Mbone stands for "multicast backbone". Although LANs such as Ethernet are capable of multicast (transmitting the same packets to multiple destinations), the Internet as a whole currently is not. The way round this is to establish a network of multicast routers around the world, linked by "tunnels". Each multicast data packet has a TTL parameter that controls how far it will travel. Packets with a TTL of 127 will cross the world.
Multicast is similar to the regular Internet protocol, except that it goes to more than one place. A block of ip addresses starting with 224 is set aside for multicast, and each address can have many "ports", so that the address space is quite large, kind of like having a few hundred thousand TV channels. There's a finite bandwidth, though, so there can't currently be more than one or two events with worldwide coverage at the same time.
Several applications have been written to use multicast addressing for remote conferencing. These include video, audio and whiteboard tools which allow users at many sites around the world to receive transmissions from a single source. Users can also participate by asking questions or writing on a whiteboard. The tools can also be used point-to-point using regular ip addresses to link two workstations. For multicast transmissions, they are usually controlled by a session tool, which displays information about available sessions, passing address and port data to the individual tools. They are quite sophisticated, supporting encryption, control of right-to-talk, and several encoding methods. While available for many flavours of Unix, including those that run on a PC, they are not yet available for Windows - but with Windows NT and Windows 95 having better Internet support and multitasking, it is surely just a matter of time.
The snapshot shows the session directory tool, an audio tool, and video tool. The video tool can accommodate multiple sources, shown as separate thumbnails. Clicking on the thumbnail produces a larger viewing window.
Normal transmissions use 128kbps for video and 64kbps for audio using PCM coding. GSM coding can reduce the audio bandwidth to about 10kbps, while the video bandwidth can be reduced by adjusting the transmitted frame rate. Don't expect to see the NASA transmissions through a 28.8 modem, though,
The Mbone is still classed as experimental, so is not available yet from most commercial ISPs, though it is possible to set up your own tunnel from your LAN to a nearby (in Net terms) Mbone router.
For more information about the Mbone, conference tools, mailing lists etc. see http://vancouver-webpages.com/mbone
