The Institute of Electrical and Electronics Engineers (IEEE), based in New Jersey, USA, promulgated EPON.
This privately run professional organization consists of individual members and their employers. Its 802 (LAN / MAN Standards) Committee took on the task of maintaining and expanding the original Ethernet standard some years ago.
The original standard was ﬁrst developed at the Xerox Palo Alto Research Center (PARC) in 1973 – 1974.
The ﬁrst 802 Ethernet standard was approved in December 1982 and, due to its wide acceptance, continued progress, and economy (partly due to scale), Ethernet became the most widely deployed data communications standard in the world. IEEE’s approach has been to keep it simple (KISS) by specifying the minimum requirements to make things work together.
The organization mostly focusses on only the two lowest layers of the ISO 7 layer model which is widely used to categorize data communications systems. It is left to industry to ﬁll in the gaps, developing or using other standards as needed. With various working groups and strong leadership, the IEEE was able to produce new versions of the standard very fast, hence the lead of several years enjoyed by 10G-EPON when compared to the ITU XG-PON standardization effort.
Although the KISS model works very well for the IEEE, it did result in some gaps that had to be plugged to make the EPON system fully functional for commercial telecommunications. The IEEE therefore formed the SIEPON Working Group II to address these issues.
This article will not make you an expert in all aspects of EPON, but it will give you enough knowledge of the characteristics so that proper systems conﬁguration will make sense. It should also give you a better understanding of why something does not work as expected.
EPON operates at 1 GB/s in both directions, although a private extension of the standard is offered by some manufacturers to allow their products to work at GPON speeds. By contrast, maximum GPON speeds are 1.244 GB/s upstream and 2.488 GB/s downstream.
The second generation of EPON (10G – EPON) operates at either 1 or 10 GB/s upstream and 10 GB/s downstream. This standard is complete, and there is already equipment on the market using it. GPON has also developed 10 GB/s standards under the name XG-PON.
Clock Synchronization and DC Elimination
It is a requirement that there should not be a DC component in a transmitted signal. That is, when measured over time, the number of zeros and the number of ones transmitted must be equal. If this criterion is not met, the optical components on each end of the link malfunction.
It is also imperative that the number of transitions occur in the transmitted signal is kept at a minimum to ensure synchronization of the data clock at the receiver.
To ensure that this happens, EPON employs a substitution method. For every 8 bits (1 byte), a 10-bit symbol is substituted, hence the name 8b/10b encoding. The substituted 10-bit symbol is chosen to have very close to an equal number of zeros and ones, and 3 to 8 transitions per symbol.
These codes satisfy the requirement of no DC component in the signal, while the large number of transitions ensures clock synchronization.
As a limited number of the available codes are used, the encoding also provides a way to detect transmission errors. The disadvantage is that, since 10 bits must be transmitted to represent 8 bits, the bandwidth required is increased by 25%. In for example a Gigabit Ethernet system, the payload plus protocol data are transmitted at 1 GB/s, but because of 8b/10b encoding, the data rate on the ﬁber is 1.25 GB/s.
A few of the many valid code-groups used in Gigabit Ethernet systems is illustrated in table below.
The two columns labeled Current RD- and Current RD+ represent two running disparity sets. The running disparity rules change the transmitted value from one column to the other based on conditions relating to the number of ones or zeros that have been transmitted in the previous code group. These rules ensure that there is no DC content and that there is not a long string of like binary digits, thus ensuring reliable clock recovery.
Ten GB/ s Ethernet systems, including 10G-EPON, use 64b/ 66b encoding that is more efﬁcient. 64 bits of data are replaced by a 66-bit transmitted data block, giving a data rate overhead of slightly more than 3%, rather than 25%. This substitution still provides data and adequate control characters, and good error detection.
The extra overhead required by 64b/ 66b encoding, and much more so with 8b/10b encoding, requires the optical components to run faster than they would have to do otherwise.
This tends to increase the signal-to-noise ratio, and also somewhat exacerbates the effects of dispersion in the ﬁber. Ethernet and other systems do however have a long history of being able to handle this overhead, and it does allow simpliﬁcations elsewhere in terms of clock synchronization time and automatic gain control (AGC) settling.
By contrast, GPON uses a scrambling technique that does not impose the speed overhead, but costs in other areas such as clock synchronization and AGC settling.