Transcript Document 7465115

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
Standards relating to interpersonal
communications
• 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 ondemand as the session progresses
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 IPbased 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 dayto-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