Introduction to GSM Basic Concept

7/31/2019 Introduction to GSM Basic Concept

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RF TrainingMicrowave Link Engineering

A communication system that utilizes the radio frequency band spanning 2 to 60

GHz. As per IEEE, electromagnetic waves between 30 and 300 GHz are calledmillimeter waves (MMW) instead of microwaves as their wavelengths are about 1to 10mm

Small capacity systems generally employ the frequencies less than 3 GHz whilemedium and large capacity systems utilize frequencies ranging from 3 to 15 GHz.Frequencies > 15 GHz are essentially used for short-haul transmission.

Advantages:

Less affected by natural calamities

Less prone to accidental damage

Links across mountains and rivers are more economically feasible

Single point installation and maintenance

Single point security

They are quickly deployed

Line of Sight Characteristic

Highly directive

Pencil like beam width

EM wave follow straight line

Link Engineering

Site selection

Frequency selection

o G = 20 log F + 20 log D 52.6

o Frequency increase ~ Diameter decreases

o While choosing frequency, two things should be noted

o interference, 2) Regulatory Issues

o f = c/ (f mean mean rain attenuation decreases)

o loss = 36.6 + 20 log f + 20 log D

o loss = 32.4 + 20 log f + 20 log D

o f mean loss mean distance

Tower Heights profile

Free Space phenomenon

o Reflection

o Refraction

o Diffraction

o Attenuation

Terrain Survey Equipment Selection (depends on choice of frequency and antenna)


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ICT (Installation, Commissioning and Testing)

Fresnel Zone

Wave front has expanding properties as it travels through space. These propertiesresult in phase transition and reflection as wave passes over an obstacle. Thisresults in increase or decrease of signal. The amount of additional clearness that

must be allowed to avoid Fresnel Phenomenon is called Fresnel Zone.

Earth Bulge: K = effective radius of earth / true radius of earth

Fading

Weakening of RF signal

Is due to Reflection, Refraction, Diffraction, Attenuation

Type:

o Multi path fading: interference between a direct wave and reflectedwave. Caused by reflection from grounds and atmospheric sheets andlayers. Temperature inversion of earth surface. Trees and water bodies.

o Flat fading: A fade where all frequencies in the channel are equallyaffected. There is barely noticeable variation of the amplitude of the signalacross the channel bandwidth

o Frequency selective fading: that disturb selected frequency

o Fast Fading: rapid fluctuation of signal over small areas.

o Slow fading: result of shadowing, mountain, hills and other non-movingobjects.

o Rain fading: rain attenuates the signal, caused by scattering andabsorption of EM waves by rain drops. It start increasing at 10 GHz andabove 15GHz

Fade Margin

The amount by which the received signal may be reduced without causing thesystem performance to fall is call Fade Margin

Path Loss

The reduction of power density of EM wave as it travel thorough space

Causes: FSL, Diffraction, Absorption, Distance between Tx and Rx, height,location of Antennas

Diversity

Used to removed fading

Method of improving of reliability of message signal by using two or morecommunication channel.

Provide equipment reliability

Types:

o Frequency diversity: two Tx and two Rx both are tuned at differentfrequencies with same information is being transmitted over bothfrequencies so one of them provides sufficient signal strength. A bitexpensive.

o Space diversity: if two antennas are placed many wavelength apart, it

has been observed that fading can not occur. One of the antennasprovides sufficient signal strength.


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o Time diversity: A time delay of two identical signals on parallel paths

o Path diversity: signal arrives on geographically separate paths.

Diversity Combiner

o Combiner combines two or more diversity paths

o Types:

Selection combiner

Equal gain combiner

Maximal ratio combiner


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AntennaAn electrical conductor used to transmit and receive EM wave

Types

Isotropic

o Ideal antenna, radiate power equally in all direction.

o Gain ___

Parabolic

o High gain reflector antenna used in radio, television and datacommunication

Directional

o Radiate greater power in one or more direction, allowing for increaseperformance on Tx and Rx and reduce interference from unwanted

sources. Dipole

o Simplest practical antenna with center fed element for transmission andreception of RF energy.

Pattern or antenna pattern or antenna radiation pattern

The directional function characterizing the relative distribution of power radiatedby antenna

Polarization

Physical orientation of electric field

Vertical and horizontal

Transmitter and receiver antenna should have same polarization.

Directivity

Maximum value of directive gain is called directivity depend on shape of radiationpattern.

Efficiency

Ration of total power radiated to the input power of antenna.

= power radiated / input power

Gain

Power output in a particular direction to that produced in any direction byisotropic antenna

G = power output by antenna / power output by isotropic antenna

Product of efficiency and directivity

G = D

Reciprocity

Antenna ability which shows that same radiation pattern for transmission andreception

Beam width

Angular separation of half power points of radiation pattern


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Reflection Coefficient

Ratio of reflected wave to incident wave

+ = V0

- / V

0

VSWR / SWR (Voltage Standing Wave Ration) The SWR have max and min voltages

The ratio of max voltage to min voltage of SW is called standing wave ratio and ifwe are dealing in term of voltage then it is called VSWR

VSWR = V max/ V min= (1 + ) / (1 - )

Antenna performance [calculated by site master]

VSWR

Distance to fault

Insertion lose

Reflection mean power transferred mean return lose


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NetworkingFunction of OSI Layers

Layer # 1 (Physical Layer)

o Physical characteristics of interfaces and media

o Representation of bits (types of encoding)

o Date rate (transmission rate (duration of bits))

o Synchronization of bits

o Line configuration (point-to-point configuration, multipoint configuration)

o Physical topology (mesh, ring, bus topology etc)

o Transmission mode (simplex, half/full duplex)

Layer # 2 (Data Link Layer)

o

Frameso Framing

o Physical addressing (source/destination address)

o Flow control (prevent overwhelming)

o Error control

o Access control

Layer # 3 (Network Layer)

o Packets

o Source to destination delivery of packets

o Logical addressing

o Routing

Layer # 4 (Transport Layer)

o Segment

o Source to destination (end-to-end) delivery of entire message

o Service point addressing

o Segmentation and reassembly

o Connection control (connection oriented / connection less)

o Flow control (perform end to end)

o Error control (perform end to end)

Layer # 5 (Session Layer)

o Establish, maintain, synchronize and terminates session between twocommunication systems

o Dialog control

o Synchronization

Layer # 6 (Presentation Layer)

o Translationo Encryption / decryption


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o Compression

Layer # 7 (Application Layer)

o Network virtual terminal

o File transfer, access and management (FTAM)

o Mail services

o Directory services

Network:

Two or more than two computers that are attached to each other andcommunication with each others.

Protocol

Set of rules that governs the communication for each layer

Difference between Protocol and Standard

Protocol is Software (Data Link Layer)

Standard is Hardware (Physical Layer)

Every protocol may be standard but every standard is not a protocol

Unicast

an individual or device can be addressed to a single one

Multicast

an individual or device can be addressed to a group

Broadcast

an individual or device can be addressed to all

Simplex

Communication is unidirectional (TV, Radio)

Half Duplex

Each station can receive and transmit but not at the same time (Walky talky)

Full Duplex

Both station can receive and transmit simultaneously (Telephone)

Broadband

The generating frequency is different from is different from transmission

frequencyBaseband

The generating and transmission frequency are same

LAN (Local Area Network)

A network system which deals within one premise or same premises.

LAN Technologies

o Ethernet: 802.3, Access Method: CSMA/CD

o Token Ring: 802.5, Access Method: Token Passing

o FDDI: Fiber Distributed Data Interface, Access Method: Token Passing


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There are four basic types of media are used in local-area networks.

o Coaxial Cable, Twisted Pair Cable, Optical Fiber, Wireless (WiFi)

Topology:

o Topology is a physical layout or distribution of network.

o There are four basic types Topologies in LAN. Bus, Ring, Star, Mesh

LAN Devices:

o Repeaters:

A repeater is a physical layer (layer 1) device

Receive a digital signal and retransmit at high power

o Hub:

A hub is a physical layer (layer 1) device

Used to connect multiple devices

Internally Bus topology

One broadcast domain and one collision domain

o Bridge:

A bridge is a Data Link layer (layer 2) device

Connect multiple network segments

One broadcast domain

No. of ports = collision domain (max 16 ports)

Software based

o

Switch: A Switch is a Data Link layer (layer 2) device

Some switches also work on Network Layer (layer 3)

Connect multiple network segments

One broadcast domain

Contain more Ethernet ports

Hardware based

WAN

Wide Area Network is a computer network that covers a broad area i.e. any

network whose communications links cross metropolitan, regional or nationalboundaries

Router

o Network layer device (layer 3)

o Works on IP

o Device that extracts a destination of packet, select the best path fordestination and forward packet to next device on selective path

o No broadcast domain

o Used to connect different networks


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o Principles of Router

Routing is always done on the network portion of destination IPaddress

Routing is always done on next hop basis

Packets from unknown destination are ignored by router

There are two types of routing.

o Static Routing

Static routing used fixed tables. It is also called default routingbecause there is only one destination network where we want tosend a message.

o Dynamic Routing

Router selects the best path for packet or data from routing tableand also depends upon metric based calculation and hop count.Routing tables hold the data for making forwarding decisions. In

order to route packets, a router communicates with other routersusing routing protocols and using this information creates andmaintains a routing table. The routing table stores the best routesto certain network destinations, the "routing metrics" associatedwith those routes, and the path to the next hop router

Switching protocol

Layer 2 protocol

o ARP (Address resolution protocol)

Conversion of IP to MAC address

o RARP (reverse Address resolution protocol)

Conversion of MAC to IP address

Metric

Criteria through which path selection is made

Administrative Distance

Assigned by CISCO

Varies between 0 ~ 255

Can be changed except directly connected network

Protocol

Administrative

DistanceRIP 120

ISIS 115

OSPF 110

IGRP 100

EIGRP 90

BGP 20

Static Route 1

Directly Connected 0

Routing protocol

Used for the selection of best path

Operates on routers only


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RIP, IGRP, EIGRP, OSPF, BGP

Distance Vector Routing Protocol

o Maintain routing table

o Update routing information after every 30 seconds

o Transmit routing table to neighboring node

o RIP, IGRP

Link State Routing Protocol

o Contain routing table, topology table and neighboring table

o Forward the routing table whenever change occur in network topology

o Fast convergence and better performance

o OSPF

RIP (Routing information protocol)

o Distance vector routing protocolo Metric is hop count

o Maximum hop count is 15

o Administrative distance is 120

IGRP (Interior Gateway Routing protocol)

o CISCO proprietary protocol and run on CISCO Routers

o Distance vector routing protocol

o Metric is Bandwidth, delay, reliability, load, MTU

o

By default is BW and delayo Administrative distance is 100

EIGRP (Extended Interior Gateway Routing protocol)

o CISCO proprietary protocol

o Hybrid protocol (contain functionality of both Link State and DistanceVector routing protocol)

o Metric is Bandwidth, delay

o Support IP, IPx, Apple Talk


OSPF (Open Shortest Path First)

o Defined by IETF (Internet Engineering Task Force)

o Metric is Cost based on BW (cost = 108 / BW)

o Link State routing protocol

o Deployed in hierarchical design

o Suitable for large network


o Decrease network overhead due to introduction of areas

BGP (Border Gateway Protocol)


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Routed protocol

Used to carry the data and reached to destination according to the best path.

Operate all over the network

IP, Apple Talk, IPx (internet packet exchange)

IP Addressing

Numeric identified assigned to each machine on a IP network

Network address uniquely identifies the network

Node address uniquely identifies each machine

Classes of IP Addressing

o Class A (1 126) used for data communication

o Class B (128 191) -do-

o Class C (192 223) -do-

o Class D (224 239) used for multi casting

o Class E (240 255) research and testing purposes

Class A

o N . H . H . H

0 xxx xxxx

o Sub net mast 255.0.0.0

o Private IP 10.0.0.0

o 27-2 (128-2=126) networks

0 000 0000 00 111 1111 127

o 224-2 (16,777,216-2= 16,777,214) hosts

Class B

o N . N . H . H

10 xx xxxx

o Sub net mast 255.255.0.0


o 214 (16,384) networks

10 00 0000 12810 11 1111 191

o 216-2 (65,536-2= 65,534) hosts

Class C

o N . N . N . H

110 x xxxx110 0 0000 192110 1 1111 223

o Sub net mast 255.255.255.0


o 221 (2,097,152) networks

o 28-2 (256-2= 254) hosts


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Subnetting

To create subnetworks, take bits from host portion to network i.e. subnet

Determine the required network ID

Determine the host required per subnet

One subnet mast for entire network

Range of host ID

Default subnet mask is used to determine the part of host address in IP address

TCP / IP Model

Layer 5 (Application Layer)

The application layer is used by most programs for network communication. Datais passed from the program in an application-specific format, and thenencapsulated into a transport layer protocol.

DNS (Domain Name System)

o Translation from Host Name to IP and vice versa.

DHCP (Dynamic Host Configuration Protocol)

o Client-Server protocol that automatically provide an IP host with its IPaddress and other related configuration information such as subnet maskand default gateway.

o Dynamic allocation of IP: Network administrator assigns a range of IPaddress to DHCP. During network initialization an IP is assigned to client.

o Automatic Allocation: Permanently assign an IP from the pool of IPaddresses to client.

o

Manual Allocation: Allocate IP on base of table of MAC address File Transfer Protocol

o Use to transfer data from one computer to another over the internet orthrough a network.

H.323

o Is an umbrella recommended by ITU-T defines the protocol to provideaudio-visual communication session on any packet network.

o Use for real time application.

o Commonly used in VoIP

o Developed for transporting multimedia applications.

RTP (Real-Time Transport Protocol)

o Defines standardized packet format for delivering audio and video overinternet.

o Port used 16384-32767

o Less sensitive for packet loss but sensitive for delay

o Uses UDP

o Header is normally of 12 bytes

o Header fields are:

Payload (7 bits): type of audio or video encoding


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Sequence No. (16 bits)

Time Stamp (32 bits): for sampling instance

SSRC (32 bits): Synchronous Source Identifier used foridentification of RTP source

RTCP (Real Time Control Protocol)o Multimedia application use with conjunction with RTP.

o RTCP Packets are sent periodically and contain sender and receiver reportsthat announce statistics.

o Used to increase QoS

o RTP/RTCP protocols are commonly used to transport audio or audio/videodata. Separate sessions are used for each media content (e.g. audio andvideo). The main advantage of this separation is to make it possible toreceive only one part of the transmission, commonly audio data, whichlowers the total bandwidth

RTSP (Real Time Streaming Protocol)

o RTSP is a protocol for use in streaming media systems which allows aclient to remotely control a streaming media server, issuing VCR-likecommands such as play and pause, and allowing time-based access tofiles on a server.

o Out of band protocol

o Use both TCP or UDP

o Does not define compression for audio and video

o Does not define how audio and video encapsulated

o Does not restrict how media player buffer audio/video SIP (Session Initiation Protocol)

o Used for creating, modifying and terminating session

o Port no is 5060

o SIP client uses UDP and TCP

o Commonly used in VoIP with H.323

o Provide signaling similar to HTTP (Human readable)


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Layer 4 (Transport Layer)

The transport layer's responsibilities include end-to-end message transfercapabilities independent of the underlying network, along with error control,fragmentation and flow control. End to end message transmission or connectingapplications at the transport layer can be categorized as either: Connection

oriented or Connectionless TCP (Transmission Control Protocol) o

Connection oriented, acknowledge

o Reliable

o 20 byte header contain:

Source port (16 bits): identifies the sending port

Destination port (16 bits): identifies the receiving port

Sequence number (32 bits): has a dual role

If the SYN flag is present then this is the initial sequencenumber and the first data byte is the sequence number plus

1

if the SYN flag is not present then the first data byte is thesequence number

Acknowledgment number (32 bits): if the ACK flag is set thenthe value of this field is the sequence number that the sender ofthe acknowledgment expects next.

Data offset (4 bits): specifies the size of the TCP header in 32-bitwords. The minimum size header is 5 words and the maximum is15 words thus giving the minimum size of 20 bytes and maximumof 60 bytes. This field gets its name from the fact that it is also theoffset from the start of the TCP packet to the data.

Reserved (4 bits): for future use and should be set to zero

Flags (aka Control bits) contains 8 bit flags

CWR Congestion Window Reduced (CWR) flag is set by thesending host to indicate that it received a TCP segment withthe ECE flag set (added to header by RFC 3168).

ECE (ECN-Echo) indicate that the TCP peer is ECN capableduring 3-way handshake (added to header by RFC 3168).


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URG indicates that the URGent pointer field is significant

ACK indicates that the ACKnowledgment field is significant

PSH Push function

RST Reset the connection

SYN Synchronize sequence numbers

FIN No more data from sender

Window (16 bits): the number of bytes that may be received onthe receiving side before being halted from sliding any further andreceiving any more bytes as a result of a packet at the beginning ofthe sliding window not having been acknowledged or received.Starts at acknowledgement field.

UDP (User Datagram Protocol)

o Connection less, unacknowledged

o Not Reliable

o No hand shaking between Tx and Rx

o Small packet overhead

o DNS, RIP and SNMP use UDP

o Used with multimedia, voice application

o Depend upper layer protocol for reliability

o Source port

This field identifies the sending port when meaningful and shouldbe assumed to be the port to reply to if needed. If not used, then itshould be zero.

o Destination port

This field identifies the destination port and is required.

o Length

A 16-bit field that specifies the length in bytes of the entiredatagram: header and data. The minimum length is 8 bytes sincethat's the length of the header. The field size sets a theoretical limitof 65,527 bytes for the data carried by a single UDP datagram. Thepractical limit for the data length which is imposed by theunderlying IPv4 protocol is 65,507 bytes.


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ICMP (Internet Control Message Protocol)

o Ping

o Error reporting

o Used by host and routers to communicate network layer information with

otherLayer 3 (Internet Layer)

Internet Protocol

o Unreliable, connection less

o Provide no error checking or tracking o

Packet in IP layer is called Datagram

o Length is variable (up to 65536 bytes)

o Header is of 20 to 60 byte

o Header contain:

version (IPv4 or IPv6) 4 bits

Internet Header Length (IHL): The second field is a 4-bit InternetHeader Length (IHL) telling the number of 32-bit words in theheader. Since an IPv4 header may contain a variable number ofoptions, this field specifies the size of the header (this alsocoincides with the offset to the data). The minimum value for thisfield is 5 (rfc791), which is a length of 532 = 160 bits. Being a 4-bit field the maximum length is 15 words or 480 bits.

Type of Service (TOS)

Total Length: This 16-bit field defines the entire datagram size,including header and data, in bytes. The minimum-length datagramis 20 bytes (20 bytes header + 0 bytes data) and the maximum is

65,535 the maximum value of a 16-bit word. The minimum sizedatagram that any host is required to be able to handle is 576bytes, but most modern hosts handle much larger packets.Sometimes subnetworks impose further restrictions on the size, inwhich case datagrams must be fragmented. Fragmentation ishandled in either the host or packet switch in IPv4 (seeFragmentation and reassembly).

Identification: This field is an identification field and is primarilyused for uniquely identifying fragments of an original IP datagram.

Some experimental work has suggested using the ID field for otherpurposes, such as for adding packet-tracing information to


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datagrams in order to help trace back datagrams with spoofedsource addresses

Flags: A 3-bit field follows and is used to control or identifyfragments. They are (in order, from high order to low order):

Reserved; must be zero.

Don't Fragment (DF)

More Fragments (MF)

If the DF flag is set and fragmentation is required to routethe packet then the packet will be dropped. This can beused when sending packets to a host that does not havesufficient resources to handle fragmentation.

When a packet is fragmented all fragments have the MF flagset except the last fragment, which does not have the MFflag set. The MF flag is also not set on packets that are notfragmented clearly an unfragmented packet can be

considered the last fragment.

Fragment Offset: The fragment offset field, measured in units of 8-byte blocks, is 13-bits long and specifies the offset of a particularfragment relative to the beginning of the original unfragmented IPdatagram. The first fragment has an offset of 0. This allows amaximum offset of 65,528 () which would exceed the maximum IPpacket length of 65,535 with the header length included.

Time To Live (TTL): An 8-bit time to live (TTL) field helps preventdatagrams from persisting (e.g. going in circles) on an internetwork.Historically the TTL field limited a datagram's lifetime in seconds,but has come to be a hop count field. Each packet switch (or router)

that a datagram crosses decrements the TTL field by one. When theTTL field hits zero, the packet is no longer forwarded by a packetswitch and is discarded. Typically, an ICMP message (specifically thetime exceeded) is sent back to the sender that it has beendiscarded. The reception of these ICMP messages

is at the heart of how traceroute works.

Layer 2 & 1 (Data Link / Physical Layer)

Cell Relay: fix length packets called Cell to be transported between nodes.

ATM is Cell Relay

No error correction or flow control

ATM (Asynchronous Transfer Mode) High speed connection oriented and multiplexing technology for transmitting

information across a wide area network

53 bytes of cell (48 byte data, 5 byte header)

Different from IP or Ethernet where variable size packet is used.

Mostly used in WAN

ADSL implements ATM

Eliminates varying delay time associated with different packet size

Can use permanent virtual circuit or switched virtual circuit

ATM standard defines three layers


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o AAL (Application Adaptation Layer): accept transmission from upper

layer services and mapped them into ATM cell

AAL is divided into two sublayer:

Convergence sublayer (CS):

o add overhead and manipulates the data stream atthe sending station

o Perform the opposite function at receiving end

Segmentation and reassembling (SAR):

o At sending station, segment the bit stream in samesized packet, add header and trailer

o Perform the opposite function at receiving end

Four different AAL

o AAL1: constant bit rate stream

o AAL2: variable bit rate stream

o AAL3/4: conventional packet switching

o AAL5: packet requiring no information from SARLayer

o ATM Layer: provide routing traffic management, switching andmultiplexing services

o Physical Layer: define the transmission medium bit transmissionencoding to electrical to optical transformation

FR (Frame Relay)

Cost effective technology that connects LAN

Used as encapsulation technique for voice and data by network service providersbetween LAN and WAN

Can handle bursty data

Eliminate extensive error checking

PVC (permanent virtual circuit) and SVC (switched virtual circuit) connections areused

DLCI (data link connection identifier) identifies VC in FR

Operate at layer 1 & 2

At layer 2, use a simplified version of HDLC (High level data link control)

Routing and switching are function of data link layer

Flow control is handled through BECN (backward explicit congestion notification)

or FECN (forward explicit congestion notification)

56 kbps, 64 kbps, 128 kbps, 256 kbps, 512 kbps and 1.5 Mbps


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Global System for Mobile CommunicationGSM Band

GSM 900

o Uplink 890-915 MHz

o Downlink 935-960 MHz

o Sub band 25 MHz with 200kHz guard band

o No. of carriers 125

GSM 1800 (DCS-Digital Cellular System)

o Uplink 1710-1785 MHz

o Downlink 1805-1880 MHz

o Sub band 75 MHz with 200kHz guard band

o No. of carriers 375

ARFCN (Absolute Radio Frequency Channel Number)

It is a number scheme used to identify RF channels in GSM radio system

BW (Bandwidth)

Information carrying capacity of a communication channel

Express in Hz for analog and bps for digital (data rate)

BER (bit error rate)

Number of bits corrupted bits / total number of bits (for digital system)

Cell

The geographic area encompassing the signal range from one base station (a sitecontaining radio transmitter/receiver and network communication equipment).Wireless transmission networks are comprised of many hexagonal, overlappingcell sites to efficiently use radio spectrum for wireless transmissions. Also, thebasis for the term "cellular phone."

Cluster

A group of cell with no frequency reuse in adjacent cell

Sector

Physical coverage area associated with base station having its own antenna, radioports and control channel.

Sec t oriz at ion


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Co channel interference

Unwanted interference within a radio channel from another transmitter using thesame channel at a different location. It is a dominant factor in frequency reusesystem.

Cell Splitting

Process of splitting cell into smaller cell.

Usually done to accommodate more traffic

Spectral Efficiency

Traffic in earlang per sq. km per MHz

GSM Architecture

MS

Physical equipment use by subscriber. Normally handheld wireless phone contain

SIM that carry all information of subscriber.

SIM The SIM carriers the following information:

o Authentication Key (Ki)

o Subscriber information

o Cipher key (Kc)

o Additional GSM Services

o IMSI: The IMSI is assigned to an MS at subscription time. It uniquelyidentifies a given MS. The IMSI will be transmitted over the radio interfaceonly if necessary. The IMSI contains 15 digits and includes

Mobile Country Code (MCC)-3 digits (home country) Mobile Network Code (MNC)-2 digits (home GSM PLMN)

Mobile Subscriber Identification (MSIN)

Nation Mobile Subscriber Identity (NMSI)

o TMSI: Temporary Mobile Subscriber identity is assigned to an MS by theVLR. The TMSI uniquely identifies an MS within the area controlled by agiven VLR. The maximum number of bits that can be used for TMSI is 32.


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OMC

G

VLROther MSCs

VLR

D

BSS

BAbis

Um A C

BM

BT S

SSS C

MSC HLR AUC

E F

Other MSC Other

NetworkEIR

BSS (Base Station Subsystem)

The Base Station Subsystem is composed of two parts:BTS (Base Transceiver Station) & BSC (Base Station Controller)

These communicate across the standardized Abis interface

BTS (Base Transceiver Station)

o The Base Transceiver Station houses the radio transceivers that define acell and handles the radio-link protocols with the Mobile Station.

o This is the radio gear that passes all calls coming in and going out of a cellsite.

o The base station is under direction of a base station controller

o BTS contain TRAU (Transcoder Rate Adaptation Unit)

o TRAU is used for speech encoding, decoding and allow lower data ratesover Abis Channel.

o TRAU can be located in MSC for more compressed transmission

BSC (Base Station Controller)

o The Base Station Controller manages the radio resources for one or more

BTSs.

o It handles radio-channel setup, frequency hopping, and handovers.

o The BSC is the connection between the mobile station and the Mobileservice Switching Center (MSC).

o One BSC may handle multiple BTSs (more than 40)

NSS (Network Switching Subsystem)

The Network Switching Subsystem, also referred to as the GSM core network

NSS carries out switching functions and manages the communications between

mobile phones and the PSTN.


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It is owned and deployed by mobile phone operators and allows mobile phones tocommunicate with each other and telephones in the wider telecommunicationsnetwork.

The architecture closely resembles a telephone exchange, but there are additionalfunctions which are needed because the phones are not fixed in one location.

MSC (Mobile Switching Center)

o MSC is a sophisticated telephone exchange which provides circuit-switchedcalling, mobility management, and GSM services to the mobile phonesroaming within the area that it serves. This means voice, data and faxservices, as well as SMS and call divert.

o MSC is basically is an ISDN switch coordinating and setting up calls to andfrom the mobile station.

GMSC (Gateway MSC)

o GMSC is the MSC that determines which visited MSC the subscriber who isbeing called is currently located.

o It also interfaces with the PSTN. All mobile to mobile calls and PSTN tomobile calls are routed through a GMSC.

HLR (Home Location Register)

o HLR is a central database that contains details of each mobile phonesubscriber that is authorized to use the GSM core network.

o There is one logical HLR per PLMN.

o The HLR stores details of every SIM card issued by the mobile phoneoperator.

o The HLR stores the IMSI, MS ISDN number, VLR address, and subscriber

data on supplementary services.

AUC (Authentication Center)

o It is a protected database that contain security information of subscriber

o Usually interacts with the HLR

VLR (Visitor Location Register)

o The Visitor Location Register or VLR is a temporary database of thesubscribers who have roamed into the particular area which it serves.

o Each Base Station in the network is served by exactly one VLR; hence asubscriber cannot be present in more than one VLR at a time.

o The data stored in the VLR has either been received from the HLR, orcollected from the MS.

o The VLR stores the MSRN, TMSI, the location area, data on supplementaryservices, IMSI, MS ISDN number, HLR address or GT, and local MSidentity, if used.

EIR (Equipment Identity Register)

o EIR is a list of valid mobile equipment on the network.

o IMEI (international mobile equipment identity) used to interrogate EIR.


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GSM Interfaces

Um Radio Interfaces (MS to BTS)

o This interface uses LAPDm protocol for signaling, to conduct call control,measurement reporting, Handover, Power control, Authentication,Authorization, Location Update and so on.

o Data rate is 13 kbps (RPE-LP)

Abis Interface (BTS to BSC)

o Generally carried by a DS1, ES1 or E1.

o Data rate 64kbps for user data or speech and 16 kbps for signaling

A interface (BSC to MSC)

o It is used for carrying Traffic channels.

o Data rate 2 Mbps or more

Security Algorithm

Authentication (A3)

Encryption (A5)

Privacy key generation (A8)

Ki is the 128-bit Individual Subscriber Authentication Key utilized as a secret keyshared between the Mobile Station and the Home Location Register of thesubscriber's home network.

RAND is 128-bit random challenge generated by the Home Location Register.

SRES is the 32-bit Signed Response generated by the Mobile Station and the

Mobile Services Switching Center.

Kc is the 64-bit ciphering key used as a Session Key for encryption of the over-the-air channel. Kc is generated by the Mobile Station from the random challengepresented by the GSM network and the Ki from the SIM utilizing the A8 algorithm

Authentication and Key Generation Mechanism

The Mobile Station (MS) signs into the network.

The Mobile Services Switching Center (MSC) requests 5 triples from the Home

Location Register (HLR).

The Home Location Register creates five triples utilizing the A8 algorithm. Thesefive triples each contain:

o A 128-bit random challenge (RAND)

o A 32-bit matching Signed Response (SRES)

o A 64-bit ciphering key used as a Session Key (Kc)

The Home Location Register sends the Mobile Services Switching Center the fivetriples.

The Mobile Services Switching Center sends the random challenge from the firsttriple to the Base Transceiver Station (BTS).

The Base Transceiver Station sends the random challenge from the first triple tothe Mobile Station.


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The Mobile Station receives the random challenge from the Base TransceiverStation and encrypts it with the Individual Subscriber Authentication Key (Ki)assigned to the Mobile Station utilizing the A3 algorithm.

The Mobile Station sends the Signed Response to the Base Transceiver Station.

The Base Transceiver Station sends the Signed Response to the Mobile Services

Switching Center.

The Mobile Services Switching Center verifies the Signed Response.

The Mobile Station generates a Session Key (Kc) utilizing the A8 algorithm, theIndividual Subscriber Authentication Key (Ki) assigned to the Mobile Station, andthe random challenge received from the Base Transceiver Station.

The Mobile Station sends the Session Key (Kc) to the Base Transceiver Station.

The Mobile Services Switching Center sends the Session Key (Kc) to the Base

Transceiver Station.

The Base Transceiver Station receives the Session Key (Kc) from the Mobile

Services Switching Center.

The Base Transceiver Station receives the Session Key (Kc) from the Mobile

Station.

The Base Transceiver Station verifies the Session Keys from the Mobile Stationand the Mobile Services switching Center.

The A5 algorithm is initialized with the Session Key (Kc) and the number of theframe to be encrypted.

Over-the-air communication channel between the Mobile Station and BaseTransceiver Station can now be encrypted utilizing the A5 algorithm.

GSM Services

Bearer services

o These services give the subscriber the capacity required to transmitappropriate signals between certain access points (i.e. user-networkinterface).

o SMS, UMS (Unified message services), Group 3 Fax, Voice mail box, Email

Teleservices

o These services provide the subscriber with necessary capabilities includingterminal equipment functions to communicate with other subscribers.

o Mobile telephony, Emergency calling

Supplementary services

o These services modify or supplement basic telecommunications servicesand are offered together or in association with basic telecommunicationsservices.

o Call waiting, call hold, call forwarding

Speech Coding

Speech coding is a process of reducing bit rate of digital speech for transmission

and storage while maintaining speech quality. Speech coding method are:


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o Wave form coding (low capacity, low delay)

o Source coding (vocoding)

o Hybrid coding (high capacity, high delay)

Speech codec attributes:

o Delay, Complexity, Bandwidth, Quality, Transmission bit rate

PCM (Pulse Code Modulation) 64 kbps

ADPCM (Adaptive Differential PCM) 32 kbps

CELP (Code Excited Linear Prediction): 16 kbps

RPE-LP (Regular Pulse Excited Linear Prediction): 13 kbps

VSELP (Vector Sum Excited Linear Prediction): 8 6.7 kbps

Quadruple-mode VSELP: 5.6 kbps

PSI (Pitch Synchronous Innovation) 3.45 kbps

Channel Coding

Used to reduces BER of channel

Convolutional coding is used as channel coding in GSM


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Channels


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Control Channel Channel Type Usage

Continually broadcasts, on the

Broadcast ControlChannel (BCCH)

Frequency CorrectionChannel (FCCH)

Synchronization Channel(SCH)

Random Access Channel(RACH)

Paging Channel (PCH)

Access Grant Channel

Broadcast downlink(Base station to

mobile)

Broadcast downlink

Common uplink

(Mobile to basestation)

Common downlink(Base station to

mobile)

downlink, information includingbase station identity, frequencyallocations, and frequency-

hopping sequences.Used to synchronies the mobileto the time slot structure of acell by defining the boundariesof burst periods, and the timeslot numbering. Every cell in aGSM network broadcasts exactlyone FCCH and one SCH, whichare by definition on time slotnumber 0 (within a TDMAframe).

Slotted Aloha channel used by

the mobile to request access tothe network.

Used to alert the mobile stationof an incoming call.

Used to allocate an SDCCH to amobile for signaling (in order to

(AGCH)Broadcast downlink

Slow Associated Control

Channel (SACCH)Uplink and downlink

Fast Associated Control

Channel (FACCH)Uplink and downlink

obtain a dedicated channel),

following a request on theRACH.

In every traffic channel. Usedfor low rate, non criticalsignaling.

A high rate signaling channel,used during call establishment,subscriber authentication, andfor handover commands

Handover

The passing of call signal from one BS to other as user move out of the range

Handoffs between time slots or frequencies in the same cell (intra-BTS);

Handoffs from BTS to BTS under control of the same BSC (inter-BTS);

BTS under one BSC to BTS of another BSC, but all under the same MSC (inter-BSC);

BTS in one network to BTS of another network, under different MSCs (inter-MSC)


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Which modulation scheme is used in GSM?

GMSK (Guassian Minimum Shift Keying)

What is AMR?

AMR is Adaptive Multi Rate Codec

AMR is a codec that can change the encoding technique according to the SNR

If SNR is better, less bits should be used for error control

If SNR is not good, error control bits may be increased

AMR is used to reduced interference

What is Equalization?

Measure taken to reduce distortion effects in a radio channel.

What is interleaving?

The process of spreading of block of data over a wider time frame by placing bitsfrom other data block in between the original data bits in original data block.

Interleaving avoid bursty fading and interference phenomenon.

What is Frequency Re-use?

A technique of reusing frequencies and channels within a communications systemto improve capacity and spectral efficiency. Frequency reuse generally utilizesregular reuse patterns.

What is DTX?

Discontinuous Transmission

Means that the transmitter is powered on only when the user is actively talkingand power should be off when is there is no burst transmitted.

What is SHF?

Slow frequency hoping

In SHF, operating frequency is changed in every TDMA frame

The mobile transmitting at one frequency during the time slot, hop to differentfrequency before the next time slot.

Used to improve signal quality in GSM

What is APC?

Adaptive Power Control

The purpose of APC is to adjust the radio transmitter and adapt to the need of anactual radio link between the BTS and MS

If MS is very near to BTS, MS should transmit less power then the MS which is atthe boundary of Cell.


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Communication FundamentalSignal Encoding Scheme

Bit Rate

Number of bits per second

Bit rate = sampling rate x number of bits / sample

Baud Rate

Number of symbol per second

Baud rate = bit rate / number of bits per sample

Digital Data, Digital Signals

NRZ

Multilevel Binary

Biphase

Scrambling Scheme

Digital Data, Analog Signals

ASK (Amplitude Shift Keying)

PSK (Phase Shift Keying)

FSK (Frequency Shift Keying)

QAM (Quadrature Amplitude Modulation)

o Logical extension of QPSK

o Combination of ASK and PSKo Two signals can be transmitted with phase quadrature

Analog Data, Digital Signals

Steps involved for converting A/D

o Sampling

o Quantization

o Encoding

PCM (Pulse Code Modulation)

o It is a digital representation of analog signal where the magnitude of signalis sampled regular intervals (twice the BW) then quantized a series tosymbol in binary code

o [Step size] = 2mp/ L

L = Number of Levels

o L = 2R

R = Number of bits used to encode one sample

o For PCM; L = 256 (R = 8 L = 28 = 256)

o Data rate = 64 kbps

o Aliasing


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WiMAXWorldwide Interoperability for Microwave Access

Describes WiMAX as "a standards-based technology enabling the delivery of lastmile wireless broadband access as an alternative to cable and DSL."

Wireless is point to point and WiMAX is point to multipoint

Range 30 miles Radius from the Base Station for LOS

Range 4 6 miles Radius from the Base Station for NLOS

Maximum data speed supported is 70 Mbps

Line of Sight is not needed between user and the base station unless very highdata rates are required at the user premises.

Licensed Frequency band: 2 11 GHz

Unlicensed Frequency band: 10 to 66 GHz

Uses Cyclic Prefix (CP) to eliminate ISI

Two main types of spectrum allocation:

o Licensed: Licensed frequencies are typically awarded through an auctionor beauty contest

o Unlicensed: Unlicensed frequencies allow multiple service providers toutilize the same section of the spectrum

802.16 Wireless MAN (WiMAX ) [Unlicensed, Licensed]

802.11 Wireless LAN WiFi (Wireless Fidelity) [unlicensed 2.4GHz, 5GHz]

802.15 Wireless PAN Bluetooth

WiFi (2.4GHz and 5GHz) are the free of cost frequencies i.e. Unlicensed

WiMAX (2GHz to 6GHz) contain both licensed and unlicensed frequencies

Aims and Objectives of WiMAX 802.16

o Superior Performance: To achieve superior throughputs in delivering IPdata services compared to other 3G technologies

o Flexibility: Allow operators to offer both fixed and mobile access inmultiple spectrum bands

o Attractive Economics: Eliminating the need for extensive and expensivewire line infrastructure and providing highly flexible and cost-effective last-

mile solutionso QoS: Defines Service Flows which can map to DiffServ code points or

MPLS flow labels that enable end-to-end IP based QoS.

Multipoint Broadband Wireless Access (BWA) systems are composed of two keyelements

o Base station and

o Subscriber equipment

CPE: customer premises equipment


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802.16a 802.16REVd 802.16e Fixed

Outdoor Fixed Outdoor Mobility

Applications Applications Applications

E1/T1 service for enterprises Indoor Broadband access for "Portable" Broadband accessBackhaul for Hotspots residential users (High Speed for consumersLimited residential Internet, VoIP,) Always Best ConnectedBroadband access

CPE CPE CPE

External box connected to PC External box connected to PC PC Cardwith outside antenna with built-in antenna


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Network Architecture: Access solution comprises of

o CPE (talks to the Access point )

o Access Point (a kind of BTS for CPE) also called DAP (diversity accesspoint) which combats fading.

o Microwave Backhaul Key Differences between 802.16d & 802.16e

802.16d 802.16e

Spectrum < 11GHz


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o BPSK is the least data efficient Method and it employed where the SS is

farthest from BASE Station

o 64QAM offers high data efficiency, requiring less Tx power.

WiMAX is broadband but nature is baseband

How to transmit a signal in wireless

o 3 techniques to avoid interference

FHSS (Frequency Hopping (jumping) Spread Spectrum)

o Channel jumping is more.

o In addition to your knowledge GSM uses slow frequency

hopping to mitigate the multipath fading and this slow

frequency hopping comes in BCH (Broadcast channel).

DSSS (Direct Sequence Spread Spectrum)

o extra chip-pin code is used

o If 1bit=10 bit chipping code

OFDM (Orthogonal Frequency Division Multiplexing)

o 1 channel is divided into 256 sub-channels and every carrier has

to modulate. OFDM in fact the technique that made WiMAX

broadband.

Range 30 miles Radius from the Base Station for LOS

Range 4 6 miles Radius from the Base Station for NLOS

Maximum data speed supported is 70 Mbps

Line of Sight is not needed between user and the base station unlessvery high data rates are required at the user premises.

Licensed Frequency band: 2 11 GHz

Unlicensed Frequency band: 10 to 66 GHz


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Multiplexing

o Sharing of medium by number of users without overlapping

Data Rates E1 = 32 x 64 kbps = 2.04 Mbps

E3 = 16 x T1

****************************

T1 = 24 x 64 kbps = 1.544 Mbps

T2 = 4 T1

T3 = 28 T1

T4 = 168 T1

****************************

DS0 = 64 kbps DS1 = 24 x DS0

DS3 = 28 DS1

****************************

OC-3 = 155.52 Mbps

OC-12 = 622 Mbps

OC-48 = 2.448 Gbps

Introduction to GSM Basic Concept

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