In the previous article we looked at how a network can be resilient and heal itself if a link fails. In this article we continue the theme of looking at a network from a broadcast engineers’ point of view so they can better communicate with the IT department, and look at configuration of host devices such as cameras and vision mixers.
DNS and DHCP
IT convention refers to user equipment connected to routers as hosts. This could be a desktop computer, laptop, camera, vision mixer or sound desk for example. All these devices share similar configuration in the way they are connected to IP/Ethernet networks.
There are two ways to set up a computer on a network, either manually, or using automatic configurations. The two key parameters that need to be configured are the IP address, and the details of domain name system. DNS is a server service to associate web names with IP addresses, and is not entirely relevant to cameras or vision mixers.
IP configuration can be automated using dynamic host configuration protocol (DHCP). This is a service running on a network server which automatically provides an IP address for a host when it boots up. This is generally not used for broadcast kit as we want to be able to identify cameras and vision mixers through their IP addresses. It’s possible for a host to change its IP address when using DHCP without warning. For these reasons we tend to use static manually configured IP addresses.
Typical PC Network Configuration
The three key parameters that will need configuration are the IP address, subnet mask, and default gateway. When setting the IP address care must be taken not to use an address which has already been used, doing so will result in IP ghosting and provides some very interesting results as other hosts may try to send and receive datagrams to a camera without realising they’re addressing the wrong device.
Two conventions of subnet are in use; classful and classless. Classful is an older rarely used system often referring to class A, B, C and D subnets, using discontinuous address ranges. This was considered too complicated in normal use and has largely been dropped in favour of the classless system. Classless uses a sequence of bits to define an address range. For example, 10.0.1.1/24 refers to the range 10.0.1.0 to 10.0.1.255. Host configuration of the subnet value tends to still use the dot notation. As an example, 10.0.1.1/24 would give a subnet of 255.255.255.0, and 10.0.1.1/20 would give a subnet of 255.255.200.0, that is the first 20 bits of the IP address.
The combination of the IP address and the subnet is often referred to as the network ID. In address 10.0.1.0/24, the part 10.0.1.xxx is the value of the subnet the camera is connected to. The camera will be able to send and receive datagrams directly to all devices in the range 10.0.1.0 to 10.0.1.255.
IP Addresses Don't Change
Default gateways are used by the host to work out what to do if they need to send data to a host outside of its subnet. For example, if we have a camera with address 10.0.1.0/24 and vision mixer with address 10.0.2.0/24, the camera would not be able to send its datagrams directly to the vision mixer as they are on different networks. The camera is on network 10.0.1.xxx and the vision mixer is on network 10.0.2.xxx. In effect they are physically separated.
Screen grab of PRINT ROUTE showing the routing table of a host PC and the default gateway is 192.168.0.1
Each host, whether it’s a camera, sound desk or desktop computer, will have its own routing table. This consists of a series of networks the host can see, and how it routes to the ones it can’t.
When the camera needs to send its datagram to the vision mixer, it will look up the vision mixers network ID in its own routing table and realise it doesn’t have a listing for network 10.0.2.0/24. The default gateway is then used to resolve this.
Ethernet MAC's change
When routing an IP datagram, the source and destination IP addresses are kept intact throughout its whole journey from source to destination. However, the source and destination Ethernet MAC addresses do change at each node.
When the camera realises it cannot send its datagram directly to the vision mixer, it will send it to the default gateway instead. It does this by finding the Ethernet MAC address of the default gateway, then setting the destination MAC address of the datagram to be the MAC address of the default gateway. It’s important to note, that the source and destination IP addresses do not change.
Screen grab of ARP -a showing the Ethernet MAC address of the some of devices on the hosts network, specifically the default gateway of 192.168.0.1.
The default gateway address must be accessible by the host so will always be within the subnet of the hosts IP address.
Lack of Tools
Configuring cameras and sound desks can be frustrating for broadcast engineers due to the lack of available tools within the equipment. For example, a PC will have command line programs such as IPCONFIG to show the connectivity of the network interface card, ARP to show the resolution of IP addresses and MAC addresses, and ROUTE PRINT to show the hosts routing table configuration. IP interfacing in broadcast kit sometimes appears to be a bit of an add-on, and the tools taken for granted in the IT world are generally not available.
Industry initiatives such as AMWA/NMOS Registration and Discovery system will help overcome this. But in the meantime the broadcast engineer has to collaborate closely with their IT colleagues to ensure broadcast IP configuration is configured correctly.
In part 5 we will apply the knowledge gained from the past four articles to look at a real life example of audio over IP.
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