Understanding IP Broadcast Production Networks: Part 4 - Host Configuration

All devices on a network are referred to as ‘hosts’ and they all need to be configured correctly.

IT convention refers to user equipment connected to routers and switches as hosts. This could be a desktop computer, laptop, camera, production switcher or sound console 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 details, 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 production switchers.

IP configuration can be automated by using Dynamic Host Configuration rotocol. 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 production switchers 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.

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 packets to a camera without realizing they’re addressing the wrong or same 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 favor 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 255.255.240.0, 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.nnn is the value of the subnet the camera is connected to. The camera will be able to send and receive packets to all devices in the range 10.0.1.0 to 10.0.1.255.

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 production switcher with address 10.0.2.0/24, the camera would not be able to send its packets directly to the production switcher as they are on different networks. The camera is on network 10.0.1.nnn and the production switcher is on network 10.0.2.nnn. In effect, they are physically separated.

Each host, whether it’s a camera, sound console 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 packets to the production switcher, it will look up the production switchers network ID in its own routing table and realize it doesn’t have a listing for network 10.0.2.0/24. The default gateway is then used to resolve this.

When routing an IP packet, the source and destination IP addresses are kept intact throughout its whole journey from source to destination (unless Network Address Translation is used). However, the source and destination Ethernet MAC addresses do change at each node.

When the camera realizes it cannot send its packets directly to the production switcher, 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 packets to be sent to be the MAC address of the default gateway. It’s important to note, that the source and destination IP addresses do not change.

Figure 1 - Screen grab of PRINT ROUTE showing the routing table of a host PC and the default gateway.

Figure 1 - Screen grab of PRINT ROUTE showing the routing table of a host PC and the default gateway.

The default gateway address must be accessible by the host so will always be within the subnet of the hosts IP address.

Figure 2 -  Screen grab of ARP -a showing the Ethernet MAC address of some of the devices on the hosts network, specifically the default gateway.

Figure 2 - Screen grab of ARP -a showing the Ethernet MAC address of some of the devices on the hosts network, specifically the default gateway.

Configuring cameras and sound desks can be frustrating for broadcast engineer’s 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 must collaborate closely with their IT colleagues to ensure broadcast IP configuration is configured correctly.

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