Question

a. Consider an audio conference call in Skype with \(N>2\) participants. Suppose each participant generates a constant stream of rate \(r\) bps. How many bits per second will the call initiator need to send? How many bits per second will each of the other \(N-1\) participants need to send? What is the total send rate, aggregated over all participants? b. Repeat part (a) for a Skype video conference call using a central server. c. Repeat part (b), but now for when each peer sends a copy of its video stream to each of the \(N-1\) other peers.

Short answer

Audio call total rate: \( N \times r \) bps; video via server total rate: \( N \times r \) bps; peer-to-peer video total rate: \( N(N-1)r \) bps.

Step-by-step solution

Audio Conference Call Initiator

In an audio conference call with Skype, the initiator needs to send a continuous stream to each of the other \( N-1 \) participants. Each stream is sent at rate \( r \) bps. Therefore, the total rate the initiator needs to send is \( (N-1) \times r \) bps.

Audio Conference Call Remaining Participants

Each of the other \( N-1 \) participants sends their stream only to the initiator. Thus, each of the other participants sends data at a rate of \( r \) bps.

Total Send Rate Aggregated Over All Participants for Audio Call

The total send rate over all participants is the sum of the rate sent by the initiator and the rate sent by each of the other \( N-1 \) participants. This is \( (N-1) \times r + (N-1) \times r = N \times r \) bps.

Video Conference Call via Server Initiator

When a Skype video call uses a central server, the initiator sends its video stream to the server at rate \( r \) bps. The server then disperses this stream to each participant.

Video Conference Call via Server Other Participants

Each of the other \( N-1 \) participants also sends their video stream to the server at rate \( r \) bps.

Total Send Rate Aggregated Over All Participants for Video Call with Server

Similar to audio, the total sending rate when using a server is \( N \times r \) bps, as each participant sends the video stream to the server.

Peer-to-Peer Video Conference Initiator

In a direct peer-to-peer setup, the initiator sends its stream individually to each of the \( N-1 \) other participants. Thus, the initiator's send rate is \( (N-1) \times r \) bps.

Peer-to-Peer Video Conference Other Participants

Each other participant also sends their stream to every other participant. Thus, each of \( N-1 \) participants sends at \( (N-1) \times r \) bps.

Total Send Rate Aggregated Over All Participants for Direct Peer-to-Peer Video Call

For a peer-to-peer video call, each participant sends data to every other participant, which results in a total send rate of \( N \times (N-1) \times r \) bps, since each of the \( N \) participants sends their stream to the \( N-1 \) others.

Key concepts

Audio Conferencing

Audio conferencing is a method that allows multiple participants to communicate simultaneously in real-time using audio signals. In a typical audio conference scenario, like with Skype, the structure operates on direct communication between participants. The initiator of the conference call must manage a more significant amount of data, as they send their stream to each of the other participants in the call. For example, in a call with three participants, the initiator sends two streams. Each stream operates at a rate of \( r \) bits per second (bps). Hence, the initiator's total bit rate is \((N-1) \times r\) bps.
The other participants, each sends their audio stream only to the initiator, resulting in a data rate of just \( r \) bps for each. If we sum up all data sent by every participant, the total send rate across the entire conference is \( N \times r \) bps. Audio conferencing relies on efficient bandwidth management to ensure clear and uninterrupted communication.

Video Conferencing

Video conferencing enables participants to connect visually apart from audio, enhancing the interaction experience. Similar to audio conferencing, if managed through a service like Skype using a central server, each participant sends their video stream to the server at a rate of \( r \) bps. The server is responsible for distributing these streams to all participants, allowing for efficient bandwidth usage per individual user.
Because each participant sends the video data to the server, rather than directly to each other, the total sending rate across all participants still remains \( N \times r \) bps. This distribution method offloads the bandwidth demand from individual participants but requires a powerful and well-configured server to handle the video streams effectively. Video conferencing via a server offers stability and scalability through centralized control.

Peer-to-Peer Networks

Peer-to-peer networks (P2P) differ significantly from centralized models by allowing direct data exchange between peers on the network. In the context of video conferencing, each participant sends their video stream to every other participant. Therefore, each sender's bit rate is \((N-1) \times r\) bps, as they transmit to every single one of the other \(N-1\) peers.
This approach can lead to a comprehensive exchange of data across the network, resulting in a total sending rate of \(N \times (N-1) \times r \) bps. The P2P model maximizes the data flow throughout the network, offering decentralized control without the need for a central server, but may demand higher bandwidth capabilities for each participant. Peer-to-peer video conferencing can enhance redundancy and resilience as each participant actively supports the network.

Centralized Server Model

The centralized server model uses a single server as the focal point for communication, processing, and distribution of data. In this model, each client connects to the server rather than directly to each other. For video conferencing, each participant sends their stream to this server at a designated rate.
This setup sees each client's sending rate equivalent to \( r \) bps, allowing the server to handle all data processing. Consequently, the total sending rate across participants remains \( N \times r \) bps. Centralized models offer several advantages, such as reducing direct traffic on individual users and maintaining consistent quality through central management. However, the server's capacity and reliability are crucial since any failure could disrupt the entire conference. The centralized server approach gives a streamlined infrastructure, often found in enterprise and large-scale applications.