Standards for Multimedia Communications

Introduction

• Most Multimedia Applications involve a number of media types that are together in some way

• Standards are necessary because it is essential that the two or more items of equipment that are used for the application interpret the integrated information stream in the same way

• Reference model is a common framework used for defining the various standards used in multimedia communications

Reference Models

• Standards are required at both the application level and the networking level

• The functionality of each set of standards is as follows: application standards, network interface standards and internal network standards

• The internal network standards are concerned solely with networking issues and are said to be within the networking environment

• The network interface standards operate over the access circuit to the network and are said to have local significance

• Application standards are independent of the network and define the communications between two terminals/computer. These communications are said to have end-to-end significance

• Each standard has a set of procedures associated with them to perform a particular function

• A procedure set will have information such as:

- content and structure of the source information stream associated with an application

- how the information stream is formatted

- the way of transmission error detection

- the procedure to obtain another copy of the corrupted block

• Both the communicating parties must adhere to the same set of procedures and collectively these form the communications protocol relating to that function

Reference Models – Standards for entertainment

TCP/IP Reference Model

• Application of the general structure of the modified model in relation to the various standards identified

• Most of the standards relating to the interpersonal communications are defined by the ITU-T and there are separate standards for the circuit and packet switched networks

• A typical session may start with a telephone conversation and during this conversion they may want to bring a third person who is not known to one of the communicating parties. Hence they may convert to a videoconferencing call.

• This shows that the initial conversation started with a speech only then speech with data, then speech with video and finally speech, video, and user data

• With ISDN each media type is allocated a fixed portion of the channel bandwidth, but with packet-switched network the appropriate amount of bandwidth is allocated on-demand as the session progresses

Standards relating to interpersonal communications

Standards Relating to Interpersonal Communications

Circuit-mode networks• The network interface standards relate primarily to the physical connection to the network termination and with the procedure to set up and clear a connection

• The basic transport layer function is provided by the multiplexer/demultiplexer

• The multiplexer merges the source information from the audio, video and user data and the system control application into a single stream for transmission over the constant bit rate channel

• The system control application is concerned with negotiating and agreeing on the operational parameters to be used with the call/session, based on the capabilities of the end system involved.

e.g: with the PSTN, the system control function includes the management of the available transmission bandwidth during a call

Circuit-mode networks• The audio and video codecs each use a particular compression algorithm which is appropriate for the application and within the bandwidth limits provided by the network

• The codecs also achieve audio and video synchronization (e.g. lip-sync)

• If the user data is to be shared between the various members of a conference, the application uses the service provided by a protocol known as a multimedia communications service (MCS)

H.320• The H.320 standard is intended for use in end systems that

support a range of multimedia applications over an ISDN• Audio: The choice of audio/speech is determined

primarily by the amount of transmission bandwidth available

• Video: A constant output bit rate from the encoder is obtained by varying the quantization threshold that is used dynamically

• User Data: The same standard is used with all the different types of circuit-mode network

• System control/call setup: Concerned primarily with the negotiation of the bandwidth/bit rate to be used for each stream

• Multiplexing: The fixed portions of the available bandwidth are allocated using a technique known as time division multiplexing (TDM). The role of H.221 is to ensure that each input stream is placed into its allocated position in the output bitstream

H.324• The H.324 standard is intended for use in end

systems that support a range of interpersonal communication applications over low bit rate switched networks such as PSTN

• Video: The H.263 standard uses the same compression technique as H.261 but contains a number of more advanced coding features in order to operate over lower bit rate channels

• Audio: The G.723.1 standard is the most common and operates at a bit rate of either 5.3 or 6.3 kbps

• Adaptive: Additional bytes are added by the transmitter to enable the receiver to detect the presence of errors

• System control: Concerned with the overall control of the end system and this involves many functions

H.324 - Multiplexing

• The channel bandwidth is divided into a number of separate logical channels each of which is identified by means of a logical channel number (LCN)

• The LCN0 is used to carry the control stream and each of the remaining channels carries a separate media stream

• The allocation of LCN is controlled by the transmitter and when it wishes to open a new channel, it sends an H.245 control message which includes the media type and the type of codec used

Circuit-mode networks – H.223 Multiplex Principles

• The role of the multiplexer is to merge the stream that are currently present into the available bandwidth by using what is known as bit-oriented protocol

H.324 - Multiplexing

• The transmitted bitstream is treated as a string of bytes and this is divided into a number of separate information fields

• Each field comprises a variable number of bytes and is separated by one or more flag bytes

• These have a bit pattern 01111110 and a technique known as zero bit insertion and deletion is used to ensure that this pattern cannot be present at the information field

• At the start of the information field is a header field which includes a multiplex code which specifies a particular mix of media and control logic channels

• The multiplex code is 4 bits in length and forms the index to a table known as the multiplex table

Circuit-mode networks – H.223 Multiplex Principles – Multiplex table usage

• At the start of each information field is the header (byte) and the combined header plus information field is known as a multiplex protocol data unit

H.321/H.310• Intended for use with terminals that provide a range of multimedia applications over a B-ISDN ( known as ATM network)

• The H.321 standard is an adaptation of the H.320 standard simplifying the interworking across both types of network and the only difference is that the network interface layers associated with H.321 relate to interfacing the end system to a B-ISDN rather than an ISDN

• The H.310 standard is intended for use with end systems that support not only interpersonal applications but also interactive and entertainment applications, it includes the H.321 set and additional standards (MPEG-1 audio and MPEG –2 video) plus their associated multiplex standard H.222

• The H.322 standard is intended for use with end systems that supports interpersonal communication applications over a LAN that provides communication channels of a guaranteed bandwidth/QoS

Packet-switched networks• Two alternative sets of protocols have been defined for providing interpersonal communication services over packet-switched networks, one defined by the ITU in recommendation H.323 and the other by the IETF• The H.323 standard pertaining to packet-switched networks relates primarily to how interpersonal communications are achieved between end systems that are attached either to the same LAN or to different LANs that are interconnected together in some way• Unlike the H.322, the H.323 standard is intended for use with LANs that provide a non-guaranteed QoS• The H.323 standard comprises components for the packetization and synchronization of the audio and video streams, an admission control procedure for end systems to join a conference, multipoint conference control, and interworking with terminals that are connected to the different types of circuit-switched networks

Packet-switched networks• The H.323 standard is independent of the underlying transport and network interface protocols and hence can be used with any type of LAN• Normally the transport layer provides both the reliable and unreliable service which in practice is the case for most LANs• With the end system connected to a LAN and communicating over an intranet or the Internet, the network layer protocol is the IP, the unreliable transport service is provided by the user datagram protocol (UDP) and the reliable transport service by the TCP

Packet-switched networks

H.323• Audio and video coding: Allows a variety of coding options to be used in order to simplify interworking with terminals/computers attached to the different types of circuit-mode networks

• prior to a call commencing an agreed coding standard must be negotiated to avoid the necessity of transcoding the audio and video streams

• The output streams of both the audio and video codecs are formatted into packets for transfer over the network using the real-time transport protocol (RTP)

H.323• Call setup: In order to limit the number of concurrent calls that involve multimedia, a device called an H.323 gatekeeper can (optionally) be used

• During the setting up of a multimedia conferencing call, each end system involved in the conference must obtain permission from the gatekeeper

• Then depending on the current level of usage of the LAN, the gatekeeper decides whether the call can take place

• If an increase in the allocated bandwidth is required during a call, again prior permission must be obtained from the gatekeeper

Packet-switched networks – Two-party call setup

H.323 gatekeeper• Setting up of the call is carried out in two stages• First the end system initiating the call obtains a permission from the gatekeeper to set up a call by sending an access request (ARQ) message to the gatekeeper and the gatekeeper responding with either an access confirm or reject• Assuming permission is received for a two-party call the initiating terminal then sends a setup request message directly to the called end system• The called system sends a call proceeding message directly to the initiating end system and then proceeds to obtain permission from the gatekeeper to take part in the call by means of the exchange of ARQ and ACF • Assuming permission is granted the called end system sends an alerting message directly to the initiating end system• Finally a connect message is sent directly to the initiating system• Above are part of the Resource Access Service (RAS) protocol

Packet-switched networks – Interworking

H.323 gateway• In addition to the operation of the end systems, the H.323 standard also defines interworking with end systems that are attached to a circuit-mode network

• This is through a device known as an H.323 gateway

• The role of the gateway is to provide translations between the different procedures associated with each network type• Translators are necessary for call-set up and clearing, system control, and for the two different multiplexing• If the two communicating end systems are using different audio and video codec standards, then transcoding between two different coding techniques must be carried out• A second function associated with a gateway relates to address translation (e.g LAN the end systems are referred to by an IP address while PSTN or an ISDN the address is a conventional telephone number

Internet Engineering Task Force (IETF)• The IETF standard provides a versatile facility supporting both multiparty conferencing and broadcast services

• It uses a different signalling protocol set from that used with H.323: session initiation protocol (SIP) and the related session description protocol (SDP), these replace the RAS and system control protocols. The SIP provides services for user location, call establishment, and call participation management

• Examples of SIP request messages (commands or methods):

- Options: sent to solicit the capabilities supported by a host

- Invite: sent to invite the user of a host to join in a call/session

- Bye: sent when the user of a host intends to leave a call/session

• Users are identified by a symbolic name similar to an email address which is converted into an actual IP address and port number by a server call the domain name server (DNS)

Electronic mail• The two basic components associated with a text-based email system that uses the Internet are the email client and the email server

• Email client is normally a desktop PC which runs a program known as the user agent (UA)

• This provides the user interface to the email system to create, send and delete messages from the user’s mailbox

• The email server is a server computer that maintains an IN and OUT mailbox for all the users/clients that are registered with it

• The server has software to transfer mail messages over the Internet. This software is known as the message transfer agent (MTA). This is responsible for the sending and receiving of messages over the Internet

Electronic mail• A copy of the user agent software runs in each client (UA client) and this communicates with a similar piece of software in the email server (the UA server) in order to log in to the server and to deposit and retrieve mail into/from the mailbox of the client

• The set of mail boxes in the server are contained in a database known as the message store (MS)

• The UA client periodically retrieves any received mail from the message store and transfer this to its own local mailbox ready for reading by the user

• An example protocol associated with the user agent function is the post office protocol, version 3, (POP3) which is defined in RFC 1939

Electronic mail• The standard structure of text-based mail associated with the Internet is defined in RFC 822

• An email message comprises a header and a message body. The structure of the header is defined in RFC 821

• The application protocol associated with the transfer of messages between the MTA in two servers is the simple mail transfer protocol (SMTP) defined in RFC 821

• A email message is transferred by the MTA in the sending server by it first establishing a TCP connection to the MTA in the recipient server

• As the email addresses are in the form of symbolic names the sending MTA requests another application protocol known as the domain name server (DNS) for the related Internet address of the recipient server

Electronic mail

• This then uses this together with its own Internet address to create an Internet packet – known as a datagram

• The routing of packets over the Internet is carried out by the IP and hence the Internet address is known as the IP address

• Each Internet packet contains the IP address of both the sending and recipient servers at its head and the email message as its contents

• The IP address of the recipient is used to route the packet over the Internet and, on receipt, the MTA in the server deposits the message contained in the packet into the recipient’s mailbox

Electronic mail

• An email client is a desktop which runs a program known as the user agent (UA) that provides the user interface to the email system and related services

Electronic mail

Email gateways

• The email gateway is used in general to overcome two problems within company/enterprise networks: the format of the mail messages and the application protocols are often different when trying to use other networks

• The email gateway has a number of interfaces, one for connecting to the local email server at the site and the others for connecting to those networks with which the employees at the site wish to communicate

• To transfer a message first the email server transfers it to the email gateway using the protocol stack associated with the company network

• The email address in the header of the message is then read by an application-level program to determine the network over which the mail should be forwarded

Email gateways

• Assuming the external network is the Internet, the program proceeds to reformat the message into the RFC 822 format and then forwards this using the TCP/IP protocol stack

• A similar procedure is followed at the reverse direction except the message format has to be changed from RFC 822 to the format used by the company network

• Using SMTP/TCP/IP protocols the email gateways can be removed

Electronic Commerce

• When you want to purchase something over the web you send your credit card details to a server

• This is an example of e-commerce• When you send your card details using a mobile handset

then this is called the m-commerce• The normal standard used to pass information from the

client to the server is via forms• The standard for use at the server for processing the

received information is known as the common gateway interface (CGI)

Electronic Commerce

Integrated Management Architecture for IP-based Networks

• There are five functional areas related to the integrated management architecture

- Performance management

- Fault management

- Configuration management

- Security management

- accounting and billing management

Performance management• This the the process of converting IP traffic measurements into

meaningful performance measures• It can be divided into real-time and long-term management• The real-time performance management process is a mechanism to

guarantee that enough bandwidth is reserved for time-sensitive IP voice traffic, while other applications sharing the same link get their share without interfacing with the mission traffic

• Another example is constant monitoring of high-priority customer services as well as customers who have been complaining about the performance of their services

• Long-term performance management supports studies that monitor the ability of the existing IP networks to meet service objectives

Performance management

• The purpose of the performance management is to identify situations where corrective planning is necessary

• This is needed when objectives are not being satisfied and, where possible, to provide early warning of potential service degradation so that a corrective plan can be formulated before service is affected

• Examples of IP performance traffic measurements include the following:

- Number of packets received per interface - Number of packets transmitted per interface - Number of packets dropped due to mild and severe congestion

per interface - Number of packets dropped due to protocol errors - Amount of time a network element is in a mild or a severe

congestion state - Number of times a network element enters a mild or severe

congestion state

Fault Management

• This process is similar to the real-tie performance process except that it uses the collected alarms and fault statistics to detect and correct problems by pointing and correlating faults through the system

• It simplifies the service provider’s ability to monitor customer services by providing the status of the subscribed services

• It is very challenging to represent the action that the network should consider in order to relieve a potential servicing problem before the service is affected

• This may include rerouting, load balancing, and congestion control

Configuration Management• It delas with the physical and geographical interconnections of

various IP network elements, such as routers, switches, multiplexers and lines

• It includes the procedure for initializing, operating, setting and modifying the set of parameters that control the day-to-day operation of the networks

• Configuration management also delas with service provisioning, user profile management and collection of operational data, which is the basis for recognizing changes in the state of the network

• The main functions are creation, deletion and modification of network elements and network resources

• This includes the action of setting up an IP network or extending an already existing network, setting various parameters, defining threshold values, allocating names to managed IP objects

Security Management• This process includes authentication, authorization and other

essential secure communication issues• Authentication establishes the identity of both the sender and the

receiver of information• Integrity checking of confidential information is often done if the

identity of the sending or receiving party is not properly established

• Authorization establishes what a user is allowed to do after the user is identified

• Issues related to authentication and authorization include the robustness of the methods used in verifying an entity’s identity, the establishment of trusted domains to define authorization boundaries and the requirement of namespace uniqueness

Accounting and Billing management

• This process deals with the generation and processing functions of end-user usage information

• This includes measuring the subscribers and possibly the network resources for auditing purposes and managing call detail information generated during the associated call processing

• The records created in the application servers are of growing importance in IP networks. They contain the contents and the services delivered by the network

• These usage details can then be transferred to a billing system to render invoices to the subscribers that use IP services

• Fraud detection and subscriber-related profile information, such as authorization to charge, are also a function of accounting and billing management