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In the world of Ethernet data, it seems that the words ‘frame’ and ‘packet’ can be swapped and it makes very little difference.
Both frames and packets are electronic containers that cart data from point-to-point by navigating LANs and WANs and, as both essentially achieve the same end-result, it can be tough to appreciate the differences between the two.
So what separates frames from packets? To begin, think of frames and packets as envelopes of information that will be sent from one person to another. The main difference is how each encapsulates the information and this depends on where the information’s target destination is located.
How an Ethernet frame works
One handy way to understand frames is to envisage a company with an internal mail department. A worker can send documents to a colleague within the confines of their organisation. The contents go into an internal envelope, the sender writes their name and department in the ‘From’ field, then writes the recipient’s name and department in the ‘To’ field.
Once sent, the mail room recognises it is an internal-use envelope, checks the destination name and department, uses a directory to convert that information into a physical location (i.e. a building or office) and delivers it to the recipient.
Never at any point does the envelope leave the organisation and all of the envelope’s movement is handled by local resources familiar with the environment. If it did, the envelope’s content would need to be in a postal envelope with a proper postal address.
This is similar to how an Ethernet frame works. It is suited to sending data between two locations on the same network. The name and department are replaced by the source and destination address of a frame, which are the MAC (Media Access Controller) address of a computer, tablet, IP Phone or IoT device. This is an ID number that is unique to every Ethernet device on the planet.
Frames are generated at Layer 2 of the TCP/IP stack by the network interface device with a payload size that depends on the type of data being transmitted. The frame goes to the network where an Ethernet switch checks the destination address of the frame against a MAC lookup table in its memory. The lookup table tells the switch which physical port, i.e. RJ45 port, corresponds to which device whose MAC address matches destination address of the frame.
The switch will send on the frame to the physical port determined by the lookup table. If the cable is connected directly to the destination device, the transmission process is complete.
If the cable is connected to another switch, the next switch will repeat the lookup and forward process until the frame reaches the intended destination.
Packets
Packets are created at Layer 3 of a network and let information to be exchanged between different LANs, typically via routers.
A router connects small networks (LANs) together allowing for information exchange on a much larger scale using IP addresses for packet forwarding instead of MAC addresses.
Layer 3 packets allow routers to transmit inter-network data using IP addresses that identify the network and the temporary address of the device on the network.
Once inside a network, intra-network (LAN) data forwarding is handled by Layer 2 switches that read the MAC address of the frame to forward it to the destination device where the Ethernet controller extracts the data payload completing the process of transferring information between devices on different networks.
