Multimedia Of IP Networks: Layered Multicast. (A Review)

Unlike stand-alone multimedia applications, in which the multimedia contents are originated and displayed on the same machine, multimedia networking has to enable multimedia data that originate on a source host to be transmitted through the IP networks (the Internet) and displayed at the destination host. The Internet, which uses IP protocols and packet switching, has become the largest network of networks in the world (it consists of a combination of manywide area and local area networks, WANs and LANs.

Multimedia over the Internet is fast growing among service providers and potential customers. Owing to the special requirements of audio and video perception, most existing and emerging real-time services need a high level of quality and impose great demands on the network. Real-time multimedia applications (e.g., live video streaming and video conferences), which are very sensitive to transmission delay and jitter and usually require a sufficiently high bandwidth, are a good example. To this end, various systems have been made available on network protocols and architectures to support the quality of service (QoS), such as the integrated services (IntServ) and differentiated services (DiffServ) models. Multiprotocol label switching (MPLS) is another technique often mentioned in the context of QoS assurance, but its real role in QoS assurance is not exactly the same as that of the IntServ and DiffServ models.

Layered Internet protocol (IP)

The IP protocol is the set (suite) of communications protocols that implement the protocol stack on which the Internet and most commercial networks run. It has also been referred to as the TCP/IP protocol, since two of the most important protocols in it are the transmission control protocol (TCP) and the Internet protocol (IP), which were also the first two networking protocols defined. The IP protocol can be viewed as a set of layers in which each layer solves a set of problems involving the transmission of data; generally, a protocol at a higher level uses a protocol at a lower level to help accomplish its aims. The IP suite uses encapsulation to provide abstraction of protocols and services. The upper layers are logically closer to the user and deal with more abstract data, relying on lower-layer protocols to translate the data into forms that can eventually be physically transmitted. The IP protocol is now commonly accepted as a top-down five-layer model, having application, transport, network, data-link, and physical layers.

Layered multicast of scalable media

With the fast deployment of Internet infrastructure, wired or wireless, the IP network is getting more and more heterogeneous. The heterogeneity of receivers under an IP multicast session significantly complicates the problem of effective data transmission. A major problem in IP multicast is the sending rate chosen by the sender. If the transmitted multimedia data rate is too high, this may cause packet loss or even a congestion collapse whereas a low transmission data rate will leave some receivers underutilized.

This problem has been studied for many years and is still an active research area in IP multicast. To solve this issue, the transmission source should have a scalable rate, i.e., multirate, which allows transmission in a layered fashion. By using multirate, slow receivers can receive data at a slow rate while fast receivers can receive data at a fast rate. In general, multirate congestion control can perform well for a large multicast group with a large number of diverse receivers. This brings us to the scheme of layered multicast.

Basically, layered multicast is based on a layered transmission scheme. In a layered transmission scheme, data is distributed across a number of layers which can be incrementally combined, thus providing progressive refinement. The scalable video coding (SVC) can easily provide such layered refinement. Thus, the idea of layered multicast is to encode the source data into a number of layers. Each layer is disseminated as a separate multicast group, and receivers decide to join or leave a group on the basis of the network condition). The more layers the receiver joins, the better quality it gets. As a consequence of this approach, different receivers within a session can receive data at different rates. Also, the sender does not need to take part in congestion control.

Layered Multicast Of Multimedia Networks

To avoid congestion, end systems are expected to be cooperative by reacting to congestion and adapting their transmission rates properly and promptly. The majority traffic in the Internet is best-effort traffic. The transport control protocol (TCP) traffic uses an additive-increase multiplicative-decrease (AIMD) mechanism, in which the sending rate is controlled by a congestion window. The congestion window is halved for every window of data containing a packet drop and increased by roughly one packet per window of data otherwise. Similarly, IP multicast for UDP traffic needs a congestion control algorithm. However, IP multicast cannot simply adopt the TCP congestion control algorithm because acknowledgements can cause an “implosion problem” in IP multicast. Owing to the use of different congestion control algorithms in TCP and multicast, the network bandwidth may not be shared fairly between the competing TCP and multicast flows. Lack of an effective and “TCP friendly” congestion control is the main barrier for the wide-ranging deployment of multicast applications. “Scalability” refers to the behavior of the protocol in relation to the number of receivers and network paths, their heterogeneity, and their ability to accommodate dynamically variable sets of receivers. The IP multicasting model provided by RFC 1112 is largely scalable, as a sender can send data to a nearly unlimited number of receivers. Therefore, layered multicast congestion control mechanisms should be designed carefully to avoid scalability degradation.