MCAST/MBONE FAQ

Frequently Asked Questions

q: what is sd?

q: how do I setup sd?

q: how do I run sd?

q: where can I get a map of the MBone?

q: what are some things broadcast over the MBone?

q: what subnets are connected?

q: what are some of the network considerations?

MBone (the Multicast Backbone) is a "virtual network" used for audio and video group conferencing. Many conferences and other events are broadcast over the MBone. A few of these are weather satelitte images of the earth, US Senate and House of Representative meetings and, "Radio Free Vat", and "internet talk radio" to name a few. The MBone Sessions are controlled throught a program called sd which lets you subscribe and unsubscribe to broadcasts.

The MBone has existed since 1992. It has grown out of an "effort to multicast audio and video from meeting of the Internet Engineering Task Force"

(Maced\ onia and Brutzman, Naval postgraduat school in article, "MBone Provides Audio and Vi deo Across the Internet")

The MBone is a part of the internet. It is network of special machines mro utersthat support multicast. These machines "tunnel" to other mrouters. They take the multicast transmission and encapsulate i t in a normal IP packet. mrouters at the end of the tunnels strips of the IP header and sends the packet as usual. Each tunnel has it's own copy of all the packets going over the line, so tunnels should be limited to a bout 2 for any given T1 link.

sd is a program that organizes and subscribes/unscribes to conferences/sessions bein g broadcast

SETTING UP sd

Setting up sd is quite easy.

• type cp /usr/local/mcast/sd.tcl ~put_your_username_here/.sd.tcl
• also add /usr/local/mcast to your path

That's it. Now all you have to do is type sd to run it.

RUNNING sd

Running sd is quite easy. Type sd. This will pull up the Session Directory. As time passes conferences will be announced. If you are interested in a specific one, click once on it and information will apear in the bottom window. If you then want to listen/watch in that "conference" double click on it.

Once in an audio conference the screen will be split in two, on the left, the usernames of the other people participating in the conference. On the right will be a speaker and microphone button. Below the speaker buttton is a volume bar which controls the volume, (there is also a mute button which will mute out sound totally when it is depressed) (If you wish to mute a certain person, click on the box next to there name and a "X" will appear and they will be muted.) The microphone slider controls the microphone input.

Many things are broadcast over MBone, including some nasa missions. UMBC has broascasted guest speakers also. Radio Free Vat is a "radio-station" were people can sign up for spots to play music.

sd is the tool used to manage your sessions, it displays active sessions and allows you to "subscribe" to one. nv (net video) and vat (video audio tool) are the packages used to send/recieve video/audio across the net. Other packages are being created, one of them is imm which allows you to view live snapshots from various satellites every half hour.

• small

• large

the subnets connected to the MBone are:
• cs.umbc.edu
• irc.umbc.edu
• acs.umbc.edu

Multicasting is bandwidth efficient as on e packet can be recieved by all machines on a network. For example 1 audio stream recieved by 1 workstation uses the same bandwidth as the same stream being subscribed to by 15 workstation. Another good feature of the MBone is that the life of packets can be limited, For example a broadcast may be set to last 30 minutes. After that time the broadcast ends and the resources are recouped for new sessions.

• DISTANCE OF TRANSMISSION OF PACKETS
  Another good feature is that the packets may be limited in there transmission radius. There is a ttl setting (time to live) that is used to limit how far a packet might go. For example a university might want to limit a transmission from a general student machine to campus machines only, this would be accomplished by a ttl setting of 16. This can happen because the ttl field is decreased each time the packet is sent through an mrouter. This limiting ability is important because transmissions such as video can use much of the resources of the connections between sites on the internet (T1 and such...)

• How much resources do audio and video use?
  audio usually eats 32 or 64kb/s and video twice that at 128kb/s. Video Multicasts should have lower ttl setting so they do not pass through links with low bandwith.

SOFTWARE LINKS

• software for SGI's

• software for suns
• sd.tcl

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