One of the topics that can be confusing when talking about optical transmission is bit rate vs. baud rate. Bit rate refers to the amount of data that can be transmitted between two devices per unit of time– usually per second. Baud rate is similar but refers to the number of symbols that can be transmitted in the same interval. Let me explain further.

Bit rate and baud rate apply to any data transmission regardless of the medium. The data transmission could be over the air like a 5G wireless transmission between your smart phone and a cell site. It could be over a hybrid fiber coax network that connects your in-home cable modem to the internet. It could also include coherent optical transmission over a fiber optic network that connects an enterprise to a data center across town, across the country or even across oceans.

When we began connecting computers, we needed a simple, cost-effective data transmission scheme. That scheme is binary transmission. In binary transmission we send a single bit of information at a time. In the case of binary transmission over fiber optics, we turn a laser on to indicate a “1” while turning it off to indicate a “0”. If we turn the laser on and off at a rate of 1 billion times per second, we achieve 1 giga-bit-per-second transmission.

If we pulse the laser at 10 billion times per second, we achieve 10 gigabit-per-second. This is in fact the transmission bit rate. With a binary transmission scheme, this is also the baud rate. But are the bit rate and baud rate always the same? No. In fact, except for binary transmission, the bit rate and baud rate are never the same.

So, what if we could not only turn the laser on and off but also change its amplitude or its phase? That’s exactly what we do with modern coherent optical transmission systems.

With quadrature phase shift keying we put the laser in one of four states. By doing so we can send two bits of information per symbol – effectively doubling the bit rate.

Modern 800 gigabit per second Coherent optical engines like Infinera’s ICE6 transmit up to 100 billion symbols per second and utilize sophisticated modulation schemes that enable up to 6 bits to be transmitted per symbol.

They also use something called polarization which enables the engine to double its transmission capacity. When combined, we can achieve over 1 terabit-per-second raw transmission.

However, a portion of this data is required for internal use – for things like framing, overhead and forward error correction and thus the usable bit rate between devices is 800 Gigabits per second. The symbol or baud rate in this case is 100 Giga-baud.

So, there you have it. Bit rate refers to the number of bits transmitted between two devices per unit of time – usually expressed in bits per second. The baud or symbol rate refers to the number of symbols that can be sent in the same amount of time. Only in binary transmission schemes do we see the bit rate equal to the baud rate. With all other more sophisticated transmission schemes, the bit rate will always exceed the baud rate because each symbol carries more than one bit of information.

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