CCNA RnS #12 : Ethernet Frame Type & FCS Field!


CCNA RnS Article – Ethernet Type and FCS

In this article let’s talk about two more fields in Ethernet Frame – EtherType and FCS.

Figure 1: Ethernet Frame Format

The EtherType field in the Ethernet frame header is to help network processing function on routers and hosts. This field identifies the type of layer 3 protocol packet that this frame carries.

The originated host inserts a value to identify the type of packet encapsulated inside the Ethernet frame. IEEE manages these EtherType values. This list can be found at IEEE’s official site

The Type value of 0800 indicates the frame carries the IPv4 information and the value 08DD indicates IPv6 information.

The FCS field in Ethernet Header is used for Error Detection. Using this field devices find out whether a frame has changed while traveling a network. The reason for errors in an Ethernet network is electrical interference or a bad NIC/cable.

Before sending the frame over a network, the sender applies a formula to the frame and stores the result of the formula in the FCS field. The receiver also does the same formula to the received frame. The receiver then compares the value it received from the sender and its own result. If the values are the same, it means the frame did not change otherwise an error occurred and the receiver discard the frame.

The FCS field is only for error detection and does not perform any error recovery. Ethernet just discards the corrupt frames and does not try to recover the lost frame. Error recovery is performed by upper layer protocols stack like TCP.

Before closing this article, let’s cover one more topic of duplex communication.

If we talk about the initial LAN setup before switches were introduced, there was LAN hubs used to build a network. Like switches, LAN hubs also provide multiple RJ-45 ports that can be used to provide connections to users or interconnect. However, they differ in terms of communication capabilities.

Devices connected to the LAN hubs must wait to send if it is currently receiving a frame. I mean a device connected to a hub can either send or receive at a time.

In contrast to a half-duplex, there is a full-duplex. In full-duplex device does not need to wait before sending. I mean it can send and receive at the same time. Hope you find it informative!

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