Question

Q.8. Explain 8B/10B,4D-PAM5 encoder in 10 Gigabit Ethernet.

Answer 1

10-Gigabit Ethernet is basically the faster-speed version of Ethernet. It will support the data rate of 10 Gb/s. It will offer similar benefits to those of the preceding Ethernet standard. However, it will not support the half-duplex operation mode. The potential applications and markets for 10-Gigabit Ethernet are enormous. There are broad groups of users who demand 10-Gigabit Ethernet, for example, enterprise users, universities, telecommunication carriers, and Internet service providers. Each market typically has different requirements for link span and cost.

There are several coding techniques that might be adopted in the 10-G Ethernet standard. 8B/10B is one such encoding technique.

This code was invented by IBM for low-cost devices and implementations. It has an excellent DC balance property with a maximum run-length of 5 and a good transition density, which simplifies the requirements on bandwidths and clock jitters for device circuits. With an input/output bit rate of 8/10, this code can improve transmission reliability by some degree. It can also provide built-in special characters for commands, synchronization, and delineation. The encoding/decoding algorithm for this code is simple and can be implemented in low-cost hardware. Apart from the aforementioned benefits, this code has another key advantage. It was adopted in the recently released, 1-Gigabit Ethernet standard (IEEE 802.3z), making it very attractive to the 10-Gigabit Ethernet. The use of 8B/10B coding will make 1-Gigabit Ethernet and 10-Gigabit Ethernet truly compatible, enabling a seamless integrated Ethernet network with less cost, effort, and time for technology migration. This advantage will likely give an edge to the 8B/10B coding in the early deployment of the 10-G Ethernet standard for LAN/MAN. The main drawback of this coding technique is the 25 % overhead and less bandwidth efficiency. That is, a line rate of 12.5 Gb/s is needed to implement a transmission rate of 10 Gb/s with the 8B/10B encoding. For a serial implementation, this is a significant disadvantage because currently there are a larger number of devices that can support 10 Gb/s, but very few at 12.5 Gb/s.

Another encoding is PAM-5. This coding technique was adopted in the1000-Base-T standard. It employs multi-level amplitude signaling to increase the number of bits per baud. In the twisted-pair lines, the PAM-5 encoding can achieve 2 bits per baud with a 3 dB coding gain, yielding a significantly lower line rate. For fiber channels, however, more works need to be done to study the impact of the ISI, SNR, and non-linearity penalty on a multi-level amplitude signaling. The PAM-5 signaling may not travel in the fiber very far, and the current devices may not support this signaling. Until these concerns are rigorously answered, the PAM-5 encoding will not compete with the 8B/10B and scrambled encoding.