Network Engineering for Audio Engineers - Part 2: Local area networks

2.3 LOCAL AREA NETWORKS
AoIP is normally contained within a LAN, so that is our focus. When audio leaves the safe and secure world of local area networks, it ceases to be AoIP and becomes streaming media.

2.3.1 Ethernet Switching
Ethernet switching has caused a revolution in data networking. With switching, each device owns all the bandwidth on its link. No sharing and no collisions. Incoming frames are forwarded only to the nodes that need them.

Despite the power of Ethernet switching, its invention was more akin to falling off a log than sawing one in two. The switch builds up a table of what addresses are attached to what ports, which it does by merely examining the source addresses of sent packets. When frames come in, the switch looks into the table, discovers what port owns the destination, and forwards the data only to that port. In the rare case that no entry exists for an address, the frames are "flooded," or broadcast to all ports, to be sure the intended recipient gets it. If a connection is unplugged or there are no data for a long time, the entry is removed. Pretty simple, eh?

The switching operation described above is for the unicast point-to-point communication that is used for typical traffic such as Web, email, etc. But Ethernet switching supports three communication types:

• Unicast means point-to-point, the usual mode for data traffic, as noted.
• Broadcast means that a source's packets are sent to all receivers.
• Multicast means that multiple receivers may "tune in" to the transmission. One source's packets input to the system can be received by any number of output nodes.

Broadcast packets are received by all devices connected to an Ethernet, without distinction or any specific distribution arrangement. The Windows file system, for example, uses broadcasts for a PC to find its partner for a file transfer. A sending device can be 100 percent sure that the intended destination will be found, if it is actively connected to the network. But this comes at a tremendous disadvantage: Bandwidth is consumed on all links, and all devices have to process the message to determine if it is needed at that location.

In effect, broadcasts don't accrue any of the benefits of switching. In a large network, this can be a significant drain on bandwidth and can cause performance to suffer. To avoid this, careful network engineering often breaks up large Ethernets into smaller ones to create isolated "broadcast domains." As we will see, virtual LANs (VLANs) are also a solution. The individual Ethernet segments are then linked together with an IP router, so they appear seamless to users.