mobile communication networks

7/28/2019 Mobile Communication Networks

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WIRELESS LAN

A fast-growing market introducing the flexibility of wireless access into office, home, or productionenvironments.

WLANs are typically restricted in their diameter to buildings, a campus, single rooms etc. and are operatedby individuals, not by large-scale network providers.

The global goal of WLANs is to replace office cabling, to enable join less access to theinternet and, to introduce a higher flexibility for ad-hoc communication in, e.g., groupmeetings.

Advantages of WLANs are:

Flexibility:

o Within radio coverage, nodes can communicate without further restriction. Radio wavescan penetrate walls, senders and receivers can be placed anywhere (also non-visible,e.g., within devices, in walls etc.).

Planning:

o No prior planning is required for connectivity as long as devices followstandard convention.

Design:

o Wireless networks allow for the design of small, independent devices which canfor example be put into a pocket.

o Wireless senders and receivers can be hidden in historic buildings, i.e., currentnetworking technology can be introduced without being visible.

Robustness:

o Wireless networks can survive disasters, e.g., earthquakes or users pulling a plug. If thewireless devices survive, people can still communicate.

Cost:

o After providing wireless access to the infrastructure via an access point for the first user,adding additional users to a wireless network will not increase the cost. This is, importantfor e.g., lecture halls, hotel lobbies or gate areas in airports.

Disadvantages of WLANs are:

Quality of service:

o WLANs typically offer lower quality than their wired counterparts.

o The main reasons for this are the lower bandwidth due to limitations in radio transmission

(e.g., only 110 Mbit/s user data rate instead of 1001,000 Mbit/s).o Higher delay in error correction and detection mechanisms.

Proprietary solutions:

o Slow standardization procedures.

IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

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Restrictions:

o All wireless products have to obey with national regulations. Several government andnon-government institutions restrict frequencies to minimize interference.

Safety and security:o Using radio waves for data transmission might interfere with other high-tech equipment,

e.g., hospitals.

Competing Requirement

Global operation:

o WLAN products should sell in all countries so, national and international frequencyregulations have to be considered.

o Wireless LAN equipment may be carried from one country into another theoperation should still be legal in this case.

Low power:o Wireless devices running on battery power. The LAN design should take this into account

and implement special power-saving modes and power management functions.

License-free operation:

o Should be able to operate the WLAN without license.

Robust transmission technology:

o Compared to their wired counterparts, WLANs operate under difficult conditions. If theyuse radio transmission, many other electrical devices can interfere with them (vacuumcleaners, hairdryers, train engines etc.).

Easy to use:

o Must be easy to use by a common man without complicated procedure

Protection of investment:

o A lot of money has already been invested into wired LANs. The new WLANs shouldprotect this investment by being interoperable with the existing networks.

Safety and security:

o Wireless LANs should be safe to operate, especially regarding low radiation if used, e.g.,in hospitals. Users cannot keep safety distances to antennas. The equipment has to besafe for leader.

o Users should not be able to read personal data during transmission, i.e., encryptionmechanisms should be integrated.

Transparency for applications:

o Existing applications should continue to run over WLANs, the only difference beinghigher delay and lower bandwidth.


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Infra red Vs Radio Transmission

Now days we have used 2 Basic Transmission Technology. The transmission Technologiesare Infra red and Radio Transmission.

Infrastructure and ad-hoc network

The two basic alternative of wireless networks are infrastructure-based and ad-hoc.

Infrastructure

Normally infrastructure is used to access other network.

Infrastructure networks not only provide access to other networks, but also include forwardingfunctions, medium access control etc.

In these infrastructure-based wireless networks, communication is done between the wireless nodesand the access point.

The access point does not just control medium access, but also acts as a bridge to other wireless orwired networks.

The design of infrastructure-based wireless networks is simpler because most of the networkfunctionality lies within the access point, whereas the wireless clients can remain quitesimple.

This type of network can use different access schemes with or without collision. Collisions may occur if medium access of the wireless nodes and the access point is not

coordinated. However, if only the access point controls medium access, no collisions arepossible.


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This setting may be useful for quality of service guarantees such as minimum bandwidth for certainnodes.

Typical cellular phone networks are infrastructure-based networks for a wide area.

Ad-hoc network

Each node can communicate directly with other nodes, so no access point controlling medium accessis necessary.

The below figure shows two ad-hoc networks with three nodes each. Nodes within an ad-hocnetwork can only communicate if they can reach each other physically, i.e., if they are withineach others radio range.

. In ad-hoc networks, the complexity of each node is higher because every node has to implement

medium access mechanisms.

The mechanisms to handle hidden or exposed terminal problems, and perhaps priority mechanisms, toprovide a certain quality of service.

This type of wireless network display the greatest possible flexibility as it is, for example, needed forunexpected meetings, quick replacements of infrastructure.

However, ad-hoc networks might only have selected nodes with the capabilities offorwarding data. Most of the nodes have to connect to such a special node first totransmit data if the receiver is out of their range.

The three WLANs presented, IEEE 802.11 and HiperLAN2 are typically infrastructure-based networks,which additionally support ad-hoc networking.

The third WLAN, Bluetooth is a typical wireless ad-hoc network. Bluetooth focusesprecisely on unplanned ad-hoc meetings or on the simple connection of two or moredevices without requiring the setup of an infrastructure.


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IEEE 802.11 ARCHITECUTRE AND SERVICES

In 1990, the IEEE 802 Committee formed a new working group, IEEE 802.11, specificallydedicated to wireless LANs.

Its agreement to develop a MAC protocol and physical medium specification.

The initial interest was in developing a wireless LAN operating in the ISM (industrial, scientific, andmedical) band.

At the same time, increasing the WLAN needed. So the team expanding the list of standards.

Wi-Fi Alliance The first 802.11 standard, the industry accepted 802.11b. Although 802.11b products are all based on the same standard.

This organization, subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance, created a test suite tocertify interoperability for 802.11b products.


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IEEE 802.11 System Architecture:

The smallest building block of a wireless LAN is a basic service set (BSS).

BSS consists of some number of stations executing the same MAC protocol and access to the same sharedwireless medium.

A BSS may be connecting to a backbone distribution system (DS) through an access point (AP).

The AP functions as a bridge and a relay point. In a BSS, client stations do not communicate directly with one another. If one station in the BSS wants to communicate with another station in the same BSS.

The MAC frame is first sent from the beginning station to the AP, and then from the AP to thedestination station.

Similarly, a MAC frame from a station in the BSS to a remote station is sent from the

local station to the AP and then relayed by the AP over the DS on its way to thedestination station. The DS can be a switch, a wired network, or a wireless network.

When all the stations in the BSS are mobile stations, with no connection to other BSSs, the BSS iscalled an independent BSS (IBSS). It also called ad hoc network.

In an IBSS, the stations all communicate directly, and no AP is involved.

An extended service set (ESS) consists of two or more basic service sets interconnected by a distributionsystem.

Normally, the distribution system is a wired or wireless.

The extended service set (ESS) appears as a single logical LAN to the logical link control (LLC) level.

Here Access point (AP) is a part of the station.


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IEEE 802.11 Protocol Architecture:

The IEEE 802.11 standard only covers the physical layer PHY and medium access layerMAC like theother 802.x LANs do.

The physical layer is subdivided into the physical layer convergence protocol (PLCP) and the physicalmedium dependent sublayerPMD as shown in the below figure.

The basic tasks of the MAC layer comprise (includes) medium access, fragmentation of user data, andencryption.

The PLCP sublayer provides a carrier sense signal, called clear channel assessment (CCA), andprovides a common PHY service access point (SAP).

Finally, the PMD sublayer handles modulation and encoding/decoding of signals.

IEEE 802.11 Physical Layer:

IEEE 802.11 supports three different physical layers:

One layer based on infra red and two (DSSS & FHSS) layers based on radio transmission(the ISM band at 2.4 GHz).

Frequency hopping spread spectrum:( FHSS)

The total bandwidth is split into many channels of smaller bandwidth.

Frequency hopping spread spectrum (FHSS) is a spread spectrum technique which allows

for the coexistence of multiple networks in the same area by separating different networksusing different hopping sequences. The frame consists of two basic parts, the PLCP part (preamble and header) and the payloadpart.

PLCP part is always transmitted at 1 Mbit/s, payload (MAC data), can use 1 or 2 Mbit/s. additionally.

The fields of the frame fulfill the following functions:


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o Synchronization: The PLCP preamble starts with 80 bit synchronization,whichis a 010101... bit pattern.

o Start frame delimiter (SFD): The following 16 bits indicate the start of the frameand provide frame synchronization. The SFD pattern is 0000110010111101.

o PLCP_PDU length word (PLW): This first field of the PLCP header indicatesthe length of the payload in bytes including the 32 bit CRC at the end of the

payload.o Header error check (HEC): Finally, the PLCP header is protected by a 16 bit

checksum.

Direct sequence spread spectrum (DSSS):

Direct sequence spread spectrum (DSSS) is the alternative spread spectrum method separating bycode and not by frequency.

In IEEE 802.11 DSSS, spreading is achieved using the 11-chip Barker sequence (+1, 1, +1, +1, 1, +1,+1, +1, 1, 1, 1).

IEEE 802.11 DSSS PHY also uses the 2.4 GHz ISM band and offers both 1 and 2 Mbit/s data rates.

A physical frame of the physical layer using DSSS. The frame consists of two basic parts, the PLCP part(preamble and header) and the payload part.

The PLCP part is always transmitted at 1 Mbit/s, payload. The fields of the frame have the following functions:

o Synchronization: The first 128 bits are not only used for synchronization, butalso gain setting, energy detection (for the CCA), and frequency offsetcompensation.

o Start frame delimiter (SFD): This 16 bit field is used for synchronization at thebeginning of a frame.

o Signal: Originally, only two values have been defined for this field to indicate thedata rate of the payload.

o Service:This field is reserved for future use.

o Length: 16 bits are used in this case for length indication of the payload inmicroseconds.

o Header error check (HEC): Signal, service, and length fields are protected bythis checksum.

Infra red:

The PHY layer, which is based on infra red (IR) transmission, uses near visible light at 850950 nm.


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This allows for point-to-multipoint communication. The maximum range is about 10 m if no sunlight orheat sources interfere with the transmission.

Typically, such a network will only work in buildings, e.g., classrooms, meeting rooms etc.

IEEE 802.11 Media Access Control (MAC) Layer: The MAC layer has to control medium access, but it can also offer support for roaming, authentication,

and power conservation.

The basic services provided by the MAC layer areo the mandatoryasynchronous data serviceando An optional time-bounded service

In ad-hoc network mode only offers the asynchronous service and an infrastructure-based network offersboth service (asynchronous data service and time-bounded service).

The asynchronous service supports broadcast and multi-cast packets, and packetexchange is based on a best effort model, i.e., no delay bounds can be given fortransmission.

Access Methods:

The three basic access mechanisms have been defined for IEEE 802.11

o DCF CSMA / CA ( Mandatory)

Based on the version of CSMA/CA oDCD with RTS/CTS (Optional)

Avoid the hidden terminal problem. oPCF (Optional)

Access point survey terminals according the list.

The first two methods are also summarized as distributed coordination function (DCF), thethird method is called point coordination function (PCF).

DCF only offers asynchronous service, but PCF offers both asynchronous and time-boundedservice.

The MAC mechanisms are also called distributed foundation wireless medium access control(DFWMAC).

Priorities:

The below figure shows the three different parameters that define the priorities of mediumaccess.


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Short inter-frame spacing (SIFS): The shortest waiting time for medium access(so the highestpriority) is defined for short control messages.

DCF inter-frame spacing (DIFS): This parameter denotes the longest waitingtime and hasthe lowest priority for medium access.

PCF inter-frame spacing (PIFS): A waiting time between DIFS and SIFS (andthus a mediumpriority) is used for a time-bounded service.

Basic DFWMAC-DCF using CSMA / CA:

CSMA/CA Principles:

Station ready to send, starts sensing the medium (Carrier Sense based on CCA, Clear ChannelAssessment).

If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending.

If the medium is busy, the station has to wait for a free IFS, and then the station must additionallywait a random back-off time (collision avoidance, multiple of slot-time).

If another station occupies the medium during the back-off time of the station, the back-off timer stops.

CSMA/CA broadcast:

The below figure explains, five stations trying to send a packet at the marked points in time.

Station3 has the first request from a higher layer to send a packet.

The station senses the medium, waits for DIFS and accesses the medium, i.e., sends the packet.

Station1, station2, and station5 have to wait at least until the medium is idle for DIFS again afterstation3 has stopped sending.


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Now all three stations choose a backoff time within the contention window and start counting downtheir backoff timers.

The backoff time of station1 as sum of bo e (the elapsed backoff time) and bor (theresidual backoff time). The same is shown for station5.

Station2 has a total backoff time of only boe and gets access to the medium first.

Now the backoff timers of station1 and station5 stop, and the stations store their residual backoff times.

While a new station has to choose its backoff time from the whole contention window, the two oldstations have statistically smaller backoff values.

Now station4 wants to send a packet as well, so after DIFS waiting time, three stations try to get access.

The two stations accidentally have the same backoff time

This results in a collision on the medium as shown, i.e., the transmitted frames are destroyed.

Station1 stores its residual backoff time again. In the last cycle shown station1 finally gets access tothe medium, while station4 and station5 have to wait.

CSMA/CA unicast:

Sending unicast packets

o station has to wait for DIFS before sending data

o receiver acknowledges at once (after waiting for SIFS) if the packet was received

correctly (CRC)o automatic retransmission of data packets in case of transmission errors


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DFWMAC-DCF with RTS/CTS extension:

Sending unicast packets

Station can send RTS with reservation parameter after waiting for DIFS(reservation determines amount of time the data packet needs the medium)

After receiving the RTS, the sender sends the CTS to the sender after SIFS.

Now the sender is ready to transmit the data. (After receiving the CTS) Now the sender can transmit the data after SIFS.

After receiving the data, the receiver sends the ACK after SIFS. other stations store medium reservations distributed via RTS and CTS

DFWMAC-PCF with polling:

The two access mechanisms presented so far cannot guarantee a maximum access delay or minimumtransmission bandwidth.

To provide a time-bounded service, the standard specifies a point coordination function(PCF) on top ofthe standard DCF mechanisms.

Using PCF requires an access point that controls medium access and polls the single nodes

The point co-ordinator in the access point splits the access time into super frame periods as shown in thefigure.


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A super frame comprises a contention free period and a contention period. The contentionperiod can be used for the two access mechanisms presented above.

The figure also shows several wireless stations (all on the same line) and the stations

NAV (again on one line).

IEEE 802.11 MAC Frame:

The general format is used for all data and control frames, but not all fields are used in all contexts.

The Data fields are as follows:

o Frame Control:Indicates the type of frame and provides control information.

o Duration/Connection ID: If used as a duration field, indicates the time. If useconnection ID, indicates the connection.


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o Addresses: The 48-bit address fields depend on context. The transmitter addressand receiver address are joined to the BSS that are transmitting and receivingframes over the wireless LAN.

o Sequence Control: A 4-bit fragment number subfield used for fragmentation andreassembly, and a 12-bit sequence number used to number frames sent between agiven transmitter and receiver.

o Frame Body: Contains an MSDU or a fragment of an MSDU. The MSDU is aLLC protocol data unit or MAC control information.

o Frame Check Sequence: A 32-bit cyclic redundancy check.

The frame control field consists of the following fields:

o Protocol Version:802.11 version, which version currently used.o Type:Identifies the frame ascontrol, management, or data.o Subtype:Further identifies the function of frame.

o To DS: The MAC coordination sets this bit to 1 in a frame destined to thedistribution system.

o From DS: The MAC coordination sets this bit to 1 in a frame leaving thedistribution system.

o More Fragments:Set to 1 if more fragments follow this one.o Retry:Set to 1 if this is a retransmission of a previous frame.

o Power Management:Set to 1 if the transmitting station is in a sleepmode. o More Data:Indicates that a station has additional data to send.o WEP:WEP is used in the exchange of encryption keys for secure data exchange.

o Order: Set to 1 in any data frame sent using the Strictly Ordered service,whichtells the receiving station that frames must be processed in order.

Control frame subtypes:

Power Save-Poll (PS-Poll) Request to Send (RTS) Clear to Send (CTS) Acknowledgment Contention-Free (CF)-End CF-End + CF-Ack

Data frame subtypes:


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Data Data + CF-Ack Data + CF-Poll Data + CF-Ack + CF-Poll

Management frame subtypes:

Used to communicate between station and AP. Association Request Association Response Reassociation Request Reassociation Response Probe Request Probe Response Beacon Announcement Traffic Indication Message Dissociation Authentication Deauthentication

IEEE 802.11 Services

IEEE 802.11 defines nine services that need to be provided by the wireless LAN to providefunctionality equivalent to that which is inbuilt to wired LANs.

The Services are

The above list of services are categorized in to two types:

o Station: Stations are implemented in every 802.11 station, including accesspoint(AP) stations.

o Distribution system: Distribution services are provided between basicservice sets (BSSs); these services may be implemented in an AP or in

another special purpose device attached to the distribution system. In the above list


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o Three of the services are used to control IEEE 802.11 LAN access andconfidentiality.

o Six of the services are used to support delivery of MAC service dataunits (MSDUs) between stations.

o The MSDU is the block of data passed down from the MAC user to the MAClayer; typically this is a LLC PDU.

o If the MSDU is too large to be transmitted in a single MAC frame, it may befragmented and transmitted in a series of MAC frames.

Distribution ofMessages within a DS:

The two services involved with the distribution of messages within a DS are oDistributiono Integration.

Distribution is the primary service used by stations to exchange MAC frames when the frame mustcross the DS to get from a station in one BSS to a station in another BSS.

For example,

Suppose a frame is to be sent from station 2 (STA 2) to STA 7 in the above figure. The frame is sent from STA 2 to STA 1, which is the AP for this BSS.

The AP gives the frame to the DS, which has the job of directing the frame to the AP associated withSTA 5 in the target BSS.

STA 5 receives the frame and forwards it to STA 7.

If the two stations that are communicating are within the same BSS, then the distribution service logicallygoes through the single AP of that BSS

The integration service enables transfer of data between a station on an IEEE 802.11 LAN and astation on an integrated IEEE 802.x LAN.


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The term integratedrefers to a wired LAN that is physically connected to the DS

The integration service takes care of any address translation and media conversion logic required for theexchange of data.

Association related Services

The main purpose of the MAC layer is to transfer MSDUs between MAC entities; this purpose is

fulfilled by the distribution service. The Distributed services required a information about the stations from the ESS, which is provided by the

association-related services.

Before the distribution service can deliver data to or accept data from a station, that station must beassociated.

The three services are related to this requirements: Association:

o Establishes an initial association between a station and an AP.

o Before a station can transmit or receive frames on a wireless LAN, its identityand address must be known.

o For this purpose, a station must establish an association with an AP withina particular BSS.

o The AP can then communicate this information to other APs within the ESSto facilitate routing and delivery of addressed frames.

Reassociation :o Enables an established association to be transferred from one AP to another,

allowing a mobile station to move from one BSS to another. Disassociation :

o A notification from either a station or an AP that an existing associationis terminated.

Access and Privacy Services

In order to transmit over a wired LAN, a station must be physically connected to the LAN.On the other hand, with a wireless LAN, any station within radio range of the other deviceson the LAN can transmit.

Similarly, in order to receive a transmission from a station that is part of a wired LAN, the

receiving station must also be attached to the wired LAN. On the other hand, with awireless LAN, any station within radio range can receive. Thus, a wired LAN provides a degree of privacy, limiting reception of data to stations connected to the

LAN. Wireless LAN provide three services:

o Authentication:Used to establish the identity of stations to each other.

o Deauthentication: This service is invoked whenever an existing authentication isto be terminated.

o Privacy: Used to stop the contents of messages from being read by other than theintended recipient.


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WIMAX AND IEEE 802.16 BROADBAND WIRELESS ACCESS

STANDARDS

In 1999, IEEE 802 committee introduces an 802.16 working group for broadband wirelessstandard.

WIMAX is a industry group (World-wide Interoperability Microwave Access).

This WIMAX Forum, has been formed to promote the 802.16 standards and to develop interoperabilityspecifications.

The charter for the group is to develop standards that

o Use wireless links with microwave or millimeter wave radioso Normally use licensed spectrumo Are metropolitan in scaleo Provide public network service to fee-paying customers

o Use point-to-multipoint architecture with stationary rooftop or tower-mountedantennas

o Are capable of broadband transmissions (>2 Mbps)

IEEE 802.16 Standards:

IEEE 802.16 Architecture:

System Reference Architecture:

The 802.16 standards are designed with respect to the abstract system reference model shown in thebelow figure.

An 802.16 wireless service provides a communications path between a subscriber site and corenetwork.

The subscribers site may be either a single subscriber device or a network on the subscriber'spremises (e.g., a LAN, IP-based network).

The core network is the public telephone network and the Internet.


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Two interfaces are defined in this service. Air interface between the subscriber's transceiver station and the base transceiver station.

The system reference model also shows interfaces between the transceiver stations and the networksbehind them (SNI and BNI). This layer working from the bottom up.

Protocol Architecture:

Protocols defined specifically for wireless transmission address issues relating to the transmission ofblocks of data over the network.

IEEE 802.16 protocols are concerned with lowest two layers of the OSI model.

The below figure shows the four protocol layers defined in the 802.16 protocol architecture tothe OSI model.


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The lowest two layers (Transmission and Physical) of the 802.16 protocol model correspond tothe physical layer of the OSI model and include such functions as

o Encoding/decoding of signals

o Preamble generation/removal (for synchronization)o Bit transmission/reception

In addition, the physical layer of the 802 model includes a specification of the transmissionmedium and the frequency band.

Medium Access Control layer include such functionso On transmission, assemble data into a frame with address and error detection fields.o On reception, disassemble frame, and perform address recognition and error detection.

o Govern access to the wireless transmission medium.

The protocol at this layer, between the base station and the subscriber station, is responsible forsharing access to the radio channel.

Specifically, the MAC protocol defines how and when a base station or subscriber station may initiatetransmission on the channel.

A convergence layer protocol may do the following functions:

o Encapsulate PDU (protocol data unit) framing of upper layers into802.16 MAC/PHY frames.

o

Map an upper layer's addresses into 802.16 addresses.o Translate upper layer QoS parameters into native 802.16 MAC format.o Equalize the upper layer time into MAC services.

IEEE 802.16 Services:

IEEE 802.16 is designed to support the following bearer services:

o Digital audio/video multicast: Transports one-way digital audio/video streams tosubscribers. The principal example of this service is a broadcast radio and video

similar to digital broadcast cable TV and digital satellite TV. A special case ofthis service is two-way video such as in teleconferencing.

o Digital telephony: Supports multiplexed digital telephony streams. This service isa classic WLL service that provides a replacement for wired access to the publictelephone network.

o ATM: Provides a communications link that supports the transfer of ATM cells aspart of an overall ATM network.

o Internet protocol: Supports the transfer of IP datagrams. The 802.16 linkmustprovide efficient timely service.

o Bridged LAN: Similar to the IP-based support. A bridge LAN service enablestransfer of data between two LANs with switching at the MAC layer.

o Back-haul:For cellular or digital wireless telephone networks.

o Frame relay: Similar to ATM. Frame relay uses variable-length framesincontrast to the fixed-length cells of ATM.


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The above bearer services are grouped in three broad categories:

o Circuit based: These services provide a circuit-switching capability, inwhichconnections are set up to subscribers across a core network.

o Variable packet: IP and frame transmit are examples of services that make use ofvariable-length PDUs. Another example is MPEG video, which is a videocompression scheme in which successive blocks of digital video information maybe of varying sizes.

o Fixed-length celUpacket:This service is for ATM.

IEEE 802.16 MAC Layer:

Data transmitted over the 802.16 air interface from or to a given subscriber are structured as asequence of MAC frames.

The MAC frame includeso MAC protocol control information ando higher-level data.

This is not to be confused with a TDMA frame, which consists of a sequence of time slots, eachdedicated to a given subscriber.

A TDMA time slot may contain exactly one MAC frame, a fraction of a MAC frame, or multiple MACframes.

The sequence of time slots across multiple TDMA frames that is dedicated to onesubscriber forms a logical channel, and MAC frames are transmitted over that logicalchannel.

Connection-oriented

o All services inherently connectionless mapped to a connection Connections referenced using a 16-bit connection identifier (CID) Management channels and transport channels for contracted services

IEEE 802.16 Frame Format

The MAC frame consists of three sections:

o Header: Contains protocol control information needed for the functioning oftheMAC protocol.

o Payload: The payload may be either higher-level data (e.g., an ATM cell, anIPpacket, a block of digital speech) or a MAC control message.

o CRC: The cyclic redundancy check field contains an error-detecting code

Three header formats are defined.

o There is a generic header format in both the uplink(toward the base station) and

downlink(toward the subscriber) directions. These formats are used forframesthat contain either higher-level data or a MAC control message.

o The third format is used for abandwidthrequest frame.


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The downlink header format is shown in the below figure.

The downlink header consists of the following fields:

o Encryption control(1 bit): Indicates whether the payload is encrypted.

o Encryption key sequence (4 bits): An index into a vector of encryption keyinformation, to be used if the payload is encrypted.

o Length(11 bits): Length in bytes of the entire MAC frame.

o Connection identifier (16 bits): A unidirectional, MAC-layer address thatidentifies a connection to equivalent peers in the subscriber and base stationMAC.

o Header type (1 bit): Indicates whether this is a generic or bandwidth requestheader.

o ARQ indicator (1 bit): Indicates whether the frame belongs to anARQ(Automatic Repeat Request) enabled connection.

o Fragment control (2 bits): Used in fragmentation and reassembly, as explainedsubsequently.

o Fragment sequence number (4 bits):Sequence number of the current fragment.o Header check sequence (8 bits): An 8-bit CRC used to detect errors in the header.

The uplink header format is shown in the below figure.

The uplink header consists of the following fields.

o It contains all of the fields of the downlink header, plus an 8-bitgrant management field.

o Grant Management field (8 bit): This field is used by the subscriber to conveybandwidth management needs to the base station.

o There are three different encodings of this field, depending on the typeof connection. The subfields within the GM field include

Slip indicator (1 bit): If set, indicates a slip of uplink grants relative to theuplink queuedepth.


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Poll-me (1 bit): If set requests a poll by the base station.

Grants per interval (7 bits): The number of grants required by a connection.

Piggyback request (8 bits): The number of bytes of uplink capacity requestedby thesubscriber for this connection.

Finally, the bandwidth request header

o is used by the subscriber to request additional bandwidth. This header is fora MAC frame with no payload.

IEEE 802.16 Physical Layer:

The 802.16 physical layer supports a different structure for

o the point-to multipoint downstream channels ando the multipoint-to-point upstream channels

Upstream channel

o Stations transmit in their assigned allocation specified in an initial map

o Uplink sub-frame may also contain contention-based allocations for initial system

accesso Uses a DAMA-TDMA techniquep Error correction uses Reed-Solomon codes

Downstream channel

o Continuous downstream mode For continuous transmission (audio/video) Simple TDM scheme is used for channel access Frequency division duplex (FDD)

o Burst downstream mode For bursty transmission (IP-based traffic) DAMA-TDMA scheme for channel access

FDD with adaptive modulation, frequency shift division duplexing (FSDD),time division duplexing (TDD)


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MOBILE AD HOC NETWORKS

Mobility support joins on the existence of at least some infrastructure.

o Mobile IPrequires, e.g., a home agent, tunnels, and default routers.

o DHCP requires servers and broadcast capabilities of the medium reaching allparticipants or relays to servers.

o Cellular phone networks require base stations, infrastructure networks, etc.

However, there may be several situations, users of a network cannot rely on an infrastructure, it is tooexpensive, or there is none at all.

In this situation mobile ad-hoc networks are the only choice. Features of ad hoc Network

o Instant infrastructure: Infrastructures need planning and administration. It

would take too long to set up this kind of infrastructure; therefore, ad-hocconnectivity has to set up.

o Disaster relief: Cyclone cut phone and power lines, floods destroy base stations,fires burn servers. But, the above things are not suffered in the ad-hoc Network.Emergency teams can only rely on an infrastructure they can set up themselves.

No forward planning can be done, and the set-up must be extremely fast andreliable.

o Remote areas: Even if infrastructures could be planned ahead, it is sometimestoo expensive to setup infrastructure in sparsely populated areas. But it ispossible in ad-hoc network.

o Effectiveness: Registration procedures might take too long, and communicationoverheads might be too high with existing networks. Application-tailored ad-hocnetworks can offer a better solution.

Fig: MANETs and mobile IP


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Over the last few years ad-hoc networking has attracted a lot of research interest. This has ledto creation of a working group at the IETF that is focusing on Mobile ad-hocnetworking(MANET).

The above figure shows the relation of MANET to Mobile IP and DHCP.

While mobile IP and DHCP handle the connection of mobile devices to a fixed infrastructure, MANETcomprises mobile routers, too.

Mobile devices can be connected with an infrastructure using Mobile IP for mobility support and DHCP asa source of many parameters, such as an IP address.

MANET research is responsible for developing protocols and components to enable ad-hoc networkingbetween mobile devices.

A variant of distance vector routing was used in this ad-hoc network.

In this approach, each node sends a routing advertisement. These advertisements contain a neighbor tablewith a list of link qualities to each neighbor.

Each node updates the local routing table according to the distance vector algorithm based on theseadvertisements.

Received packets also help to update the routing table.

Routing

In wireless networks with infrastructure support a base station always reaches all mobilenodes, this is not always the case in an ad-hoc network. A destination node might be out

of range of a source node transmitting packets. Routing is needed to find a path betweensource and destination and to forward the packets appropriately.

In wireless networks using an infrastructure, cells have been defined. Within a cell, the basestation can reach all mobile nodes without routing via a broadcast. In the case of ad-hocnetworks, each node must be able to forward data for other nodes.

The below figure gives a simple example of an ad-hoc network.

At a certain time t1 the network topology might look as illustrated on the left side of thefigure. Five nodes, N1 to N5, are connected depending on the current transmissioncharacteristics between them.

In this snapshot of the network, N4 can receive N1 over a good link, but N1 receives N4 only via a weaklink.


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Links do not necessarily have the same characteristics in both directions. The reasons for this are, e.g.,different antenna characteristics or transmit power.

N1 cannot receive N2 at all, N2 receives a signal from N1. This creates many additional problems that are discussed in the following paragraphs.

Asymmetric links:

o Node A receives a signal from node B. But this does not tell us anythingabout the quality of the connection in reverse.

o Node B might receive nothing, have a weak link, or even have a better linkthan the reverse direction. Routing information collected for one directionis of almost no use for the other direction.

Redundant links:

o In ad-hoc networks nobody controls redundancy, so there might be manyredundant links up to the extreme of a completely meshed topology.

o Routing algorithms for wired networks can handle redundancy, but a highredundancy can cause a large computational overhead for routing tableupdates.

Interference:

o Links come and go depending on transmission characteristics, onetransmission might interfere with another, and nodes might overhear thetransmissions of other nodes.

o Interference creates new problems by unplanned links between nodes: iftwo close-by nodes forward transmissions, they might interfere anddestroy each other, interference might also help routing.

Dynamic topology:

o The mobile nodes might move or medium characteristics might change.These results in frequent changes in topology, so snapshots are valid onlyfor a very short period of time.

o In ad-hoc networks, routing tables must somehow reflect these frequentchanges in topology, and routing algorithms have to be adapted. Routingalgorithms used in wired networks would either react much too slowly orgenerate many updates to reflect all changes in topology.

Considering all the additional difficulties in comparison to wired networks, the following observationsconcerning routing can be made for ad-hoc networks with moving nodes.

Traditional routing algorithms known from wired networks will not work efficiently.Routing in wireless ad-hoc networks cannot rely on layer three knowledge alone.Centralized approaches will not really work, because it takes too long to collect thecurrent status and circulate it again. Within this time the topology has alreadychanged.

Many nodes need routing capabilities.

The concept of a connection with certain characteristics cannot work properly.

A last alternative to forward a packet across an unknown topology is known as

flooding.


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Destination Sequence Distance Vector (DSDV)

Destination sequence distance vector (DSDV) routing is an improvement to distance vector routing for ad-hoc networks.

Distance vector routing is used as routing information protocol (RIP) in wired networks. It performs extremely poorly (very poor) with certain network changes due to the count-to- infinity

problem. DSDV now adds two things to the distance vector algorithm:

Sequence numbers: Each routing advertisement comes with a sequence number.

Sequence numbers help to apply the advertisements in correct order. This avoids the

loops that are likely with the unchanged distance vector algorithm.

Damping: Transient changes in topology that are of short duration should not

destabilize the routing mechanisms. Advertisements containing changes in the

topology currently stored as therefore not disseminated further. A node waits with

dissemination if these changes are probably unstable.

Dynamic source routing

In ad-hoc network, nodes are transmitting the packets from time to time and Destination sequence distancevector (DSDV) is used for updating the routing tables.

Although only some user data has to be transmitted, the nodes exchange routinginformation to keep track of the topology. These algorithms maintain routes between allnodes, although there may currently be no data exchange at all. This causes unnecessarytraffic and prevents nodes from saving battery power.

Dynamic source routing (DSR), therefore, divides the task of routing into two separate

problems.Route discovery: A node only tries to discover (find out) a route to a destination if it

has to send something to this destination and there is currently no known route.

Route maintenance: If a node is continuously sending packets via a route, it has tomake sure that the route is held upright. As soon as a node detects problems with the

current route, it has to find an alternative.

If a node needs to discover a route, it broadcasts a route request with a unique identifier and the destinationaddress as parameters. Any node that receives a route request does thefollowing:

If the node has already received the request, it drops the request packet.If the node recognizes its own address as the destination, the request has reached its target.

Otherwise, the node appends its own address to a list of traversed hops in the packet

and broadcasts this updated route request.


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Example:

The below figure, find the route between N1 and N3

o N1 broadcasts the request ((N1), id = 42, target = N3), N2 and N4 receive thisrequest.

o N2 then broadcasts ((N1, N2), id = 42, target = N3), N4 broadcasts ((N1, N4), id= 42, target = N3). N3 and N5 receive N2s broadcast, N1, N2, and N5 receive

N4s broadcast.

o N3 recognizes itself as target, N5 broadcasts ((N1, N2, N5), id = 42, target = N3).N3 and N4 receive N5s broadcast. N1, N2, and N5 drop N4s broadcast packet,

because they all recognize an already received route request.

o N4 drops N5s broadcast, N3 recognizes (N1, N2, N5) as an alternate, butlonger route.

o N3 now has to return the path (N1, N2, N3) to N1. This is simple assumingsymmetric links working in both directions. N3 can forward the information usingthe list in reverse order.

The assumption of bi-directional links holds for many ad-hoc networks. However, iflinks are not bi-directional, the scenario gets more complicated. The algorithm has to beapplied again, in the reverse direction if the target does not maintain a current path tothe source of the route request.

o N3 has to broadcast a route request ((N3), id = 17, target = N1). Only N5 receivesthis request.

o N5 now broadcasts ((N3, N5), id = 17, target = N1), N3 and N4 receivethe broadcast.

o N3 drops the request because it recognizes an already known id. N4 broadcasts((N3, N5, N4), id = 17, target = N1), N5, N2, and N1 receive the broadcast.

o N5 drops the request packet, N1 recognizes itself as target, and N2 broadcasts((N3, N5, N4, N2), id = 17, target = N1). N3 and N5 receive N2s broadcast.

o N3 and N5 drop the request packet.

Now N3 holds the list for a path from N1 to N3, (N1, N2, N3), and N1 knows the pathfrom N3 to N1, (N3, N5, N4, N1). But N1 still does not know how to send data to N3!

The only solution is to send the list (N1, N2, N3) with the broadcasts initiated by N3in the reverse direction.


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Alternative metrics

Even for fixed networks, example bandwidth can also be a factor for the routing metric.Due to the varying link quality and the fact that different transmissions can interfere,other metrices can be more useful. One other metric, called least interference routing(LIR), takes possible interference into account. The below figure shows an ad-hoc

network topology.

With both transmissions taking place simultaneously, there would have been interference between them.

In this case, least interference routing helped to avoid interference. Taking only localdecisions and not knowing what paths other transmissions take, this scheme can just lowerthe probability of interference.

Routing can take several metrices into account at the same time and weigh them. Metrices could be thenumber of hops h, interference i, reliability r, error rate e, etc.

The cost of a path could then be determined as:

Cost = h + i + r + e +

It is not at all easy to choose the weights , , , , to achieve the desired routing behavior.


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WIRELESS LOCAL LOOP (WLL)

Introduction:

Traditionally, the provision of voice and data communications to the end user, over the local loop, orsubscriber loop, has been provided by wired systems.

The residential subscribers, twisted pair has been and continues to be the standard means of connection.

For business and government subscribers, twisted pair, coaxial cable, and optical fiber are in use.

As subscribers have demanded greater capacity, particularly to support Internet use,traditional twisted pair technology has become inadequate. Telecommunicationsproviders have developed a number of technologies to meet the need, includingISDN (integrated services digital network).

In addition, cable operators have introduced two-way highspeed service using cablemodem technology. Thus, wired technologies are responding to the need for reliable,high-speed access by residential, business, and government subscribers.

However, increasing interest is being shown in competing wireless technologies forsubscriber access. These approaches are generally referred to as wireless local loop(WLL), or fixed wireless access.

WLL alternatives are narrowband, which offer a replacement for existing telephony services, andbroadband, which provide high-speed two-way voice and data service.

The Role of WLL

The below figure illustrates a simple WLL configuration. A WLL provider services one or more cells.

Each cell includes a base station antenna, mounted on top of a tall building or tower.Individual subscribers have a fixed antenna mounted on a building or pole that has anunobstructed line of sight to the base station antenna.

From the base station, there is a link, which may either be wired or wireless, to aswitching center. The switching center is typically a telephone company local office,which provides connections to the local and long-distance telephone networks.

An Internet service provider (ISP) may be collocated at the switch or connected to the switch by ahigh-speed link.

The below figure shows the WLL configuration.

The WLL has a number of advantages over a wired approach to subscriber loop support: o Cost:Wireless systems are less expensive than wired systems. Although the

electronics of the wireless transmitter/receiver may be more expensive than thoseused for wired communications, with WLL the cost of installing kilometers ofcable, either underground or on poles, is avoided, as well as the cost of maintainingthe wired infrastructure.

o Installation time:WLL system can be installed in a small fraction of thetimerequired for a new wired system.

o Selective installation:Radio units are installed only for those subscribers whowantthe service at a given time.

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WLL needs to be evaluated with respect to two alternatives:

o Wired scheme using existing installed cable:WLL has become cost-competitivewith wired schemes; new installations face a genuine choice between the wired andwireless approaches.

o Mobile cellular technology:A major advantage of WLL over mobile cellular is that,because the subscriber unit is fixed, the subscriber can use a directional antennapointedat the base station antenna, providing improved signal quality in both directions.

In the United States, the Federal Communications Commission (FCC) has set aside 15frequency bands for use in commercial fixed wireless service, at frequencies 2 to 40 GHz.Two approaches of most interest for the WLL application are:

o Local multipoint distribution service (LMDS) ando Multichannel multipoint distribution service (MMDS).

Multichannel Multipoint Distribution Service: (MMDS)

The below table shows five frequency bands in the range 2.15 GHz to 2.68 GHz that have been allocated inthe United States for fixed wireless access using MMDS.

The first two bands were licensed in the 1970s when they were called multipoint distributionsservices (MDSs), for broadcast of 6 MHz TV channels.

In 1996, the FCC increased the allocation to its present range and allowed formultichannel services, called MMDS. MMDS has been used to struggle with cable TVproviders and to provide service in rural areas not reached by broadcast TV or cable. Forthis reason, MMDS is also referred to as wireless cable.

The transmitted power allowed by the FCC enables an MMDS base station to service an area with aradius of 50 km, but subscriber antennas must be in the line of sight.

MMDS can be used to support two-way services. MMDS is also used in other countries fortwo-way access. Thus, MMDS is an alternative for broadband data services, such asInternet access.

The Disadvantage of MMDS, compared to LMDS. MMDS is offers much less bandwidth than LMDS.

With current technology, a single MMDS channel can offer upstream transfer rates of 27 Mbps, withindividual subscriber rates of 300 kbps to 3 Mbps.


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The advantages of MMDS over LMDS include the following:

MMDS signals have larger wavelengths (greater than 10 cm) and can travel farther without losingsignificant power.

Equipment at lower frequencies is less expensive, yielding cost savings at both the subscriber andbase station.

MMDS signals don't get blocked as easily by objects and are less susceptible to rain absorption.

Local Multipoint Distribution Service: (LMDS)

LMDS is a relatively new WLL service to deliver TV signals and two-way broadbandcommunications, operating at millimeter frequencies.

In the United States, LMDS will be offered at frequencies near 30 GHz; in Europe and some otherareas, frequencies near 40 GHz will be used..

LMDS has the following advantages:

Relatively high data rates, in the Mbps range Capable of providing video, telephony, and data

Relatively low cost in comparison with cable alternatives


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The principal disadvantage of LMDS

The short range from the base station, requiring a relatively large number of base stations toservice a given area.

In a typical system, the base station antenna is located on top of a tall building orhigh pole overlooking the service area, with line of sight to subscribers with thepossible exception of tree canopies.

The base station antenna covers a sector 60 to 90 wide. Thus, full coverage requires 4 to 6antennas.

Propagation Considerations for WLL:

For most high-speed WLL schemes, frequencies are referred to as the millimeter waveregion are used. Although the term millimeter wave is not precisely defined, a commonboundary is 10 GHz.

The reasons for using frequencies in this range for WLL include the following:o There are wide unused frequency bands available above 25 GHz.

o At these high frequencies, wide channel bandwidths can be used, providing highdata rates.

o Small size transceivers and adaptive antenna arrays can be used.

However, millimeter wave systems have some unwanted propagation (spread)characteristics:

o Free space loss increases with the square of the frequency.

o Generally, below 10 GHz, we can ignore attenuation due to rainfall andatmospheric or gaseous absorption. Above 10 GHz, these attenuation effectsare large.

o Multipath losses can be quite high.

Because of these negative propagation characteristics, WLL systems can only serve cells of a limitedradius, usually just a few kilometers.

Finally, rainfall and humidity effects limit the range and availability of WLL systems.

Fresnel Zone:o For effective communication at millimeter wavelengths, there should be an

unobstructed line of sight between transmitter and receiver.

o The definition of Fresnel zones is, any small element of space in the path of anelectromagnetic wave may be considered the source of secondary wavelet, andthat the radiated field can be built up by the superposition of all these wavelets.

o It can be shown that objects lying within a series of concentric circles around thedirect line of sight between two transceivers have constructive or destructive effectson communication.

o Those that fall within the first circle, the first Fresnel zone, have the most seriousnegative effects.

o Consider a point along the direct path between a transmitter and receiver, that is, a

distance S from the transmitter and a distanceD from the receiver, with the total


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distance along the path equal to S +D (The below figure show)

o Then the radius of the first Fresnel zone at that point is

whereR, S,D, and are in the same units, and

A is the wavelength of the signal along the path.whereR is expressed in meters,

the two distances are in kilometers, andthe signal frequency is in gigahertz.

Atmospheric Absorption

o At frequencies above 10 GHz, radio waves propagating through the atmosphere aresubject to molecular absorption. The absorption as a function of frequency is veryuneven.

o There is a peak of water vapor absorption at around 22 GHz and a peak of oxygenabsorption near 60 GHz.

Effect of Rain

o One of the most serious concerns for millimeter wave propagation is attenuation due

to rain. The presence of raindrops can severely degrade the reliability andperformance of communication links.

o The effect of rain on millimeter wave propagation is complex, depending on dropshape, drop size, rain rate, and frequency.

Effects of Vegetation

o Through part of its path, a WLL link may run through vegetation, particularly foliageof tall trees. In some suburban and small town areas, such obstacles may beunavoidable for some subscribers, even with rooftop subscriber antennas.

o A study reported in reached the following conclusions: The presence of trees near subscriber sites can lead to multipath fading. The principal multipath effects from the tree canopy are diffraction and scattering.

Measurements in regularly planted orchards have found attenuation values between 12and 20 dB.

The multipath effects are highly variable due to wind.


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