GROUP COMMUNICATION
Chapters

GROUP COMMUNICATION

1.0 INTRODUCTION

                  Groups of computers to communicate with one another is referred to as group communication. It is considerably more efficient to convey information to an entire group than separately to each member of the group. The typical group communication applications are video conferencing, distance learning, distributed databases, data replication, multi-party games, distributed simulation, network broadcast services and many others.

1.1 TYPES OF COMMUNICATION

                  Within the context of group communication, various types of communication can be differentiated, depending on the number of senders and receivers involved. A distinction is made between the following basic types of communication:

·        Unicast(1:1)

·        Multicast(1:n)

·        Concast(m:1)

·        Multipeer/Multipoint(m:n)

                  The notation in the brackets should be interpreted as follows: The first number (m) refers to the number of senders; the second (n) refers to the number of receivers. The special case of a single sender or receiver is denoted by the number 1.

1.2             UNICAST COMMUNICATION

                  Unicast is equivalent to traditional point-to-point communication in which there is exactly one sender and one receiver. Point-to-point communication between two people in a group can be viewed as a special case of group communication. Figure 1.1 illustrates a simple unicast communication within a group. The nodes shaded in green (receivers) and red (sender) represent members of a group. The nodes in yellow represent non-members of a group.

Figure 1.1 Unicast Communication

                  In this example, the sender (S) communicates receiver (R) using unicast communication. The communication may be a simplex wherein one participant is the sender of data and the other participant is receiver of the data or duplex communication wherein both the participants are active senders as well as active receivers. Traditional applications like File Transfer Protocol (FTP), electronic mail, chat etc. make use of unicast communication.

                  If unicast is used to support group communication, then two unicast communication relationships have to be established between two members in a group. The number of transmissions for a packet delivery linearly increases with group size. Since there is no scalability with respect to group size, unicast communication is not feasible option for large groups.  The detailed comparison is explained in section 1.3.

1.3             MULTICAST COMMUNICATION

                  With the new emerging applications like video-conferencing, shared whiteboards, multi-user games etc. getting into the life of an average computer user, a mode of communication called multicast has been used. These applications require that the data should be sent only to a set of participants (green shaded nodes), not to every other end points (yellow shaded nodes) as shown in Figure 1.2. Hence, to fulfill the requirements where the communication is restricted to only a set of participants (a group), the multicast communication emerged in the computer communications arena.

                 

Figure 1.2 Multicast Communication

                  When compared to traditional IP unicasting and broadcasting, IP multicasting  is more efficient and economical, consumes less bandwidth and processing power, scales better, and does not lead to network congestion as the number of clients grows. Multicasting operates over any network technology that can support Transmission Control Protocol/Internet Protocol (TCP/IP), including Ethernet, Asynchronous Transfer Mode (ATM), frame relay, and satellite (Tanenbaum 1988).

1.4             CONCAST COMMUNICATION

                  In a concast communication several senders are able to send user data to a single receiver. This involves an m:1 communication in which data is sent on a unidirectional basis from the senders to the receiver. Concast communication is used in the field of open distance learning when students forward their homework assignments to their teachers or tutors.

1.5             MULTIPEER COMMUNICATION

                  Multipeer communication takes place when several senders are able to send user data to the group members. This corresponds to an m:n type of communication and is frequently referred to as multipoint communication.  Multipeer is the most diverse form of group communication because it places no restrictions on the number of senders and receivers that can communicate. Multipeer communication is very difficult to implement but can be emulated through the simultaneous operation of several multicast communications. To this end, a multicast communication is established for each sender to all the other members of the group. This technical implementation is frequently selected as an option today.

1.5             OTHER TYPES OF COMMUNICATION

                  Unicast can be viewed as an exception in group communication types since it does not really involve more than two communication partners. Two other types of communication are also used today:

·        Anycast

·        Broadcast

                  Anycast also makes use of the group concept. However, in this case the group is not used for the actual exchange of data; this takes place with the available unicast mechanisms or with new anycast mechanisms. The receiver is selected from a group of potential candidates. In anycast, there is also a one-to-many association between network addresses and network endpoints. The destination address identifies a set of receiver endpoints, but only one of them is chosen at any given time to receive information from any given sender. This is shown in Figure 1.3.


Figure 1.3 Anycast Communication

                  In this example, though the receivers are identified by single address, sender S sends data to only one receiver. Anycast is generally used as a way to provide high availability and load balancing for stateless services such as access to replicated data; for example, Domain Name System (DNS) service is a distributed service over geographically dispersed servers.

                  In broadcast communication, there are one or more senders that send data to every other receiver in the network as shown in Figure 1.4, and there is no restriction with respect to the group of receivers. In this sense broadcast is a simplified version of multicast because it does not require the establishment, addressing or administration of a group.  There are packet radio networks, satellite networks and bus local networks that use broadcast communication. However, IP broadcasting sends a copy of a message simultaneously to every client on the network, including those who don’t want to receive it.

Figure 1.4 Broadcast Communication

                  For example in Figure 1.4, yellow shaded nodes also receive the packets without cause. In data communication, broadcast tends to play a less important role, at least for wide-area networks. Applications of group communication are listed in Table 1.1.

Table 1.1 Applications of Group Communication

Types of Communi-cation

No. of Senders

No. of  Receivers

Applications

Unicast

1

1

FTP, electronic mail, chat.

Multicast

1

n

Database Updates, Push media, Live Concerts, Newsfeeds, Lectures and Announcements.

Concast

M

1

Resource Discovery, Auctions, Data Collection and Polling.

Multipeer

M

n

Multimedia Conferencing , Synchronized Resources , Concurrent Processing, Chat Groups, Distance Learning ,   Multi-player Games and Distributed Interactive Simulations.

Broadcast

1

All         All

Television, Radio, Ethernet and Address Resolution Protocol (ARP).

Anycast

1

1(anyone of  the designated receiver in a group)

Domain Name System.

 

Reference
  • Ralph Wittmann and Martina Zitterbart, “Multicast communication protocols and applications”, Harcourt India Pvt. Ltd, 2001.
  • http://en.wikipedia.org/wiki/Multicast