By leveraging multicast, organizations can optimize network bandwidth usage and support a larger number of concurrent users in video conferencing, multimedia broadcasting, and other similar scenarios. This makes multicast a key component of UC and VoIP solutions, offering a more efficient and cost-effective way to manage communications at scale.
In this article, we examine multicast communications in the context of networking, how it compares with unicast and broadcast communication, and practical steps for leveraging it on your network.
What is multicast?
In an IP-based infrastructure, multicast refers to the underlying capability of a network to deliver information or data from a single source to multiple (but not all) destinations simultaneously.
It contrasts with unicast, where a network sends data from one source to a single destination, and broadcast sends it to all possible recipients, regardless of whether they requested it or not. The following diagrams further illustrate these three forms of communication.
The purpose of multicast is to distribute data more efficiently on a network by ensuring that the sender sends only a single stream of data and that only the receivers that have requested the data receive it. To better comprehend the usefulness of multicast, let’s perform the following thought experiment.
Imagine you have a network with 200 hosts and a server on the network that wants to send a stream of data to 80 of those hosts. Both unicast and broadcast will be inefficient for different reasons and in different parts of the network:
Unicast: The server must create 80 separate data streams, one for each recipient. The server would have to process 80 separate simultaneous communication streams, including overhead (such as addressing, session maintenance, and coordination) for each one. Similarly, the network would be burdened with 80 streams of identical, duplicated information, taking up valuable bandwidth, CPU, and memory resources on the server and the intervening network devices.
Broadcast: Here, the server sends a single stream of data destined for all hosts on the network. However, this means that all 200 hosts receive the information. If it is required, they process it; if not, they discard it. This means that the 120 hosts that didn’t ask for this information must still actively examine, process, and discard the arriving packets.
To make matters worse, as the number of hosts and the scale of the delivered network service increase, the inefficiencies also increase. The most efficient solution is to create a framework where a single stream of data is sent from the source only to those hosts that have requested the data. This is what multicast does.
How does multicast work?
Multicast capabilities are built into the IPv4 and IPv6 protocols themselves. Using reserved ranges of IPv4 and IPv6 addresses, a network can assign a particular multicast address to a specific service or multicast source.
This address represents a multicast group. A host can join various multicast groups and become one of the destinations of each group’s multicast data. Similarly, hosts can leave a group without needing to receive its data.
By actively joining and/or leaving multicast groups, hosts can choose to receive or not receive specific multicast data from particular sources. Network devices, such as routers and switches, route the proper multicast traffic to the hosts that have joined the applicable multicast groups.
Similarly, the network also ensures that the hosts that have not joined a multicast group will not receive that group’s data.
The reserved ranges of network addresses used for multicast are:
IPv4: 224.0.0.0/4
IPv6: FF00::/8
Multicast protocols
Several protocols are involved in enabling multicast to operate correctly on a network. Some of these protocols include the following:
Internet Group Management Protocol (IGMP): This protocol is used in IPv4 networks to allow hosts to join and leave specific multicast groups.
Multicast Listener Discovery (MLD): This is IPv6’s counterpart to IGMP. It is used to discover multicast listeners for particular multicast groups in an IPv6 environment.
Protocol Independent Multicast (PIM): This multicast routing framework is used in IPv4 and IPv6 to route multicast traffic. Routers involved in routing multicast traffic must “speak” to each other using PIM to ensure that the multicast traffic of particular groups is routed to the network areas where hosts requesting that traffic exist.
Multicast in VoIP, UC, and video communications
As you probably have surmised, the multicast capabilities of a network are useful for situations where we have communications from one source to many recipients. Within the context of VoIP, UC, and video communications, you would typically employ multicast in scenarios where the same content needs to be delivered to multiple recipients simultaneously, such as the following:
Large-scale meetings or conferences: When one or a few sources need to send audio or video streams to many participants, multicast allows these streams to be sent as a single stream over the network and be received by multiple (but not all) recipients.
Webinars and online training: Multicast is ideal for webinars and online training sessions where the content (audio, video, and presentations) needs to be distributed to many viewers simultaneously. Since the content is the same for all participants, multicast efficiently distributes the data without overburdening the network.
Corporate announcements: For live broadcasts of corporate announcements or updates where employees across different locations are the recipients, multicast offers an efficient way to reach all employees without requiring individual streams for each location.
Paging: Some VoIP and UC systems incorporate a paging capability that allows announcements to be heard throughout the facilities or the organization. Such announcements are ideal to leverage multicast capabilities since the same message is sent to multiple end devices.
Music on hold (MoH): MoH is a service often used in VoIP systems, call centers, and interactive voice response (IVR) systems. Since there are often multiple callers on hold or in a queue, it is more efficient to provide a single multicast audio stream to all callers rather than sending a separate stream for each caller.
Emergency notifications: Multicast technology can be extremely effective for distributing emergency notifications in scenarios where rapid, simultaneous communication to many recipients is crucial. This includes campus-wide alerts and public safety announcements.
How to leverage multicast
As an implementer, it is important to understand the multicast capabilities of the underlying network and how your UC or VoIP solution can leverage those capabilities. In almost all cases, if multicast is not configured on the underlying network, communications systems will typically revert to unicast functionality.
This may result in network inefficiencies, as explained above; in some rare cases, failures in certain services can occur if they require multicast mode.
Typically, VoIP and UC systems do have some parameters that can be configured to leverage multicast whenever it is available, and you should see those in some of the configuration interfaces provided by each system.
These are typically check boxes that you can enable to allow the use of multicast for certain services. Beyond this, there isn’t much involved in configuring parameters for multicast for business communications applications.
The magic of multicast and managing the complexity of its implementation are dependent on the proper configuration and parameterization of the underlying network.
If it is configured correctly, multicast functionality takes place largely behind the scenes because servers, network devices, and end devices simply leverage the use of IGMP, MLD, and PIM to make it work.
Where is multicast implemented?
Multicast is particularly advantageous in networks where bandwidth conservation is critical, and the infrastructure supports multicast routing and management. However, its implementation requires careful planning and consideration of network capabilities because not all network equipment supports multicast or is configured to handle it efficiently.
Additionally, multicast is more beneficial in controlled network environments like corporate, enterprise, or closed university or educational networks. It is generally unavailable on the public internet, although some ISPs may implement it within their networks.
There are many reasons for this, but they are beyond the scope of this article. Essentially, this means that UC and VoIP systems that use the public internet to interconnect end devices (such as in a mobile or distributed workforce) won’t benefit greatly from multicast.
Conclusion
Multicast holds significant promise for enhancing UC and VoIP systems by enabling the efficient, scalable distribution of real-time media streams across multiple recipients.
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