Ethernet and streaming

Ethernet clock

There is no specific clock signal passed on an Ethernet cable. The Ethernet specification requires that the transmitted data be self-clocking. This is accomplished through several mechanisms. First is that each Ethernet frame begins with a predefined set of 64 bits. Fifty six bits are the preamble which consists of alternating zero and one values. The next 8 bits indicate start of frame and contain the values 10101011.

The receiving station uses the preamble to sync its clock to the transmitting stations clock. This synchronization is performed for every frame received. The receiving station maintains synchronization by recognizing at least two voltage changes over five bit times.

If synchronization is lost the receiving station will discard bits until it synchronizes at the start of the next frame. After a frame has been transmitted the transmitting station will wait 96 bit times (Inter-Packet gap) before it starts transmitting the next frame.

Ethernet Switch

Ethernet switches connect multiple devices together by physically cabling devices to the same switch or devices connected to another switch.

Every Ethernet compatible device has a hard coded physical address called a MAC address that the connecting switch uses to uniquely identify a device. When a switch receives an Ethernet frame, it stores the sending device's MAC address and the port it is connected to in a locally held table called a MAC address table. The switch then checks the MAC address table to see if the destination MAC address is connected to the same switch. If it is, the switch forwards the frame to the known destination port. If not, the switch broadcasts the frame to all ports.

Ethernet switches can not and do not change the physical properties of the data being transmitted.

Summary

Ethernet cables only transmit two values 0 or 1.

The bits transmitted on the cable are not the actual data but a representation of the data.

Copper Ethernet cables can transmit electromagnetic noise, absorb electromagnetic noise from the environment and generate electromagnetic noise.

Ethernet design increases immunity to noise, thus reducing transmission bit errors.

Fiber optic Ethernet cables do not transmit electromagnetic noise, absorb electromagnetic noise from the environment or generate electromagnetic noise.

There is no specific clock signal passed on an Ethernet cable the data is self-clocking. An Ethernet receiver re-clocks with the transmitter at the start of every frame.

Ethernet, IP and TCP operate on a block basis (frame/packet). A checksum is used to detect bit errors. If an error is detected the entire block is discarded and eventually retransmitted.

Ethernet switches can not and do not change the physical properties of the data being transmitted.

The music file is transmitted in small discrete chunks and reassembled in destinations system memory. The analog signal is generated by transferring bits from system memory and not from the Ethernet cable.