How Routers & Switches reduce traffic congestion and improve security.
Other articles in this series:
Routers and switches reduce traffic congestion by sending packets and frames only to the devices that require the data and not spraying the whole network with them. As well as reducing congestion this will help improve security as administrators can limit which hosts can exchange data; IP-cameras, IP-production switchers, and computers etc.
An IP packet consists of two parts, the header, and the payload. In the case of video over IP, the camera will break its video stream into smaller chunks until they fit into the payload of an IP packet. The header consists of the source IP address, that is the address of the device sending the packet such as a camera, and the destination address of where the camera wants to send its’ video stream to, for example a production switcher.
If we assume that the network we are using in the studio is CAT6/Ethernet, the camera will have its’ own media access control (MAC) address and the production switcher will also have its own MAC address. These are unique Ethernet addresses that are hard coded into the devices during manufacture and are different for every Ethernet enabled unit that leaves the production line.
A network often consists of routers and switches where switches distribute traffic within the locality of the connected devices, and routers route packets between localized networks. For example, studio-1 consisting of four cameras and a production switcher will only distribute data within studio-1, however, if camera-1 from studio-1 needs to be sent to studio-2, then a router is required.
Ethernet networks are defined as a network which share the same Ethernet broadcast address. It is possible to have multiple switches cascaded across many studios but whenever an Ethernet broadcast message is sent, for example, when using the ARP protocol, then every device in every studio will receive this message, thus creating unnecessary traffic and potentially increasing latency. To stop this from happening, routers are often used to separate the networks, so the broadcast traffic is only sent to the devices within the locality of a studio.
VLANS also solve this problem and are discussed in a later chapter.
In diagram two we have a simple studio network where a production switcher is connected to three cameras via two different layer-2 switches. It is possible to connect all these devices together on one switch but that would leave us with a single point of failure and probably over-load the switch due to the high bandwidths video-over-IP demands.
The router uses a combination of the IP address and netmask, called the Network-ID to route packets to other routers. A typical network ID in the class-less system would be 10.1.1.0/24. The “24” refers to the subnet mask and the 10.1.1.0 is the network. In this instance the router would route any packet in the address range 10.1.1.000 to 10.1.1.255, as it only looks at the first 24 bits of the IP address, and each number within the dot is 8 bits. As another example, a network ID such as 10.1.1.0/8 will contain all hosts with an IP address range from 10.000.000.000 to 10.255.255.255. Subnets are often aligned with Ethernet networks to aid administration and security.
Static routers work on a next-hop system, that is each router only knows of the existence of subnets connected to its ports, this simplifies the network design and keeps the router database to a manageable size.
In diagram two, Ethernet is being used as the physical interface and Camera-1 will have to set the source and destination MAC addresses of the Ethernet frame leaving camera and entering the switch port. A MAC address is a unique number issued by the IEEE for every piece of equipment that is manufactured. When the IP packet leaves camera-1 it must also set the Ethernet MAC source and destination addresses for the encapsulating Ethernet frame, as well as those for the IP datagram.
A system called address resolution protocol (ARP) is instigated by camera-1 which broadcasts an Ethernet message saying “who has IP address 10.0.3.1 (the production switcher)? And what is your Ethernet address?”. The broadcasting of ARP messages is restricted to local subnets and routers to stop the entire network being flooded with ARP query messages.
At this point switch A answers the ARP query from camera-1 with the address of the MAC address for port-1. Camera-1 sets its own MAC address as the source MAC address, and the destination address is the MAC address of port-1 on switch A. Port 5 on switch A will have a different MAC address than ports 1 to 3 as it’s connected to a different network segment.
Each switch is able to build a database of the connected devices so will substitute the destination MAC addresses as required. The switches in diagram one provide an Ethernet network so, if the studio technical director wants to send camera-1 from studio-1 to studio-2, then a router with the appropriate forwarding tables will need to be provided. This stops unnecessary traffic from studio-1 flooding studio-2, and vice versa.
Figure 3 - Showing the changing Ethernet Addresses and fixed IP addresses as they datagram moves from the camera to the vision mixer.
Throughout the whole process of sending a packet from camera-1 to the production switcher via switch A and switch B, the source and destination IP addresses did not change at all. However, the source and destination Ethernet MAC addresses changed at each network segment.
It’s entirely possible that the cameras on switch A were at the Superbowl stadium, with switch B and the production switcher at the studio in New York, with an IP satellite link connecting them. The source and destination IP addresses did not change, but the MAC and physical connectivity mapping automatically changed as the frames moved between devices. From an IP point of view, we do not know or care how the packet travelled between switch A and B, the underlying complexity of the physical, electrical and optical routing was abstracted away from us. And this is one of the greatest strengths of IP.
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