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Network requirements

NOT Cable runs on the local network. An internet connection is not required for the broadcast itself. This page covers the few things the network has to provide.

Ports#

Two UDP ports are used.

Port	What it carries	Direction
49000	Clock requests and responses	Between each receiver and the Studio
49100	Audio packets	From the Studio to every receiver

Both are UDP, not TCP. If a firewall asks, open both ports for the local network. On Windows that means marking the network profile as private, not public.

Multicast#

The broadcast is delivered by multicast. The default group address is 239.255.77.77 on port 49100.

What this means: the Studio sends each packet once and the network delivers it to every joined receiver. Four points or three hundred, the load on the network stays the same.

Only the clock exchange is per-receiver, and it is very small.

Multicast puts one stream on the network; unicast puts one per receiver.

On a network where multicast does not pass, a receiver falls back to unicast. The broadcast keeps working, but each receiver then means a separate stream on the network.

Switches and routers#

Most home and office networks need no extra setup. On installations with managed switches, look at two things.

If IGMP snooping is on, the ports joining the multicast group must be learned correctly. A switch that cannot do this never delivers the broadcast to some ports.
A querier must be present on the network. Without one, some switches time out group membership and the sound cuts off after a few minutes.

Receivers and the Studio must be on the same subnet. Multicast does not cross between subnets by default.

Wireless#

Cable is always better, but wireless works. At wireless points watch the following.

Prefer 5 GHz. In crowded places 2.4 GHz raises packet loss.
Do not leave the multicast rate low on the access point. Some devices send multicast at the lowest rate, which slows the whole cell.
Client isolation must be off. With it on, a receiver cannot find the Studio.

An occasional drop at one wireless point does not break the whole broadcast. The receiver finds the clock again and returns to place on the next packet.

Bandwidth#

One broadcast stream takes as much room as its chosen bitrate. With multicast that figure is not multiplied by the number of receivers.

A rough example: a 128 kbit/s broadcast takes about the same room on the network with four receivers as with three hundred. Falling back to unicast changes this, and the stream multiplies per receiver.

Discovery#

The Studio sends small announcement packets onto the network. A Receiver listens for them and finds the Studio by itself. No address has to be typed in.

If a receiver never appears in the list, check in order: same subnet, firewall allowing UDP 49000 and 49100, client isolation off on wireless. See Troubleshooting for more.

Rough arithmetic#

What a broadcast takes on the network comes from its bitrate. As long as multicast is in use, that figure is not multiplied by the number of receivers.

bytes/s  = bitrate / 8
per hour = bitrate · 3600 / 8

128 kbit/s  →  16 kB/s  →  about 57.6 MB per hour
64  kbit/s  →   8 kB/s  →  about 28.8 MB per hour

That is the audio payload itself. Each packet also carries the 48-byte NAP header, the 4-byte checksum and the UDP and IP headers beneath them, which comes to roughly 80 bytes of overhead per packet. On a broadcast sending fifty packets a second, that overhead is about four kilobytes a second.

Next to this the clock channel is negligible: one request and one response per receiver per second, eighty bytes in total.