Tom Knowles, Broadcast systems product manager at SSL knows networking and with kind permission from SSL we reproduce a part of his excellent white paper "Professional AoIP for Broadcast - The Way Forward".
Some people in Pro Audio have found the introduction of networking terminology difficult. The terminology is unfamiliar and for those of us used to a discrete, point to point world, networking is uncomfortably different. If that is anything like you then read this white paper. It offers an accessible but authoritative overview of networking and AoIP from a pro audio perspective.
Here is a selection of entries from the Glossary from the white paper:
Network Layers And The OSI Model
The Open Systems Interconnect model is a standardised model which defines the functions of a communication system by partitioning it into seven abstraction layers. It was published in 1984 in ISO 7498 and also in parallel by the ITU in the X200 standard. If a network device e.g. a switch is “Layer n managed”, it can be assumed that it is capable of performing functions that relate to the given OSI layer as shown below. It may also be assumed that if a switch is layer 3 managed, it is also capable of managing layer 2 functionality.
- Layer 1 – Physical Layer • Defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fibre optic cable).
- Layer 2 – Data Link Layer • The data link layer provides a reliable link between two directly connected nodes. It detects and possibly corrects errors that may occur in the physical layer. VLANs are layer 2 constructs.
- Layer 3 – Network Layer • The network layer provides the means of transferring datagrams. Datagram delivery at the network layer is not guaranteed to be reliable. A number of layer-management protocols belong to the network layer. These include routing protocols and multicast group management.
- Layer 4 – Transport Layer • The transport layer provides reliable transmission of data packets between nodes (with addresses) located on a network.
- Layer 5 - Session Layer • Establishes, manages and terminates the connections between the local and remote application.
- Layer 6 - Presentation Layer • Transforms data into the form that the application accepts.
- Layer 7 – Application Layer • Interacts directly with software applications running on a host e.g. determining identity and availability
Classification Of Traffic
- Unicast - Unicast packets are sent from one source to one destination, identified by its IP address. In unicast communications, every copy of a signal – even identical signals – is a point to point connection with its own bandwidth requirement.
- Multicast - Multicast packets are sent from one source to many destinations on a network subnet. Multicast communication allows a saving in network bandwidth to be made when one source is being sent to many destinations. Using multicast communication, a sending device requires only one unit of bandwidth per discrete signal sent. E.g. a one-to-many distribution system. Replication and distribution of a multicast signal is performed by network switches or routers. Devices may subscribe or unsubscribe from receiving multicast traffic.
- Broadcast - Broadcast packets are automatically forwarded to all devices on the subnet to which they are connected. Broadcast traffic is usually reserved for discovery and DHCP services which, by their nature, must be able to contact every device on a network segment without initially knowing their address.
VLANs - In computer networking, a single network may be partitioned to create multiple distinct domains, which are mutually isolated so that packets can only pass between them via one or more routers. These domains are referred to as Virtual Local Area Networks, or VLANs. VLANs are layer 2 constructs, compared with subnets, which are layer 3. It is possible to have multiple subnets on one VLAN but not multiple VLANs on one subnet.
Subnets - Subnets divide a large network into smaller networks for performance and security reasons. subnetting involves the separation of the network and subnet portion of an IP address from the host identifier. Devices on one subnet cannot communicate with devices on a different subnet without passing through a gateway.
MAC Address - A Mac address is a globally unique hardware identifier assigned usually by the hardware vendor and stored in ROM. It is notated as a six digit Hex number, e.g. 0a:1b:2c:3e:4e:5f
- Ethernet Hub - A multi-port repeater operating on OSI Layer 1. A hub forwards any signal received to all its ports except the originating port. It contains no memory or routing logic but may send jam signals if a collision (more than one device transmitting at once) is detected.
- Ethernet Switch - A ‘smart’ repeater. Buffers, processes and forwards packets only to devices who request or require them at a link speed which has been predefined or negotiated. A Switch may be managed and perform security or traffic management functions.
- Gateway - A gateway is a type of router which allows data to be sent from one subnet to a different subnet. If a device sends data to a device outside its own subnet, it will be automatically routed to the gateway (assuming IP settings on the originating device are set correctly).
- Router - A router may also direct traffic between networks, but is responsible for finding the best physical route for traffic to take to reach its destination. Routers prioritise traffic based upon type, network load and policy.
Clocking And Time Protocols
- NTP - (Network Time Protocol) Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency networks. NTP is intend - ed to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC).
- PTP - (Precision Time Protocol) Precision Time Protocol (PTP) is a protocol used to synchro - nize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range. In audio Over IP systems, each device has its own highly accurate internal clock, whose drift relative to the master clock in the system is controlled by PTP messages.
- NTP vs PTP Using NTP vs PTP for network/system timing all comes down to the accuracy needed. If the system accuracy needed is measured in microseconds or nanoseconds then PTP (IEEE 1588) is required. If the accuracy needed is only required to milliseconds or seconds, then NTP is sufficiently accurate.
Why is PTP so accurate? Because hardware timestamping is commonly implemented in PTP technology, but not in NTP. Hardware timestamping is allowed in the client and server devices which are running NTP, but not many devices im - plement this. The largest source of error in network timing is often due to the variations in queuing time in switches and routers. NTP does not have a solution for this, PTP does.
QOS - Quality Of Service
QOS is an industry-wide set of standards and mechanisms for ensuring high-quality network performance for critical applications. By using QoS, network administrators can prioritise allocation of existing resources efficiently and ensure the required level of service for defined classes of network traffic at the expense of less time-critical services.
QoS must be enabled to allow Dante to share network infrastructure with other types of data and signals. In many cases QoS in installed network switches may already be enabled, if not the network switches need to have the Basic mode of QoS enabled, checking that the switch is using DSCP (Differentiated Services Code Point). Education and use of common language between broadcast and network - ing engineers is key in successfully leveraging the advan - tages in AoIP technology. The network specialist may want to know more about the DSCP labels Dante uses. Audinate publish the DSCP priority values for Dante.
IGMP - (Internet Group Management Protocol) IGMP is a network protocol which provides a way for a network device to report its multicast group membership to adjacent switches and routers. Multicasting allows one computer on the Internet to send content to multiple other computers that have identified themselves as interested in receiving the originating computer’s content. If it is desir - able to send the same data to many devices at the same time, a single multicast stream provides significant savings in network bandwidth over using multiple unicast streams all sending the same data.
Flows - Flows are a construct of Dante and can be visualised as a bundle of audio channels streaming across a network. Routing audio in a Dante network automatically creates flows, which carry one or more channels of audio from a transmitting device to one or more receiving devices. Flows may be either unicast or multicast. Unicast routing creates flows to a single receiving device; a unicast flow typically assigns space for 4 channels of audio. Multicast routing creates flows that can be received by multiple receivers. Multicast flows are assigned IDs, enabling them to be identified in Dante Controller.
Find Out More
Head over to the SSL website to read the entire white paper. Thanks to SSL for permission to reproduce an extract here.