A question popped into my head today and after a quick few searches on google, I have no idea. When a device is sent a frame there is a layer 1 header/preamble saying the length and layer 2 protocol (I think please correct me if I'm wrong), but how does the NIC know where to start reading the data from? If it starts reading from the first 1 what happens if that 1 was just noise on the line? If any clarification is needed please ask. (Also no idea what tags to put for this.)
NICs – How They Detect the Start of a Frame
ethernetlayer1layer2osiprotocol-theory
Related Solutions
A Layer 2 switch learns most of its information about the location of other endpoints via "listening" to ingressing frames, and when it is not aware of the location, it uses floodingand will learn from the answer. Lets say the topology is:
(Host A) <--> (Switch A) <--> (Switch B) <-->(Host B).
Also important to note, a L2 Switch forwards, it does not route. By that I mean, when it receives an encapsulated frame, it forwards it out interfaces.
- Host A wants to get to Host B. Host A knows Host B's MAC address. So Host A sends a frame out to Switch A.
- Switch A does not have Host B in its MAC address Table. Switch A then flood out the frame on all of its ports, except for the one Host A is located. Among all those ports there is the ports that Switch B is plugged into, so Switch B receives the frame.
- Switch B then flood that frame out all of its ports (considering Switch B does not have Host B in its MAC address Table).
- Host B will respond to the frame, Switch B will then record Host B MAC address and send the response to Switch A. Switch A then records the MAC address of Host B in its table and send the response to Host A
Here's a related question: How does a switch learn a switch table?
Dear you must know that there is no thing called NIC frames because NIC is OSI layer 1 device such like hubs and cables so it simple generate bits (0s and 1s) , each OSI layer generate different type of Data called protocol data unit PDU and special device can understand it as next
- L1>>Bits (Cables,NIC,HUB)
- L2>>Frames (L2 Switches)
- L3>>Packets (L3 switches and Routers)
- L4>>Segment (Fire walls)
- L5:L7>>Data (PC and Servers)
Where each layer adds its header on the previous PDU and generates its own PDU in the encapsulation process and vise versa in the decapsulation process
For example during the encapsulation process L5 (Data) encapsulated in L4 header and generate new PDU called Segment and during the decapsulation process L4 segment header will be striped off to get Data from it as the next
now we can answer your question , each Layer add functional header to the previous layer for example L3 will add header indicate source and distination L3 address (IPs) or some times called logical address and other informations provide end to end delivery insurance and so on as next
Best Answer
Ethernet sends seven octets of preamble (
10101010) to synchronize, and then a one octet SoF (Start of Frame) delimiter (10101011). The next bit is the first bit of the frame. The frame has a four octet FCS (Frame Check Sequence) that is a CRC, and if the received CRC doesn't match the calculated CRC, then the frame is dropped as damaged.