As consumer demands for digital communication technology continue to shift and evolve, it can be challenging for some manufacturers and software vendors to keep up: from desktop to mobile, analog to digital, and faster this to more portable that. Networked audio is among those forms of digital communication technology that is being adopted more and more.
While most AV professionals understand what networks and audio are, explaining how networked audio works, and what its benefits are, requires a deeper dive into the realm typically inhabited by IT.
In an effort to provide an educational tool for AV professionals who want to understand some of the IT in AV, this series of blog posts will discuss the general components of networked audio, its real world applications, and its promise as a mature digital communication technology.
The Anatomy of a Network
Think about a network like the postal service, and computers or network devices as houses with individual addresses. When networked devices talk to each other, they are communicating by putting data/mail into a proverbial mailbox. As the postal service, the network takes the data/mail, processes it through its sorting machines (the layers of a network), and carries it to its designated location (another device).
In order to equip you with as many technical tools as possible to understand networks and networked audio, you’ll need to know the two models for describing the process of networking: the seven-layered Open Systems Interconnection (OSI) model and the five-layered Transmission Control Protocol and Internet Protocol (TCP/IP) stack. Both models were developed around the same time by different organizations as a means of organizing the processes required for communication of devices via a network into abstract layers.
The main difference between the two configurations is that TCP/IP Layer 5 (“Application”) encompasses Layers 5-7 of the OSI model.
TCP/IP Network Stack & OSI Model
Each of the layers passes data to the layer above it and is served by the layer below it, adding bits to each packet for proper routing over the network, and processing by the end device or program. When data reaches the top layer, it passes over the Cat-5/6 cabling to its designated location. Here’s a description of the purpose of Layers 1-5 of the TCP/IP stack below.
Layer 1: Physical
The physical layer consists of the basic networking hardware of a network (Cat¬5/6 cable, fiber, switches, routers, and network interface cards, for example). It defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. Another way to describe its purpose is that the physical layer translates logical communications requests from the Data Link layer (Layer 2) into hardware-specific operations that affect transmission or reception of electronic signals.
Layer 2: Data Link
When a data/mail packet is sent over the physical network components, that sending computer adds its device-specific MAC address to the packet. This addition to the sent data packet now allows devices on the local network to talk to each other. It’s like stamping an envelope before putting it in the mailbox: it means this message is ready to be sent.
MAC addresses only operate on Local Area Networks (LANs). They cannot go through a router because a router will only accept the addressing scheme from Layer 3. So, the MAC address is used specifically for local communications on the network.
Layer 3: Internet (Network Layer in OSI model)
The third layer of a network is responsible for delivering information from one network, through the necessary routers (nodes), and over to another network.
Whenever a node needs to send data to another node on a network, it must first know where to send it. If the node cannot directly connect to the destination node, it has to send it via other nodes along a proper route to the destination node. Most nodes do not try to figure out which route(s) might work; instead, a node will send an IP packet to a gateway in the LAN, which then decides how to route the “package” of data to the correct destination. Each gateway keeps track of which way to deliver various packages of data, and for this it uses a routing table. A routing table is a database that keeps track of paths, like a map, and allows the gateway to provide this information to the node requesting the information.
For example, when you send an email, the data goes to a router at your location, and the router says, “The destination IP address isn’t in my routing table. I’m going to pass you to the outside network.” That email keeps passing through router after router until it finally gets to the destination IP address (the email recipient, for example). The primary responsibility of a router is to do just that: route information from one network to another.
Layer 4: Transport
Continuing with the mail analogy, the Transport layer deals with how the packet is delivered from one point to another. IP only provides a best effort delivery, so the Transport layer is the first layer of the TCP/IP stack to offer reliability.
The Transport layer handles data flow and error checking. If there are any errors in the packet, the receiver signals the sender and a replacement packet is sent. This process is repeated until the piece of data being transferred is complete. Like the postal service, if a piece of mail has incorrect information on it (wrong name or zip code, for example) it is sent back through the system for correction, and sent out again when the errors are fixed.
Layer 5: Application
The fifth layer of the TCP/IP model comprises Layers 5-7 of the OSI model: Session, Presentation, and Application.
This encompassing layer controls the connections between computer applications. It bridges the communication gap between layers by translating the data received from one “language” to another, from the back-end coding of the data/mail to what we actually see on our computer screen.
And there you have the anatomy of a network. In next week’s post about networked audio, I’ll be talking about what exactly networked audio is and why you’d want it. To learn more about what networked audio is, check out the Networked Media Resource Guide in this month’s issue of the Sound & Video Contractor magazine.
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