Application Layer
The protocols at the application layer handle file transfer, virtual terminals, network management, and fulfilling networking requests of applications. A few of the protocols
that work at this layer include:
• File Transfer Protocol (FTP)
• Trivial File Transfer Protocol (TFTP)
• Simple Network Management Protocol (SNMP)
• Simple Mail Transfer Protocol (SMTP)
• Telnet
• Hypertext Transfer Protocol (HTTP)
Presentation
The services of the presentation layer handle translation into standard formats, data compression and decompression, and data encryption and decryption. No protocols work at this layer, just services. The following lists some of the presentation layer standards:
• American Standard Code for Information Interchange (ASCII)
• Extended Binary-Coded Decimal Interchange Mode (EBCDIC)
• Tagged Image File Format (TIFF)
• Joint Photographic Experts Group (JPEG)
• Motion Picture Experts Group (MPEG)
• Musical Instrument Digital Interface (MIDI)
Session
The session layer protocols set up connections between applications, maintain dialog control, and negotiate, establish, maintain, and tear down the communication channel.
Some of the protocols that work at this layer include:
• Network File System (NFS)
• NetBIOS
• Structured Query Language (SQL)
• Remote procedure call (RPC)
Transport
The protocols at the transport layer handle end-to-end transmission and segmentation into a data stream. The following protocols work at this layer:
• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)
• Secure Sockets Layer (SSL)/Transport Layer Security (TLS)
• Sequenced Packet Exchange (SPX)
Network
The responsibilities of the network layer protocols include internetworking service, addressing, and routing. The following lists some of the protocols that work at this layer:
• Internet Protocol (IP)
• Internet Control Message Protocol (ICMP)
• Internet Group Management Protocol (IGMP)
• Routing Information Protocol (RIP)
• Open Shortest Path First (OSPF)
• Novel Internetwork Packet Exchange (IPX)
Data Link
The protocols at the data link layer convert data into LAN or WAN frames for transmission, convert messages into bits, and define how a computer accesses a network. This layer is divided into the Logical Link Control (LLC) and the Media Access Control (MAC) sublayers. Some protocols that work at this layer include the following:
• Address Resolution Protocol (ARP)
• Reverse Address Resolution Protocol (RARP)
• Point-to-Point Protocol (PPP)
• Serial Line Internet Protocol (SLIP)
Physical
Network interface cards and drivers convert bits into electrical signals and control the physical aspects of data transmission, including optical, electrical, and mechanical requirements.
The following are some of the standard interfaces at this layer:
• High-Speed Serial Interface (HSSI)
• X.21
• EIA/TIA-232 and EIA/TIA-449
Showing posts with label telecommunication. Show all posts
Showing posts with label telecommunication. Show all posts
Wednesday, August 24, 2011
Sunday, July 10, 2011
Binary data modulation with coding
When we design a communication system, we need to consider transmitter and receiver structures, probability of error, bandwidth occupancy of the modulated signal and bandwidth efficiency. Communication performance is critical factor in achieving error free transmission.
Let’s discuss the main blocks in a communication system and the way to organize them to achieve higher performance. Data to be sent is generated in the data source and fed in to the channel encoder. The purpose of the channel encoder is introducing redundancy bits to combat the effects of noise and interference over the channel. Channel coding is referred as a signal transformation designed to improve communication performance. Convolution coding is a type of channel coding. The importance characteristic of convolution coding is that coder has memory. K is a parameter called constraint length in the convolution coder. The output n-tuple emitted by coder is not only a function of input k-tuple, but also it is a function of the previous K-1 input k-tuple.
The output of the channel encoder fed in to the digital modulator. The primary purpose of the digital modulator is to map the binary information sequence into signals suitable for transmission over the channel. We have to test Coherent phase shift keying (PSK) and Coherent frequency shift keying (FSK) modulation techniques in our communication system. Coherent receiver means receiver has a phase recovery circuitry. Receiver knows both the frequency and the phase of the carrier signal used in the transmission. PSK means signals carries information in the phase, and FSK means signal carries information in the frequency.
Modulated signal is transmitted via communication channel. Communication channel is the physical channel we use to transmit data for transmitter to the receiver. The essential feature in this physical medium is the transmitted signals are corrupted in a random manner due to various mechanisms. Here we have used Additive White Gaussian Noise channel for our simulations. Modulated signal transmitted over the channel and then converted back to sequence of binary data in the demodulator. Chanel decoder attempts to decode the channel encoded sequence. As the channel decoder in the receiver we can use maximum likelihood decoding and Viterbi decoding. We used Viterbi decoding for our simulations. The signal at the channel decoder in our scenario is the approximation of the original data.
We need to consider probability of error, bandwidth occupancy of the modulated signal and bandwidth efficiency of the communication system in designing. With coding we can achieve lower probability of error without increasing the signal to noise ratio. By introducing channel coding to the modulation, we raised the performance of the communication system. But when we used coding we need higher bandwidth for data transmission. Bandwidth the price we have to pay for the higher performance with channel coding.
We can achieve lower bit error probability in using BPSK modulation over BFSK modulation. BPSK showed higher performance than BFSK for same signal to noise ratio.With coding we can increase the performance of the communication system. When we use channel coding we can achieve the same probability of error level that we have in modulation with lower signal to noise ratio. We saw that code rate ½ perform well over code rate 1/3 at lower probability of error levels. But there is a price that we have to pay when we use channel coding in communication systems. When we use channel coding we need higher bandwidth, because we transmit more bits in the same bit duration. So higher code rates will result higher spectral occupancy. Normally, BPSK has less spectral occupancy than BFSK.
BPSK is the best modulation technique for binary data transmission. BPSK with channel coding raised the performance level of the communication system, resulting higher utilization of the bandwidth.
Monday, February 4, 2008
SMS messages
An SMS center (SMSC) is responsible for handling the SMS operations of a wireless network. When an SMS message is sent from a mobile phone, it will reach an SMS center first. The SMS center then forwards the SMS message towards the destination.If the recipient is unavailable (for example, when the mobile phone is switched off), the SMSC will store the SMS message. It will forward the SMS message when the recipient is available.It is possible to specify the period after which the SMS message will be deleted from the SMS center so that the SMS message will not be forwarded to the recipient mobile phone when it becomes online. This period is called the validity period.
We must sepecify the address of the wireless network operator's SMSC to use SMS messaging with our mobile phone. Typically an SMSC address is an ordinary phone number in the international format.Normally, the SMSC address is pre-set in the SIM card by the wireless network operator.In Sri lanka for Dialog +9477000003 and for mobitel +9471000003.
If we want to know whether an SMS message has reached the recipient mobile phone successfully or not.We must set a flag in the SMS message to notify the SMS center that we want a status report about the delivery of this SMS message. The status report is sent to us in the form of an SMS message(error or failure e.g. incorrect SMS message format, busy SMS center, etc).
If the mobile phone does not receive the message submission report after a period of time, it concludes that the message submission report has been lost. The mobile phone may then re-send the SMS message to the SMS center. A flag will be set in the new SMS message to inform the SMS center that this SMS message has been sent before. If the previous message submission was successful, the SMS center will ignore the new SMS message but send back a message submission report to the mobile phone. This mechanism prevents the sending of the same SMS message to the recipient multiple times.
After receiving an SMS message, the recipient mobile phone will send back a message delivery report to the SMS center to inform whether there is any error or failure (example causes: unsupported SMS message format, not enough storage space, etc). This process is transparent to the mobile user. If there is no error or failure, the recipient mobile phone sends back a positive delivery report to the SMS center. Otherwise it sends back a negative delivery report to the SMS center.
If the sender requested a status report earlier, the SMS center sends a status report to the sender when it receives the message delivery report from the recipient.If the SMS center does not receive the message delivery report after a period of time, it concludes that the message delivery report has been lost. The SMS center then re-sends the SMS message to the recipient.
Transmission Process of Intra-operator SMS Messages
Transmission Process of Inter-operator SMS Messages
When the originator SMS center receives an inter-operator SMS message, it gets the routing information from the recipient wireless network and delivers the SMS message to the recipient mobile phone directly.This can be used if the two wireless networks involved in the transmission of the inter-operator SMS message are based on similar technologies.
when an SMS message is sent from a GSM network to a CDMA network(if two networks are different).The originator SMS center and the recipient SMS center are interconnected through an SMS gateway or with a communication protocol that is supported by both SMS centers.The recipient SMS center will be responsible for sending the SMS message to the recipient mobile phone and storing the SMS message if the recipient mobile phone is offline.
We must sepecify the address of the wireless network operator's SMSC to use SMS messaging with our mobile phone. Typically an SMSC address is an ordinary phone number in the international format.Normally, the SMSC address is pre-set in the SIM card by the wireless network operator.In Sri lanka for Dialog +9477000003 and for mobitel +9471000003.
If we want to know whether an SMS message has reached the recipient mobile phone successfully or not.We must set a flag in the SMS message to notify the SMS center that we want a status report about the delivery of this SMS message. The status report is sent to us in the form of an SMS message(error or failure e.g. incorrect SMS message format, busy SMS center, etc).
If the mobile phone does not receive the message submission report after a period of time, it concludes that the message submission report has been lost. The mobile phone may then re-send the SMS message to the SMS center. A flag will be set in the new SMS message to inform the SMS center that this SMS message has been sent before. If the previous message submission was successful, the SMS center will ignore the new SMS message but send back a message submission report to the mobile phone. This mechanism prevents the sending of the same SMS message to the recipient multiple times.
After receiving an SMS message, the recipient mobile phone will send back a message delivery report to the SMS center to inform whether there is any error or failure (example causes: unsupported SMS message format, not enough storage space, etc). This process is transparent to the mobile user. If there is no error or failure, the recipient mobile phone sends back a positive delivery report to the SMS center. Otherwise it sends back a negative delivery report to the SMS center.
If the sender requested a status report earlier, the SMS center sends a status report to the sender when it receives the message delivery report from the recipient.If the SMS center does not receive the message delivery report after a period of time, it concludes that the message delivery report has been lost. The SMS center then re-sends the SMS message to the recipient.
Transmission Process of Intra-operator SMS Messages
Transmission Process of Inter-operator SMS Messages
When the originator SMS center receives an inter-operator SMS message, it gets the routing information from the recipient wireless network and delivers the SMS message to the recipient mobile phone directly.This can be used if the two wireless networks involved in the transmission of the inter-operator SMS message are based on similar technologies.
when an SMS message is sent from a GSM network to a CDMA network(if two networks are different).The originator SMS center and the recipient SMS center are interconnected through an SMS gateway or with a communication protocol that is supported by both SMS centers.The recipient SMS center will be responsible for sending the SMS message to the recipient mobile phone and storing the SMS message if the recipient mobile phone is offline.
Wednesday, January 9, 2008
Global positioning system(GPS)
Mobile that we using can be used to locate all the people in the case of emergency.that's why GPS(global positioning receiver) located chip was inserted in new phone.Newer phones can be used to track people in 30ft range.cell phone essentially radio.When cell phone is on it sends radio signals to it's cell tower to know where the phone is,this will help a person to move one cell to another cell with out loosing a call.At the same time GPS system receives signal from outer space specifically at least 3 geosynchronous satellites.Using geometric method called tridimentional trilateration,the GPS chip using the satellite signal to calculate its exact location on earth.
If we put a sniffer on a node we can track someone.Node is something which takes all calls coming to a cell tower and release those signals to where they are going.but sniffer is a electronic device that detect specific calls then send that signal to a phone that we are using to tracking.
Global positioning system(GPS)
GPS is a satellite based navigation system.GPS works anywhere in the world under any weather conditions.GPS constellation consist of 24 satellites in 6 near circular orbits.An on board GPS receiver must locked in to at least 3 satellite to calculate 2D position or the movement,if 4 satellites or more receiver can determine the 3D position.Once the user position has been determined GPS unit will start to calculate other information such as speed,track,trip distance,time.These parameters are continually updated by a master control station,which transmits them up to the satellites where they are broadcast as part of a navigational message from each satellite.Civilian GPS uses L1 frequency of 1.5754 GHz in the UHF band(signal travels by line of sight,it can pass clouds,glass and plastic but fail to pass through solid objects).Defense GPS uses L2 frequency of 1.2276 GHz in the UHF band.In the GPS system the modulation used is direct sequence spread spectrum and it is one way transmission system(from satellite to user only,user does not require a transmitter).Modulation method makes GPS system jam resistant,secure and addressable.The in phase component is being C/A (coarse acquisition) code while quadrature component is P-code(precision code).
GPS signal contains 3 different bits of information:
If we put a sniffer on a node we can track someone.Node is something which takes all calls coming to a cell tower and release those signals to where they are going.but sniffer is a electronic device that detect specific calls then send that signal to a phone that we are using to tracking.
Global positioning system(GPS)
GPS is a satellite based navigation system.GPS works anywhere in the world under any weather conditions.GPS constellation consist of 24 satellites in 6 near circular orbits.An on board GPS receiver must locked in to at least 3 satellite to calculate 2D position or the movement,if 4 satellites or more receiver can determine the 3D position.Once the user position has been determined GPS unit will start to calculate other information such as speed,track,trip distance,time.These parameters are continually updated by a master control station,which transmits them up to the satellites where they are broadcast as part of a navigational message from each satellite.Civilian GPS uses L1 frequency of 1.5754 GHz in the UHF band(signal travels by line of sight,it can pass clouds,glass and plastic but fail to pass through solid objects).Defense GPS uses L2 frequency of 1.2276 GHz in the UHF band.In the GPS system the modulation used is direct sequence spread spectrum and it is one way transmission system(from satellite to user only,user does not require a transmitter).Modulation method makes GPS system jam resistant,secure and addressable.The in phase component is being C/A (coarse acquisition) code while quadrature component is P-code(precision code).
GPS signal contains 3 different bits of information:
- Pseudorandom code,simply an ID code to identify which satellite is transmitting th signal.
- Ephemeris data,which is constantly transmitted by each satellite,contains information sbout its status and this is essential to determine its position.
- Almanac data,tells the GPS receiver about the position(orbital information) of each GPS satellite throughout the day.
Friday, January 4, 2008
Wireless networks
Modern wireless communication systems
Wireless communication networks have become much more pervasive than any one could have imagine when the cellular concept was first developed in the 1960s and 1970s.The wide spread success of cellular has led to the development of newer wireless systems and standards for many other types of telecommunication traffic.
Personal communication networks (PCN) refers to a wireless networking concept where any user can make or receive call,no matter where they are, using a light weight,personalized communicator.Personal communication services (PCS) refers to new wireless system that incoperate more network features and more personalized than existing cellular radio systems.
Second Generation(2G) Cellular networks
Unlike first generation cellular systems that relied on FDMA/FDD and analog FM,second generation standards use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques.The most popular 2G standard includes 3 TDMA standards and one CDMA standards:
1.Global System Mobile (GSM),which supports 8 time slotted users for each 200kHz radio channel.(in the PCS spectrum band only)
2.Interim Standard 136 (IS-136),wich support 3 time slotted users for each 30kHz radio channel.
3.Pacific Digital cellular(PDC), a japanese TDMA standards that is similar to IS-136.
4.Interim Standard 95 Code devision Multiple access(IS-95),also known as cdmaone, which supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 channel.
All 2G technologies offer at least a 3 times increase in spectrum efficiency as compared to the first generation analog technologies.
Evolution to 2.5 Mobile Radio Networks
The 2G technology use circuit switched data modems that limit data users to single circuit-switched voice channel.2G networks only support single user data rates on the order of 10 kilobits per second,which is too low for rapid email and internet browsing applications.This is because 2G standards were designed before the wide spread use of internet.
Even with relatively small user data rates, 2G standards are able to support limited internet browsing and sophisticated short messaging(SMS) capabilities using circuit-switched approach.The new technology 2.5G allow existing 2G equipment to be modified and supplemented with new base station add-ons and subscriber unit software upgrades to support higher data rate transmission for web browsing and e-mail traffic. 2.5G also support new web browsing format language,called wireless application protocol(WAP),that allow standard web pages to be viewed in compressed format specially designed for small hand held potable wireless devices.Japan introduced its own propriety wireless data service and internet micro browser technology called I-mode,on its PDC.I-mode supports games,color graphics and interactive web page browsing using 2G PDC data rate 0f 9.6 kilobits per second.
Evolution for 2.5G TDMA Standards
Three TDMA upgrade option include:
1.High Speed Circuit Switched Data (HSCSD)
2.General Packet Radio Service (GPRS)
3.Enhanced Data rates for GSM Evaluation (EDGE)
These option provide significant improvement in internet access speed over GSM and IS-136 and support the creation of new internet-ready cell phones.
HSCSD for 2.5G GSM
HSCSD allows single mobile subscriber to use consecutive user time slots in GSM standard,in order to obtain high speed data access.HSCSD relaxes the error control coding algorithm that specified in GSM for data transmission and increase data rate to 14.4 kbps.By running up to 4 consecutive time slots,HSCSD able to provide rate up to 57.6 kbps to individual users.Service provider has to implement a software change at existing GSM base station to activate HSCSD.HSCSD is ideal for streaming internet access and real-time interactive web sessions.
GPRS for 2.5G
GPRS is packet based data network,wich is well suited for non-real time internet usage,and applications where the user downloads much more data than its uploads(like retrieval of email,faxes).GPRS supports multi-user network sharing of individual radio channels and time slots.GPRS can support many users than HSCSD but in bursty manner.GPRS subscriber units are automatically instructed to tune to dedicated GPRS radio channels and particular time slot for "always on"access to the network.Implementation of GPRS merely requires the GSM operator to install new router and internet gateway at the base station,along with new software that redefines the base station air interface and time slots,no new RF hardware needed.As the case of packet network throughput experience by user may decreases as more users attempt to use the network or due to poor propagation conditions.GPRS allow dedicated peak 21.4kbps per channel data rate,if all 8 time slots of GSM used for GPRS,an individual user is able to achieve 171.2 kbps (8x21.4).GPRS originally designed to provide packet data access over GSM networks,but later extended to work with IS-136 as well.
EDGE for 2.5G
EDGE requires new hardware and software upgrades to the existing base station.EDGE introduce a new digital format,8-PSK(octal phase shift keying) in addition to existing GMSK modulation.EDGE allows for 9 different air interface formats,known as multiple modulation and coding schemes(MCS),with varying degree of error control protection.So each user connection may adaptively determine the best MCS setting(incremental redundancy) for the particular radio propagation condition and data access requirement of the user.EDGE is sometimes referred to as Enhansed GPRS,EGPRS.Incremental redundancy,is packets transmitted first with maximum error protection and data rate throughput,but subsequent packets are transmitted with less error protection and less throughput.If all 8 time slots are dedicated to EDGE,raw peak throughput data rate of 547.2kbps can be provided.
IS-95B for 2.5 CDMA
IS-95B provides high speed packet and circuit switched data access on a common CDMA radio channel by dedicating multiple orthogonal user channels(Walsh function).IS-95B supports medium data rate(MDR) service by allowing a dedicated user to command up to 8 different user Walsh code simultaneously and in parallel throughput of 115.2kbps per user(8 x 14.4kbps).IS-95B also specifies hard hand off procedure that allow subscriber unit to search different radio channels in the network with out instruction from the switch,and maintain link quality.
Wireless communication networks have become much more pervasive than any one could have imagine when the cellular concept was first developed in the 1960s and 1970s.The wide spread success of cellular has led to the development of newer wireless systems and standards for many other types of telecommunication traffic.
Personal communication networks (PCN) refers to a wireless networking concept where any user can make or receive call,no matter where they are, using a light weight,personalized communicator.Personal communication services (PCS) refers to new wireless system that incoperate more network features and more personalized than existing cellular radio systems.
Second Generation(2G) Cellular networks
Unlike first generation cellular systems that relied on FDMA/FDD and analog FM,second generation standards use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques.The most popular 2G standard includes 3 TDMA standards and one CDMA standards:
1.Global System Mobile (GSM),which supports 8 time slotted users for each 200kHz radio channel.(in the PCS spectrum band only)
2.Interim Standard 136 (IS-136),wich support 3 time slotted users for each 30kHz radio channel.
3.Pacific Digital cellular(PDC), a japanese TDMA standards that is similar to IS-136.
4.Interim Standard 95 Code devision Multiple access(IS-95),also known as cdmaone, which supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 channel.
All 2G technologies offer at least a 3 times increase in spectrum efficiency as compared to the first generation analog technologies.
Evolution to 2.5 Mobile Radio Networks
The 2G technology use circuit switched data modems that limit data users to single circuit-switched voice channel.2G networks only support single user data rates on the order of 10 kilobits per second,which is too low for rapid email and internet browsing applications.This is because 2G standards were designed before the wide spread use of internet.
Even with relatively small user data rates, 2G standards are able to support limited internet browsing and sophisticated short messaging(SMS) capabilities using circuit-switched approach.The new technology 2.5G allow existing 2G equipment to be modified and supplemented with new base station add-ons and subscriber unit software upgrades to support higher data rate transmission for web browsing and e-mail traffic. 2.5G also support new web browsing format language,called wireless application protocol(WAP),that allow standard web pages to be viewed in compressed format specially designed for small hand held potable wireless devices.Japan introduced its own propriety wireless data service and internet micro browser technology called I-mode,on its PDC.I-mode supports games,color graphics and interactive web page browsing using 2G PDC data rate 0f 9.6 kilobits per second.
Evolution for 2.5G TDMA Standards
Three TDMA upgrade option include:
1.High Speed Circuit Switched Data (HSCSD)
2.General Packet Radio Service (GPRS)
3.Enhanced Data rates for GSM Evaluation (EDGE)
These option provide significant improvement in internet access speed over GSM and IS-136 and support the creation of new internet-ready cell phones.
HSCSD for 2.5G GSM
HSCSD allows single mobile subscriber to use consecutive user time slots in GSM standard,in order to obtain high speed data access.HSCSD relaxes the error control coding algorithm that specified in GSM for data transmission and increase data rate to 14.4 kbps.By running up to 4 consecutive time slots,HSCSD able to provide rate up to 57.6 kbps to individual users.Service provider has to implement a software change at existing GSM base station to activate HSCSD.HSCSD is ideal for streaming internet access and real-time interactive web sessions.
GPRS for 2.5G
GPRS is packet based data network,wich is well suited for non-real time internet usage,and applications where the user downloads much more data than its uploads(like retrieval of email,faxes).GPRS supports multi-user network sharing of individual radio channels and time slots.GPRS can support many users than HSCSD but in bursty manner.GPRS subscriber units are automatically instructed to tune to dedicated GPRS radio channels and particular time slot for "always on"access to the network.Implementation of GPRS merely requires the GSM operator to install new router and internet gateway at the base station,along with new software that redefines the base station air interface and time slots,no new RF hardware needed.As the case of packet network throughput experience by user may decreases as more users attempt to use the network or due to poor propagation conditions.GPRS allow dedicated peak 21.4kbps per channel data rate,if all 8 time slots of GSM used for GPRS,an individual user is able to achieve 171.2 kbps (8x21.4).GPRS originally designed to provide packet data access over GSM networks,but later extended to work with IS-136 as well.
EDGE for 2.5G
EDGE requires new hardware and software upgrades to the existing base station.EDGE introduce a new digital format,8-PSK(octal phase shift keying) in addition to existing GMSK modulation.EDGE allows for 9 different air interface formats,known as multiple modulation and coding schemes(MCS),with varying degree of error control protection.So each user connection may adaptively determine the best MCS setting(incremental redundancy) for the particular radio propagation condition and data access requirement of the user.EDGE is sometimes referred to as Enhansed GPRS,EGPRS.Incremental redundancy,is packets transmitted first with maximum error protection and data rate throughput,but subsequent packets are transmitted with less error protection and less throughput.If all 8 time slots are dedicated to EDGE,raw peak throughput data rate of 547.2kbps can be provided.
IS-95B for 2.5 CDMA
IS-95B provides high speed packet and circuit switched data access on a common CDMA radio channel by dedicating multiple orthogonal user channels(Walsh function).IS-95B supports medium data rate(MDR) service by allowing a dedicated user to command up to 8 different user Walsh code simultaneously and in parallel throughput of 115.2kbps per user(8 x 14.4kbps).IS-95B also specifies hard hand off procedure that allow subscriber unit to search different radio channels in the network with out instruction from the switch,and maintain link quality.
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