature of AMPS called autonomous registration.Autonomous registration is a process by which mobile notifies aserving MSC of its presence and location.

•The mobile accomplishes this by periodically keying up andtransmitting its identity information, which allows the MSC toconstantly update its customer list.

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•The registration command is sent in the overhead message ofeach control channel at five or ten minute intervals, and includes atimer value which each mobile uses to determining the precisetime at which it should respond to the serving base station with aregistration transmission.

•Each mobile reports its MIN and ESN during the brief registrationtransmission so that the MSC can validate and update the customerlist within the market.

•The MSC is able to distinguish home users from roaming usersbased on the MIN of each active user, and maintain a real-timeuser list in the home location register (HLR) and visitor locationregister (VLR).

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Second Generation Wireless Networks

•Second generation wireless systems employ digital modulation andadvanced call processing capabilities. Examples of second generationwireless systems includes the GSM, the TDMA and CDMA US digitalstandards, second generation Cordless Telephone (CT2), the Britishstandard for cordless telephone, the Personal AccessCommunication System (PACS) local loop standard, and DigitalEuropean Cordless Telephone (DECT), which is the Europeanstandard for cordless for cordless and office telephone.

•Second generation wireless networks have introduced newnetwork architectures that have reduced to the computationalburden of the MSC.

•GSM introduced the concepts of a base station controller (BSC),which is inserted between several base stations and the MSC.

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•In PACS/WACS, the BSC is called a radio port control unit. Thearchitectural change has allowed the data interface between the basestation controller and the MSC to be standardized, thereby allowingcarriers to use different manufacturers for MSC and BSC components.

•This trend in standardization and inter operability is new to secondgeneration wireless networks.

•Eventually, wireless network components, such as the MSC and BSC,will be available as off-the shelf components, much like there wire linetelephone counter parts.

•All second generation systems use digital voice coding and digitalmodulation. The systems employ dedicated control channels with inthe air interface for simultaneously exchanging voice and controlinformation between the subscribers, the base station, and the MSCwhile a call is in progress.

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•Second generation systems also provide dedicated voice andsignaling trunk between MSCs and between each MSC and the PSTN.

•Second generation wireless networks have been specificallydesigned to provide paging, and other data services such as facsimileand high-data rate network access.

•The network controlling structure is more distributed in secondgeneration wireless systems, since mobile stations assume greatercontrol functions.

•In second generation wireless networks, the handoff process ismobile-controlled and is known as mobile assisted handoff (MAHO).DECT is an example of a second generation cordless telephonestandard which allows each cordless phone to communicate with anynumber of base stations, by automatically selecting the base stationwith the greatest signal level.

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Third Generation Wireless Networks

•The aim of third generation wireless networks is to provide a singleset of standards that can meet a wide range of wireless applicationsand provide universal access throughout the world.

•In third generation wireless systems, the distinctions betweencordless telephone and cellular telephones will disappear, and auniversal personal communicator (a personal handset) will provideaccess to a variety of voice, data, and video communication services.

•Third generation systems will use the B-ISDN to provide access toinformation networks. Third generation networks will carry manytypes of information (voice, data and video), and will serve bothstationary users and vehicular users travelling at high speeds.

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•Packet radio communications will likely be used to distributenetwork control while providing a reliable information transfer.

•The terms 3G Personal Communication Systems (PCS) and 3GPersonal Communication Network (PCN) are used to imply emergingthird generation wireless systems for hand-held devices.

•Other names for PCS include Future Public Land MobileTelecommunication Systems (FPLMTS) for worldwide use which hasmore recently been called International Mobile Telecommunication(IMT-2000), and Universal Mobile Telecommunication Systems(UMTS) for advanced mobile personal services in Europe.

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Traffic Routing In Wireless Networks

•The amount of traffic capacity required in a wireless network ishighly dependent upon the type of traffic carried.

•For example, a subscribers telephone call (voice traffic) requiresdedicated network access to provide real time communications,whereas control and signaling traffic may be in nature and may beable to share network resources with other bursty users.

•Alternatively, some traffic may have an urgent delivery schedulewhile some may have no need to be sent in real-time. The type oftraffic carried by a network determines the routing services,protocols, and call handling techniques which must be employed.

•Two general routing services are provided by networks. These areconnection oriented services (virtual circuit routing), andconnectionless services (datagram services).

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•In connection oriented routing, the communications path between the message source and destination is fixed for the entire duration of the message, and a call set-up procedure is required to dedicate network resources to both the called and calling parties.

•Since the path through the network is fixed, traffic in connection-oriented routing arrives at the receiver in the exact order it was transmitted.

•A connection oriented service relies heavily on error control coding to provide data protection in case the network connection becomes noisy.

•If coding is not sufficient to protect the traffic, the call is broken, and the entire message must be retransmitted from the beginning.

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•Connectionless routing, on the other hand, does not establish a firmconnection for the traffic, and instead relies on packet-basedtransmissions.

•Several packets form a message, and individual packets in aconnection less service is routed separately.

•Successive packets within the same message might travel completelydifferent routes and encounter widely varying delays throughout thenetwork.

•Packet sent using connectionless routing do not necessarily arrive inthe order of transmission and must be reorder at the receiver.

•Because packets take different routes in a connectionless service,some packets may be lost due to network or link failure, howeverothers may get through with sufficient redundancy to enable theentire message to be recreated at the receiver.

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•Thus, connectionless routing often avoids having to retransmit anentire message, but requires more overhead information for eachpacket.

•Typical packet overhead information includes the packet sourceaddress, the destination address, the routing information, andinformation needed to properly order packets at the receiver.

•In a connectionless service, a call set-up procedure is not required atthe beginning of a call, and each message burst is treatedindependently by the network.

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Circuit Switching

•MSC dedicates a voice channel connection between the base stationand the PSTN for the duration of a cellular telephone call.

•Furthermore, a call initiation sequences is required to connect thecalled and calling parties on a cellular system.

•When used in conjunction with radio channels, connection-orientedservices are provided by a technique called circuit switching, since aphysical radio channel is dedicated for two-way traffic between themobile user and the MSC, and the PSTN dedicates a voice circuitbetween the MSC and the end-user.

•Circuit switching establishes a dedicated connection for the entireduration of a call. There is always a dedicated radio channel toprovide service to the user, and the MSC dedicates a fixed, full duplexconnection to the PSTN.

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•Wireless data networks are not well supported by circuit switching,due to their short, bursty transmissions which are often followed byperiods of inactivity.

•Often, the time required to establish a circuit exceeds the durationof the data transmission. Circuit switching is best suited for dedicatedvoice-only traffic, or for instances where data is continuously sentover long periods of time.

A private road all for yourself

Dedicated end to end connection

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Packet Switching

•Packet switching (also called virtual switching) is the most commontechnique used to implement connectionless services and allows alarge number of data users to remain virtually connected to the samephysical channel in the network.

•Since all users may access the network randomly and at will, call set-up procedures are not needed dedicated specific circuits whenparticular user needs to send the data.

•Packet switching breaks each message into smaller units fortransmission and recovery.

•When a message is broken into packets, a certain amount of controlinformation is added to packet to provide source and destinationidentification, as well as error recovery provisions.

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•The packet consists of header information, the user data, and a trailer.

•The trailer contains a cyclic redundancy checksum which is used forerror detection at the receiver.

•The structure of a transmitted packet, which typically consists of fivefields; the flag bits, the address field, the control field, theinformation field, and the frame check sequence field.

•The flag bits are specific (or reserved) bit sequence that indicate thebeginning and the end of each packet.

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•The address field contains the source and the destination addressfor transmitting messages and for receiving acknowledgements.

•The control field defines functions such as transfer ofacknowledgements, automatic repeat requests (ARQs), and packetsequencing.

•The information field contains the user data and may have variablelength.

•The final field is the frame check sequence field or the CRC that isused for error detection.

•In contrast to circuit switching, packet switching (also called packetradio when used over a wireless link) provides excellent channelefficiency for bursty data transmissions of short length.

•An advantage of packet switched data is that the channel is utilizedonly when sending or receiving bursts of information.

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Divided packets can take different paths and times

A shared highway

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The X.25 Protocol•X.25 was developed by CCITT (now ITU-T) to provide standardconnectionless network access (packet switching) protocols for thethree lowest layers (layers 1,2, and 3) of the open systemsinterconnection (OSI) model.

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X.25

PAD DCE PSE DCE PAD

X.25 Wide Area Network

Conceptual View of X.25

DTE DTE

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X.25 Network Devices

X.25 network devices fall into 3 general categories:

Data terminal equipment (DTE).

Data circuit-terminating equipment (DCE).

Packet switching exchange (PSE).

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Data terminal equipment (DTE)– End systems that communicate with one another across the X.25 data

network and include terminals, PCs, and network hosts

Data circuit-terminating equipment (DCE)– Communications devices such as modems and packet switches, that

provide the interface between DTEs and PSE.

Packet switching exchanges (PSE)– Constitute the majority of the network

– Transfers data from one DTE to another through the X.25 network

Packet Assembler/Disassembler (PAD)

- PADs provide buffering (data storage), packet assembly and

packet disassembly.

- This operation includes adding an X.25 header.

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Terminal

DTEModem

DCE

PAD

Buffer

Data

Assembly/

Disassembly

Data

X.25 Packet

PSE

PSE

PAD in ActionASRao

7 Layersof

OSI

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X.25 Protocol Layers

Layer 3

Layer 2

Layer 1

OSI Network Layer

OSI Data-link Layer

OSI Physical Layer

X.25

Packet Layer

X.25

Frame Layer

X.25

Physical Layer

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X.25 Protocol Layers

X.25 defines how packet mode terminals can be connected to apacket network.

It also describes the procedures required to establish, maintain,and terminate a connection as well as a set of services that provideadditional functions.

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X.25 mapping to OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

PLP

LAPB

x.21 bis, EIA/TIA-232, EIA/TIA-449,EIA-530, G.703

Other Services

X.25 Protocol Suite

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X.25 Physical Layer Protocol

• Called the X.21 digital interface.

•Designed to enable all-digital communications between DTEs andDCEs and to address the problems inherent in many of thepreexisting EIA interface standards.

•It specifies how a DTE and DCE exchange signals to set up and clearcalls.

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Several well-known standards are used for X.25 networks– X.21bis – supports up to 2 Mbps

• 15-pin connector– RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps

• 25-pin connector– RS-449 (EIA/TIA-449) – supports up to 64 Kbps

• 37-pin connector– V.35 – supports up to 2 Mbps

• 34-pin connectorUses serial communications in either asynchronous orsynchronous modes

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X.25 frame-layer protocol

Layer 2 protocol intended to provide reliable data transfer betweenthe DTE and DCE by transmitting data as a sequence of frames.

X.25 Frame Format

FlagField8 bits

AddressField8 bits

Frame checkSequence(CRC-16)

FlagField8 bits

Data field(variable length

In 8-bit groupings)

ControlField8 bits

F A C DCRC code

F

01111110

7E hex

01111110

7E hex

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LAPB Frame Format

Flag FlagAddress Control Data FCS

Flag: (8 bits) Indicates start and end of frame (01111110)

Address: (8 bits) DTE address is maintained in higher layer so thisfield is used to identify command and responses between DTE andDCE. A value of 0x01 indicates a command from DTE and responsesfrom DCE while a value of 0x03 indicates commands from DCE andresponses from DTE.

Control: (8 bits) Contains sequence numbers, commands andresponses for controlling data flow

Data: (varies is size) Contains upper layer data

FCS: (16 bits) Frame Check Sequence used to determine if an errorhas occurred in transmission (variation of CRC)

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X.25 frame-layer protocol functions

• Transfer data efficiently and in a timely manner

• Synchronize the link, ensuring that the receive is synchronized tothe transmitter

• Provide error detection and recovery

• Identify and report procedural errors to a higher layer forrecovery

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X.25 Packet Layer Protocol (PLP)

• A layer 3 protocol

• Creates network data units called packets that contain userinformation as well as control information.

• Responsible for establishing a connection, transferring dataover the connection, and then terminating the connection.

• Responsible for creating virtual circuits and negotiating networkservices between a DTE and DCE.

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PLP Operates in Five Modes

Call Setup

– Used to setup virtual circuit for SVC

Data Transfer

– Used for transferring data with both SVC and PVC

Idle

– Used when SVC call has been established but no data iscurrently being transferred

Call Clearing

– Used to end communication between DTEs for a SVC

Restarting

– Used to synchronize DTE and DCE for all virtual circuits thatexist between them

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Call Setup

Local

DCE

Remote

DCE

Local

DTE

Remote

DTE

Call Request

Incoming Call

Call Accepted

Call

Connected

Internal

Protocol

Locate Remote

DCE

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Call Clearing

Local

DCE

Remote

DCE

Local

DTE

Remote

DTE

Clear Request

Clear

Indication

Clear Confirm

Clear Confirm

Internal

Protocol

Remote DCE

from Call Setup

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Data Transfer w/End to End Ack

Local

DCE

Remote

DCE

Local

DTE

Remote

DTE

Data Packet

#1

Data Packet

#1

RR P(R)=2

RR P(R)=2

Internal

Protocol

Remote DCE

from Call Setup

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Data Transfer w/Local Ack

Local

DCE

Remote

DCE

Local

DTE

Remote

DTE

Data Packet

#1

Data Packet

#1

RR P(R)=2

Internal

Protocol RR P(R)=2

Data Packet

#2

RR P(R)=3 Data Packet

#2RR P(R)=3

Remote DCE

from Call Setup

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Cellular Digital Packet Data (CDPD)

•In 1993, the first version of standard of CDPD came into view.

•In 1994, in America, the first CDPD experimental network run.

•CDPD offer one of the most advanced means of wireless datatransmission technology.

•CDPD is a technique for quick transfer of data.

•It transmitt the packet over the cellular network in a reliablemanner.

•CDPD transmits digital packet data at 19.2 Kbps. (idle case)

•CDPD system allow user to transmit data with a higher degree ofaccuracy, strong security, and few service interruption.

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•CDPD is a service uses a full 30 KHz AMPS channel on a sharedbasis.

•CDPD provides mobile packet data connectivity to existing datanetworks and other cellular systems without any additionalbandwidth requirements.

•It also capitalizes on the unused air time which occurs betweensuccessive radio channel assignments by the MSC (packet data maybe transmitted until that channel is selected by the MSC to provide avoice circuit).

•CDPD directly overlay with existing cellular infrastructure and usesexisting base station equipment

•CDPD doesn’t use the MSC

•GMSK BT=0.5 modulation is used so that existing analog FM cellularreceivers can easily detect the CDPD format without redesign.

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Key CDPD Characteristics

1. Availability: Identifies whether CDPD services can be acquiredfrom a carrier in a given region

2. Coverage: Identifies whether CDPD transmission can reach usersin a given region.

3. Reliability: Identifies whether user can access and use CDPDservices during congestion or network disruption

4. Transmission Speed: Describe the end to end data speed,including call setup time & transmission speed

5. Privacy and Security: Describe the level of inherent privacy andsecurity of the service and the capability to add security measure

6. Cost: Characterizes the cost typical of CDPD service.

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Features

1. Same Coverage as cellular system

2. Schedule data packets to unused voice channel

3. 19.2 Kbps raw data transfer rate.

4. Full duplex transreciever

5. 600 mw transmit power

6. Integrated TCP/IP protocol stack

7. Compact size

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Working Of CDPD

• It uses Channel Hopping Technique.

• Location of available channel to establish data link.

• Mobile data unit transmit data packets.

• CDPD technology transmits chunks or packets of data

• Equipment :

1) CDPD Modem

2) User terminal

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User Equipment

1 .CDPD Modem:

– It Includes wireless antenna and modem that providescompatibility with network.

– It can be internal or external.

2 .User Terminal:

– Any device that supports IP-based data communication canuse CDPD .

– Examples are as notebook or laptop Personal Computers(PCs), handheld computers or PCs.

CDPD transmissions are carried out using fixed-length blocks. Userdata is protected using a Reed-Solomon (63,47) block code with 6-bit symbols. For each packet, 282 user bits are coded into 378 bitblocks, which provide correction for up to eight symbols.

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Interfaces

• A Interface : Air interface• I Interface : Intermediate system interface• E Interface : External interface

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Components

1. Mobile End System ( M-ES)

– Mobile computing device which has a CDPD modem built-in or attached

– It transmit data over the airlink to MDBS

– It should provide transparent interface to the user application

– Example is CDPD modem

2. Mobile data base station (MDBS)

– It located at the cell site.

– It is responsible for R.F management

– It provide hop to hop control over the air interface

– It controls the hop & RF segment between the M-ES and the CDPD network

– Cellular geographic area control by MDBS

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3. Mobile Data Intermediate System(MDIS)– MDIS routs data packet to and from the CDPD N.W & the M-ES

appropriately– It validates M-ES using encryption keys for secure transmission

of data– It is responsible for IP routing– An MDIS may serve single or multiple CGSA cells

4. INTERMEDIATE SYSTEM (IS)– It is the standard IP router – It routes the data through IP & CLNP, N.W– The IS components are the backbone of CDPD mesh

5. FIXED END SYSTEM (F- ES)– F-ES is the final destination of message sent from M-ES– F-ES received the data & process it appropriately– The F-Es can be one of many stationary computing devices such

as host computer , UNIX workstation

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Two lower layer protocols are used in CDPD:

Mobile Data Link Protocol (MDLP)

•It provides logical link control services between Mobile EndSystems and Mobile Data Intermediate Systems.

•The purpose of MDLP is to transmit information betweennetwork layer entities across the CDPD Air link interface.

•Identifies destination mobile - virtual address.

•The MDLP also provides sequence control to maintain thesequential order of frames across a data link connection, as wellas error detection and flow control.

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Radio Resource Management Protocol (RRMP)

•RRMP is a higher, layer 3 protocol used to manage the radiochannel resources of the CDPD system and enables an M-ES tofind and utilize a duplex radio channel without interfering withstandard voice services.

•The RRMP handles base station identification and configurationmessages for all M-ES stations

•The objective of radio resource management is to maximize thesystem spectral efficiency in bit/s/Hz/base station site orErlang/MHz/site.

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Application of CDPD

• Credit card verification

• ATM network

• Transportation: Ambulance, dispatch system.

• Public security: Policemen can access the latest database in realtime.

• E.g. Check suspected person, Verify identity, Check the carcurrent status.

• Wireless ATM, Wireless Credits Card Authentication, MobilePayment.

• High speed. CDPD modem set up time is much faster than wiredial up modem. Even CDPD is 19.2kbps, while dial up modem is33.6kbps.

• Reliable and Safety with strong encryption algorithm.

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ADVANCED RADIO DATA INFORMATION SYSTEMS (ARDIS)

•ARDIS is a private network service provided by Motorola and IBMand is based on MDC 4800 and RD-LAP (Radio Data Link AccessProcedure) protocols developed at Motorola.

•ARDIS provides 800 MHz two way mobile data communications forshort-length radio messages in urban and in-building environments,and for users travelling at low speeds.

•Short ARDIS messages have low retry rates but high packetoverhead, while long messages spread the overhead over the lengthof the packet but have a higher retry rate.

•ARDIS has been deployed to provide excellent in-buildingpenetration, and large-scale spatial antenna diversity is used toreceive messages form mobile users.

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•When a mobile sends a packet, many base stations which are turnedto the transmission frequency attempt to detect and decode thetransmission, in order to provide diversity reception for the case whenmultiple mobiles contend for the reverse link.

•In this manner, ARDIS base stations are able to insure detection ofsimultaneous transmissions, as long as the users are sufficientlyseparated in space.

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RAM MOBILE DATA (RMD)

•RAM Mobile Data (RMD) is a public, two-way data service basedupon the Mobitex protocol (open & non-proprietary protocol)developed by Ericsson.

•RAM provides street level coverage for short and long messages forusers moving in an urban environment.

•RAM has capability for voice and data transmission, but has beendesigned primarily for data and facsimile.

•Fax messages are transmitted as normal text to a gateway processor,which then converts the radio message to an appropriate format.

•Thus, the packet-switched wireless transmission consists of a normallength message instead of a much larger fax image, even though theend-user receives what appears to be a standard fax.

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•RMD uses frequencies in the specialized Mobile Radio band. The mobileunits transmit between 896 MHz and 901 MHz, and the base stationstransmit 39 MHz higher, between 935 MHz and 940 MHz.

•Data is transmitted at 8000 bps and uses automatic repeat and forwarderror correction to combat noise and fading typical in mobile radioenvironment.

•Messages may be sent to an individual mobile unit or to a group. If asubscriber is not reachable, because the modem is turned off or the unit isout of a coverage area, the message may be stored in the network for upto 72 hours. When the modem is turned on or reenters a coverage area,stored messages are automatically delivered. As an option, the sender canreceive a confirmation notice when the original message is delivered to therecipient.

•Mobile units stay in contact with the radio network through the roamingcapabilities of the system, in many ways similar to cellular telephonenetwork. Mobile units monitor signal strength from nearby base stationand determine if and when a transfer to another base station is necessary.

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ARDIS RMD

Coverage Nation wide Nation wide

Data rate (bps) 4800 (19,200 in limitedareas)

8000

Packet size (in bytes) 240 512

Mobile transmit power (W) 3 2 to 10

Response time (sec) 2 to 7 4 to 8

Transmit frequencies (in MHz) 800 Mobile: 896 to 901Base: 935 to 941

Channels 600 200

Channel spacing 25 KHz 12 KHz

Pricing $ 20/20 kilobytes $ 30/100 kilobytes

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COMMON CHANNEL SIGNALING (CCS)

•Common channel signaling (CCS) is a digital communicationstechnique that provides simultaneous transmission of user data,signaling data, and other related traffic throughout a network.

•This is accomplished by using out-of-band signaling channels whichlogically separate the network data from the user information (voiceand data) on the same channel.

•For second generation wireless communication systems, CCS is usedto pass user data and control/supervisory signals between thesubscriber and the base station, between the base station and theMSC, and between MSCs.

•Even though the concept of CCS implies dedicated, parallelchannels, it is implemented in a TDM format for serial datacommunication.

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•Before the introduction of CCS in the 1980s, signaling traffic betweenthe MSC and subscriber was carried in the same band as the end-user’s audio.

•The network control data passed between MSCs in the PSTN was alsocarried in band, requiring that network information be carried withinthe same channel as the subscriber’s voice traffic throughout thePSTN.

•This technique, called in-band signaling, reduced the capacity of thePSTN since the network signaling data rates were greatly constrainedby the limitations of the carried voice channels, and the PSTN wasforced to sequentially (not simultaneously) handle signaling and userdata for each call.

•CCS is an out-of-band signaling technique which allows much fastercommunications between two nodes within the PSTN. Instead ofbeing constrained to signaling data rates which are on order of audiofrequencies.

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•CCS supports signaling data rates from 56 Kbps to many Mbps.

•Thus, network signaling data is carries in a seemingly parallel, out-of-band, signaling channel while only user data is carried on the PSTN.

•CCS provides a substantial increase in the number of users which are served by trunked PSTN lines, but requires that a dedicated portion of the trunk time be used to provide a signaling channel used for network traffic.

•In first generation cellular systems, the SS7 families of protocols, as defined by the ISDN are used to provide CCS.

•Since network signaling traffic is bursty and of short duration, the signaling channel may be operated in a connectionless fashion where packet data transfer techniques are efficiently used.

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•CCS generally uses variable length packet sizes and a layeredprotocol structure. The expense of a parallel signaling channel isminor compared to the capacity improvement offered by CCSthroughout the PSTN, and often the same physical networkconnection (i.e. a fiber optic cable) carries both the user traffic andthe network signaling data.

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The Distributed Central Switching Office for CCS

•CCS forms the foundation of network control and managementfunctions in second and third generation networks.

•Out-of-band signaling networks which connect MSCs throughoutthe world enable the entire wireless network to update and keeptrack of specific mobile users, wherever they happen to be.

•CCS network architecture is composed of geographically distributedcentral switching offices, each with embedded switching end points(SEPs), signaling transfer points (STPs), a service managementsystem (SMS), and a database service management system (DBAS).

•The MSC provides subscriber access to the PSTN via the SEP. TheSEP implement a stored-program-control switching system known asthe service control point (SCP) that uses CCS to set up calls and toaccess a network database.

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•The SCP instructs the SEP to create billing records based on the callinformation recorded by the SCP.

•The STP controls the switching of messages between nodes in theCCS network. For higher reliability of transmission (redundancy), SEPsare required to be connected to the SS7 network via at least two STPs.

•This combination of two STPs in parallel is known as the mated pair,and provides connectivity to the network in the event one STP fails.

•The SMS contains all subscriber records, and also houses toll-freedatabase which may be accessed by the subscribers.

•The DBAS is the administrative database that maintains servicerecords and investigates fraud throughout the network.

•The SMS and DBAS work in tandem to provide a wide range ofcustomer and network provider services, based on SS7.

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