Messaging systems
In this section we first consider the basic operation of traditional phone and fax systems operating over the PSTN. We then consider enhancements to the traditional phone system to include voice mail and information services. We then consider VoIP, a system for making phone calls using the Internet. Finally we examine the characteristics of email and how it is transmitted and received.
1. Traditional Phone and Fax
Telephones and the PSTN network connecting homes and organisations operate using similar principles as the original system first implemented over 100 years ago. Essentially all telephones have a microphone, a speaker, some sort of bell and a simple switch to connect the phone to the telephone network. A 100-year-old phone will still operate on most of today’s phone lines. The only significant difference being the signals used to dial numbers – older phones use pulse dialling whereas current phones use tone dialling. When pulse dialling, the phone switch is rapidly disconnected and connected the same number of times as the number being dialled – techniques included tapping the hook the required number of times or rotating a dial. Tone dialling transmits different frequencies to represent each number. In many older homes the copper wires connecting the phone to the PSTN network have been in place for many more years than originally intended, it is what happens once the wires reach the local telephone exchange that has changed. In the past, actual mechanical switches were used to connect the copper wire from your home phone directly with the copper wires connected to the phone being called. Circuit switching creates a direct connection or circuit between the two phones. In the days of manual switchboards, operators would manually connect the wires running from your home with the wires running to the person’s phone you wished to call. Although manual switching has now been completely replaced by electronic switching, the PSTN circuit switched network operates using this very same connection-based principle that is, a direct connection is setup and maintained whilst each conversation takes place. During a typical conversation we spend less than half the time listening, less than half the time speaking and the remaining time in relative silence. This is not such a concern between a phone and its local exchange; however over longer distances the inefficiencies are significant. Today, apart from the connection between telephones and their local exchange, the remainder of the PSTN is essentially digital. Digital networks make much more efficient use of the lines. By digitising the analog voice signals it becomes possible to compress the bits and also to combine (multiplex) many conversations on a single physical connection. This means many conversations share the same line simultaneously. Various different modulation schemes are used depending on the range of frequencies used and the physical attributes of the cable. For example time division multiplexing (TDM), used on tier 1 (T1) lines, samples each voice 8000 times per second and each of these samples is coded into 7-bits. A total of 24 voice channels are combined onto a single copper circuit. Most medium to large organisations do away with analog lines altogether, rather they have one or moreT1 lines that directly enters their premises.It is the digital nature of most of the PSTN that has allowed most phone companies to provide their customers with additional features, such as call waiting, caller id, three- way calls, call diversion and voice mail. The processing required to implement these features occurs at the telephone exchange – the customer sends commands to access and control the feature using tones generated by their phone’s keypad. Furthermore much of the PSTN’s digital infrastructure is used to transmit IP data across the Internet.
Facsimile (Fax)
Alexander Bain first patented the basic principle of the facsimile, or fax machine, in1843. Incredibly this is some 33 years before the telephone was invented. It was some twenty years later that the first operational fax machines and transmissions commenced. Initially it seems odd that fax pre-dates telephones, however in fact it makes sense. At this time the telegraph system using Morse code was in operation. Morse code was transmitted by opening and closing a circuit, which is similar to the binary ones and zeros used by today’s fax machines. It wasn’t until the late 1960s that fax machines became commercially viable; these machines adhered to the CCITT Group 1 standard, which used analogy signals and took some 6 minutes to send each page. The message was sent as a series of tones, one for white and another for black, these tones were then converted to an image using heat sensitive paper. By the late 1970s the fax machine had become standard inclusion in most offices. A new Group 2standard was introduced; these Group 2 machines generated digital signals and used light sensors to read images on plain paper originals. Soon after machines were developed that used inkjet and laser printer technologies to print directly onto plain paper. The Group 3 standard was introduced in 1983; it contained various different resolutions together with methods of compressing the digital data. Today computers are routinely used to produce, send and receive faxes; in fact most dial-up modems have built in fax capabilities. There are even Internet sites that allow single fax to be broadcast to many thousands of fax machines simultaneously. It is common today for a single device to integrate scanning, faxing and printing.
VOICE MAIL AND PHONE INFORMATION SERVICES
Voice mail, in its simplest form, is much like a digital version of a traditional answering machine. Calls that are not answered after a predefined number of rings are diverted to the voice mail system. The voice mail system answers the call and plays a pre-recorded outgoing message (OGM). The OGM welcomes the caller and provides instruction on how to leave a message – for residential phones the OGM may be as simple as “Hi, you’ve reached Sam, please leave a message after the tone and I’ll get back to you ASAP.” The voice mail system then digitally records the users voice and stores it within the customers voice mailbox. At some later time the customer rings the voice mail system, verifies their identity using a numeric password and listens to the voice messages held in their voice mailbox. During message retrieval the customer uses their phone keypad to enter commands that control the voice mail system. No doubt we are all familiar with such systems. The familiar voice mail system described above is normally a service provided by the customer’s local telephone service provider – Telstra, Optus, Orange, etc. The servers used to process messages are located and owned by the telephone company. More sophisticated voice mail systems are used by business and government organisations. These organisations maintain their own systems. Such systems include a multitude of features designed to meet the needs of the individual organisation and its customers. They do a lot more than maintaining voice mail for many users. Commonly such systems integrate with other messaging systems such as email and fax, and they provide automated information services and call forwarding functionality to customers. For our purposes we more accurately describe such systems as Phone Information Services. The majority of phone information systems include a hierarchical audio menu whereby customers navigate down through the hierarchy of menus to locate information or be directed to specific personnel. The available options at each level of the hierarchy are read out as an OGM, the customer responds using their phone’s keypad or using voice commands to progress to the next level. Some of the features present within Phone Information Services include:•Voice mail management for many users. Customers enter the extension number of the required person and if not answered the system records the message to the person’s mailbox.
•Support for multiple incoming and outgoing lines of different types. Today large organisations will have many digital T1 lines connected directly to the PSTN and also VoIP (voice over IP) lines connected to the Internet via broadband connection.
•Fax on demand where customers navigate a menu system to locate and request particular documents to be faxed back.
•Call attendant functions where the menu system filters callers through to the correct department based on the caller’s selections. Some systems can forward calls tother external lines.
•Text to speech (TTS) capabilities that allow text to be read to users over the phone. For example, TTS can be used to read emails and other text documents or more simply it is often used to read numbers and currency amounts back to customers to verify their data entry.
•Call logging to databases. For example records commonly include the caller id, time and length of call. This data is analysed to provide management information tithe organisation.
•Provision of information to customers. The OGMs include information rather than just details of how to navigate the menu system. For example, in Australia numbers with the prefix 1900 provide such information on a user pays basis
Voice over Internet Protocol (VOIP)
Voice over Internet Protocol, as the name suggests, transfers voice calls over the public Internet. VoIP is also known as IP Telephony, Voice over broadband and Internet telephony. All these names indicate some of the basics of VoIP – a broadband Internet connection is used to transfer telephone calls using IP. However it is possible to transfer voice over the Internet using any Internet connection combined with a microphone, speakers and one of the many free instant messaging applications, such as MS Messenger, for example. So how is VoIP different? VoIP goes one step further and provides an interface to the PSTN. This is the defining feature of VoIP – it allows VoIP calls to be made to any normal telephone across the globe. Furthermore calls are significantly cheaper as the public Internet carries the data for free regardless of distance. If both ends of the call are using VoIP then the commercial PSTN is not used at all – such calls are often free, apart from the cost of the Internet connection. VoIP is not a single protocol rather it is suite of protocols. For instance, audio codeshare included to digitise and compress the analogy voice data, and then decompress and convert it back to analogy at the receiving end. Once the data has been converted from analogy to digital it passes through a stack of protocols – commonly RTP (Real Time Protocol) and UDP (User Datagram Protocol) at the OSI Transport Layer 4 and thein at the OSI network Layer 3. RTP is used to control streaming of data packets, including maintaining a constant speed and also keeping packets in the correct sequence. UDP is used rather than TCP as UDP fires off packets more rapidly without the overhead of error checking and flow control. There is various hardware combinations that are all commonly used to connect VoIP users – five possibilities are shown in
. The VoIP provider maintains one or more servers whose central task is to translate normal telephone numbers into I addresses. VoIP providers also maintain gateway servers which convert analogy phone calls to IP packets and vice versa – a gateway is a device that connects two different networks. Users who sign up with a VoIP provider commonly connect using their existing broadband modem and Internet connection. Broadband modems are also available with built-in support for VoIP; in this case a standard analogy telephone is simply plugged into the modem. Other possibilities include soft phones, where a VoIP software application operates on an existing Internet connected computer. Voice boxes are also available that connect existing analogy handsets to existing broadband modems. Now consider users, who don’t have an account with a VoIP provider, rather they have traditional PSTN phone line. VoIP providers must maintain a network that allows their customers to connect to phones on the PSTN. To implement this functionality and still save money on long distance calls requires VoIP gateway servers to be installed in locations throughout the world. Clearly it would not be economically viable for each VoIP provider to install gateway servers in every country. Therefore VoIP providers share their gateway servers with other international VoIP providers. Each local VoIP provider enters into an agreement with their local PSTN phone company. The local PSTN then creates a circuit between the PSTN users and the local VoIP gateway server for the duration of each VoIP call. The VoIP gateway server manages the packet switched side of the connection and the conversion of data between the Internet and the local PSTN
Electronic Mail (EMAIL)
In this section we describe the characteristics and organisation of email messages. This includes the components or fields within an email message as well as how the message data and any attachments are encoded. We also identify and briefly discuss the application/presentation layer protocols used to transmit and receive email messages across the Internet – all email is ultimately transmitted as ASCII text. During transmission all email messages are composed of two broad components, an envelope and a contents component. The envelope contains the information required to transfer the message to its destination – much like a paper envelope. The envelope data is examined and used by SMTP (Simple Mail Transfer Protocol) servers to relay email messages to other SMTP servers and finally to their destination. The contents component contains various headers together with the actual message. SMTP examines and adds to these headers, however it does not alter the actual message. The contents component contains the actual message data together with various header fields used to specify the sender, receiver, date/time, subject and also the relationship of the message to other related messages. RFC2822 “Internet Message Format” is the current standard that specifies how the content of all email messages are organised. From a user’s perspective creating an email involves specifying header fields for the recipients (receivers) and a subject – as well as entering the body of the message. The email client application adds the sender’s address, date/time and various other headers.
In this section we first consider the basic operation of traditional phone and fax systems operating over the PSTN. We then consider enhancements to the traditional phone system to include voice mail and information services. We then consider VoIP, a system for making phone calls using the Internet. Finally we examine the characteristics of email and how it is transmitted and received.
1. Traditional Phone and Fax
Telephones and the PSTN network connecting homes and organisations operate using similar principles as the original system first implemented over 100 years ago. Essentially all telephones have a microphone, a speaker, some sort of bell and a simple switch to connect the phone to the telephone network. A 100-year-old phone will still operate on most of today’s phone lines. The only significant difference being the signals used to dial numbers – older phones use pulse dialling whereas current phones use tone dialling. When pulse dialling, the phone switch is rapidly disconnected and connected the same number of times as the number being dialled – techniques included tapping the hook the required number of times or rotating a dial. Tone dialling transmits different frequencies to represent each number. In many older homes the copper wires connecting the phone to the PSTN network have been in place for many more years than originally intended, it is what happens once the wires reach the local telephone exchange that has changed. In the past, actual mechanical switches were used to connect the copper wire from your home phone directly with the copper wires connected to the phone being called. Circuit switching creates a direct connection or circuit between the two phones. In the days of manual switchboards, operators would manually connect the wires running from your home with the wires running to the person’s phone you wished to call. Although manual switching has now been completely replaced by electronic switching, the PSTN circuit switched network operates using this very same connection-based principle that is, a direct connection is setup and maintained whilst each conversation takes place. During a typical conversation we spend less than half the time listening, less than half the time speaking and the remaining time in relative silence. This is not such a concern between a phone and its local exchange; however over longer distances the inefficiencies are significant. Today, apart from the connection between telephones and their local exchange, the remainder of the PSTN is essentially digital. Digital networks make much more efficient use of the lines. By digitising the analog voice signals it becomes possible to compress the bits and also to combine (multiplex) many conversations on a single physical connection. This means many conversations share the same line simultaneously. Various different modulation schemes are used depending on the range of frequencies used and the physical attributes of the cable. For example time division multiplexing (TDM), used on tier 1 (T1) lines, samples each voice 8000 times per second and each of these samples is coded into 7-bits. A total of 24 voice channels are combined onto a single copper circuit. Most medium to large organisations do away with analog lines altogether, rather they have one or moreT1 lines that directly enters their premises.It is the digital nature of most of the PSTN that has allowed most phone companies to provide their customers with additional features, such as call waiting, caller id, three- way calls, call diversion and voice mail. The processing required to implement these features occurs at the telephone exchange – the customer sends commands to access and control the feature using tones generated by their phone’s keypad. Furthermore much of the PSTN’s digital infrastructure is used to transmit IP data across the Internet.
Facsimile (Fax)
Alexander Bain first patented the basic principle of the facsimile, or fax machine, in1843. Incredibly this is some 33 years before the telephone was invented. It was some twenty years later that the first operational fax machines and transmissions commenced. Initially it seems odd that fax pre-dates telephones, however in fact it makes sense. At this time the telegraph system using Morse code was in operation. Morse code was transmitted by opening and closing a circuit, which is similar to the binary ones and zeros used by today’s fax machines. It wasn’t until the late 1960s that fax machines became commercially viable; these machines adhered to the CCITT Group 1 standard, which used analogy signals and took some 6 minutes to send each page. The message was sent as a series of tones, one for white and another for black, these tones were then converted to an image using heat sensitive paper. By the late 1970s the fax machine had become standard inclusion in most offices. A new Group 2standard was introduced; these Group 2 machines generated digital signals and used light sensors to read images on plain paper originals. Soon after machines were developed that used inkjet and laser printer technologies to print directly onto plain paper. The Group 3 standard was introduced in 1983; it contained various different resolutions together with methods of compressing the digital data. Today computers are routinely used to produce, send and receive faxes; in fact most dial-up modems have built in fax capabilities. There are even Internet sites that allow single fax to be broadcast to many thousands of fax machines simultaneously. It is common today for a single device to integrate scanning, faxing and printing.
VOICE MAIL AND PHONE INFORMATION SERVICES
Voice mail, in its simplest form, is much like a digital version of a traditional answering machine. Calls that are not answered after a predefined number of rings are diverted to the voice mail system. The voice mail system answers the call and plays a pre-recorded outgoing message (OGM). The OGM welcomes the caller and provides instruction on how to leave a message – for residential phones the OGM may be as simple as “Hi, you’ve reached Sam, please leave a message after the tone and I’ll get back to you ASAP.” The voice mail system then digitally records the users voice and stores it within the customers voice mailbox. At some later time the customer rings the voice mail system, verifies their identity using a numeric password and listens to the voice messages held in their voice mailbox. During message retrieval the customer uses their phone keypad to enter commands that control the voice mail system. No doubt we are all familiar with such systems. The familiar voice mail system described above is normally a service provided by the customer’s local telephone service provider – Telstra, Optus, Orange, etc. The servers used to process messages are located and owned by the telephone company. More sophisticated voice mail systems are used by business and government organisations. These organisations maintain their own systems. Such systems include a multitude of features designed to meet the needs of the individual organisation and its customers. They do a lot more than maintaining voice mail for many users. Commonly such systems integrate with other messaging systems such as email and fax, and they provide automated information services and call forwarding functionality to customers. For our purposes we more accurately describe such systems as Phone Information Services. The majority of phone information systems include a hierarchical audio menu whereby customers navigate down through the hierarchy of menus to locate information or be directed to specific personnel. The available options at each level of the hierarchy are read out as an OGM, the customer responds using their phone’s keypad or using voice commands to progress to the next level. Some of the features present within Phone Information Services include:•Voice mail management for many users. Customers enter the extension number of the required person and if not answered the system records the message to the person’s mailbox.
•Support for multiple incoming and outgoing lines of different types. Today large organisations will have many digital T1 lines connected directly to the PSTN and also VoIP (voice over IP) lines connected to the Internet via broadband connection.
•Fax on demand where customers navigate a menu system to locate and request particular documents to be faxed back.
•Call attendant functions where the menu system filters callers through to the correct department based on the caller’s selections. Some systems can forward calls tother external lines.
•Text to speech (TTS) capabilities that allow text to be read to users over the phone. For example, TTS can be used to read emails and other text documents or more simply it is often used to read numbers and currency amounts back to customers to verify their data entry.
•Call logging to databases. For example records commonly include the caller id, time and length of call. This data is analysed to provide management information tithe organisation.
•Provision of information to customers. The OGMs include information rather than just details of how to navigate the menu system. For example, in Australia numbers with the prefix 1900 provide such information on a user pays basis
Voice over Internet Protocol (VOIP)
Voice over Internet Protocol, as the name suggests, transfers voice calls over the public Internet. VoIP is also known as IP Telephony, Voice over broadband and Internet telephony. All these names indicate some of the basics of VoIP – a broadband Internet connection is used to transfer telephone calls using IP. However it is possible to transfer voice over the Internet using any Internet connection combined with a microphone, speakers and one of the many free instant messaging applications, such as MS Messenger, for example. So how is VoIP different? VoIP goes one step further and provides an interface to the PSTN. This is the defining feature of VoIP – it allows VoIP calls to be made to any normal telephone across the globe. Furthermore calls are significantly cheaper as the public Internet carries the data for free regardless of distance. If both ends of the call are using VoIP then the commercial PSTN is not used at all – such calls are often free, apart from the cost of the Internet connection. VoIP is not a single protocol rather it is suite of protocols. For instance, audio codeshare included to digitise and compress the analogy voice data, and then decompress and convert it back to analogy at the receiving end. Once the data has been converted from analogy to digital it passes through a stack of protocols – commonly RTP (Real Time Protocol) and UDP (User Datagram Protocol) at the OSI Transport Layer 4 and thein at the OSI network Layer 3. RTP is used to control streaming of data packets, including maintaining a constant speed and also keeping packets in the correct sequence. UDP is used rather than TCP as UDP fires off packets more rapidly without the overhead of error checking and flow control. There is various hardware combinations that are all commonly used to connect VoIP users – five possibilities are shown in
. The VoIP provider maintains one or more servers whose central task is to translate normal telephone numbers into I addresses. VoIP providers also maintain gateway servers which convert analogy phone calls to IP packets and vice versa – a gateway is a device that connects two different networks. Users who sign up with a VoIP provider commonly connect using their existing broadband modem and Internet connection. Broadband modems are also available with built-in support for VoIP; in this case a standard analogy telephone is simply plugged into the modem. Other possibilities include soft phones, where a VoIP software application operates on an existing Internet connected computer. Voice boxes are also available that connect existing analogy handsets to existing broadband modems. Now consider users, who don’t have an account with a VoIP provider, rather they have traditional PSTN phone line. VoIP providers must maintain a network that allows their customers to connect to phones on the PSTN. To implement this functionality and still save money on long distance calls requires VoIP gateway servers to be installed in locations throughout the world. Clearly it would not be economically viable for each VoIP provider to install gateway servers in every country. Therefore VoIP providers share their gateway servers with other international VoIP providers. Each local VoIP provider enters into an agreement with their local PSTN phone company. The local PSTN then creates a circuit between the PSTN users and the local VoIP gateway server for the duration of each VoIP call. The VoIP gateway server manages the packet switched side of the connection and the conversion of data between the Internet and the local PSTN
Electronic Mail (EMAIL)
In this section we describe the characteristics and organisation of email messages. This includes the components or fields within an email message as well as how the message data and any attachments are encoded. We also identify and briefly discuss the application/presentation layer protocols used to transmit and receive email messages across the Internet – all email is ultimately transmitted as ASCII text. During transmission all email messages are composed of two broad components, an envelope and a contents component. The envelope contains the information required to transfer the message to its destination – much like a paper envelope. The envelope data is examined and used by SMTP (Simple Mail Transfer Protocol) servers to relay email messages to other SMTP servers and finally to their destination. The contents component contains various headers together with the actual message. SMTP examines and adds to these headers, however it does not alter the actual message. The contents component contains the actual message data together with various header fields used to specify the sender, receiver, date/time, subject and also the relationship of the message to other related messages. RFC2822 “Internet Message Format” is the current standard that specifies how the content of all email messages are organised. From a user’s perspective creating an email involves specifying header fields for the recipients (receivers) and a subject – as well as entering the body of the message. The email client application adds the sender’s address, date/time and various other headers.