Exploring the efficiency of half-duplex Gigabit Ethernet Switches

Explore the efficiency of half-duplex Gigabit Ethernet switches. Recently, GE Ethernet switches have been favored by many friends. Today, let's take a closer look at the charm of GE Ethernet switches! To solve the efficiency problem of half-duplex Gigabit Ethernet switches, IEEE introduces the frame burst technology.

For example, if a DTE sends a 64-byte frame, MAC will add a 512-64 = 448byte carrier extension sequence to it. If the frame length sent by DTE is greater than 512 bytes, the MAC will not change. In the case of carrier expansion, the problem of half-duplex Gigabit Ethernet switch distance coverage is solved, but a new problem is introduced: the transmission efficiency of Ethernet frames with smaller length is reduced.

For a 64-byte frame, although the transmission speed is 10 times faster than that of Ethernet, the transmission time is increased by 8 times. This is not much more efficient than fast Ethernet. To solve the problem of half-duplex Gigabit Ethernet switch efficiency, IEEE introduced the frame burst technology.

Frame bursts work as follows: For the first frame sent by a DTE smaller than 512 bytes, the carrier is still extended to bytes, however, the shorter frames sent less than 65536 bytes later do not use carrier extension. Instead, the shorter frames are sent consecutively after the 96-bit frame interval sequence is added, and the maximum burst length is bits.

The reason for this is that, in a correctly configured network environment, if a DTE starts to send data, other DTE can detect its signal through the carrier listening protocol and suppress its own data transmission. The efficiency of a half-duplex Gigabit Ethernet switch with burst frames is improved. When a DTE continuously bursts 64-byte frames and the burst lasts 65536 bits, the efficiency is about 72%. 1.2.2 full-duplex Gigabit Ethernet switch MAC layer protocol in full-duplex Gigabit Ethernet switch.

Because each Gigabit Ethernet switch DTE exclusively occupies one channel during communication, you do not need to consider Ethernet conflicts. Naturally, full-duplex Gigabit Ethernet switches are not limited by the length of the time slot, and thus there is no distance coverage limit. Compared with the half-duplex mode, the MAC layer of the full-duplex Gigabit Ethernet switch has the following differences:
◆ Frame sending during the reception activity will not be postponed
◆ Conflicting indication in full duplex mode will be ignored
◆ No carrier extension, the minimum frame length is still 64 bytes
◆ No frame burst

In full-duplex switched Ethernet, if multiple input ports output data to one output port at the same time, congestion will occur on the output port, at this time, some frames sent by the input and exit will be discarded. If a TCP/IP packet is carried on an Ethernet frame, the TCP transmission mechanism automatically resends the discarded packet, we can imagine that every input port that generates packet loss will resend the packet, resulting in a new round of congestion and packet loss. As a result, the network throughput is greatly reduced.

To avoid Frame loss and retransmission, IEEE introduces the 802.3x traffic control protocol on the MAC layer to avoid packet loss. The principle of traffic control is that when a Gigabit Ethernet switch detects a congested port, it will send a pause frame to the input port to notify it to suppress the sent traffic and eliminate the congestion.