What is the GT / s unit?
In many materials, for example PCIe 5.0 interface if systems equipped with this interconnect unit appears GT / s - gigatransfer per second. What does it really mean? It appeared with the presentation of the PCI Express (PCIe) standard developed by PCI-SIG (PCI Special Interest Group). With each subsequent iteration, for example, "doubling the transmission speed from 16 GT / s to 32 GT / s" was regularly repeated. But what does gigatransfer mean?
We are used to giving transfers in gigabits per second (Gbps, Gb / s), thanks to this it is easy to imagine how many "0's and 1's" can be transferred to a given interface per unit of time. In the case of PCIe, however, the situation is different due to the data encoding used. In PCI Express, transmissions are serial and the clock is integrated into the data transfer itself. In order for the receiver to be able to successfully "recover" the clock signal from the data stream, the coding system must ensure that there are enough edges in a given packet (go from 1 to 0 and from 0 to 1). To achieve increased number of passes, PCIe uses 8b / 10b encoding - each eight bits are encoded as a 10-bit symbol, which is then decoded back to 8 bits by the receiver. This means that 10 bits are required for every eight uncoded bits. Let's look at PCIe 1.1 - a single channel of this interface has a bandwidth of 2.5 Gbps on each side, so a total of 5 Gbps. Since the bus must transmit 10 bits for each data byte sent, the effective baud rate is:
5 Gbps * 8/10 = 4 Gbps
Thus, a 16-channel PCIe 1.1 interconnect can transmit either 80 Gbps of encoded or 64 Gbps of unencrypted data. PCIe 2.0 doubled these parameters so that one channel was able to transmit 8 Gbps of uncoded data, and the 16-channel PCIe 2.0 interconnect could transmit up to 128 Gbps of uncoded data, it 16 gigabytes per second . Gigatransfers therefore refer to the amount of raw data that is transferred over an interface. In order to determine the real effective bandwidth from gigatransfers, data encoding must also be taken into account.
Taking the data encoding into account for the first two generations of PCI Express is quite simple - 8b / 10b encoding means that the speed in GT / s is multiplied by 0.8 to get the effective throughput in Gbps. However, since PCIe 3.0 the 128b / 130b encoding is used, this means that every 128 bits (16 bytes) is encoded in a character that is 130 bits. So in the case of standards from version 3 onwards, the number of gigatransfers is multiplied by 128/130 (?0.985) to get the effective interface bandwidth. This means that the 5.0 version of the PCIe interconnect, which reaches 32 GT / s in each direction - 64 GT / s in total - has a bandwidth of 64 * 128/130, or about 63 Gbps - almost 7.88 gigabytes per second. With 16 interface channels, this means that it is possible to send via this interconnect 126 gigabytes of data per second .
Source: https://www.edn.com/electronics-news/4380071/What-does-GT-s-mean-anyway-
We are used to giving transfers in gigabits per second (Gbps, Gb / s), thanks to this it is easy to imagine how many "0's and 1's" can be transferred to a given interface per unit of time. In the case of PCIe, however, the situation is different due to the data encoding used. In PCI Express, transmissions are serial and the clock is integrated into the data transfer itself. In order for the receiver to be able to successfully "recover" the clock signal from the data stream, the coding system must ensure that there are enough edges in a given packet (go from 1 to 0 and from 0 to 1). To achieve increased number of passes, PCIe uses 8b / 10b encoding - each eight bits are encoded as a 10-bit symbol, which is then decoded back to 8 bits by the receiver. This means that 10 bits are required for every eight uncoded bits. Let's look at PCIe 1.1 - a single channel of this interface has a bandwidth of 2.5 Gbps on each side, so a total of 5 Gbps. Since the bus must transmit 10 bits for each data byte sent, the effective baud rate is:
5 Gbps * 8/10 = 4 Gbps
Thus, a 16-channel PCIe 1.1 interconnect can transmit either 80 Gbps of encoded or 64 Gbps of unencrypted data. PCIe 2.0 doubled these parameters so that one channel was able to transmit 8 Gbps of uncoded data, and the 16-channel PCIe 2.0 interconnect could transmit up to 128 Gbps of uncoded data, it 16 gigabytes per second . Gigatransfers therefore refer to the amount of raw data that is transferred over an interface. In order to determine the real effective bandwidth from gigatransfers, data encoding must also be taken into account.
Taking the data encoding into account for the first two generations of PCI Express is quite simple - 8b / 10b encoding means that the speed in GT / s is multiplied by 0.8 to get the effective throughput in Gbps. However, since PCIe 3.0 the 128b / 130b encoding is used, this means that every 128 bits (16 bytes) is encoded in a character that is 130 bits. So in the case of standards from version 3 onwards, the number of gigatransfers is multiplied by 128/130 (?0.985) to get the effective interface bandwidth. This means that the 5.0 version of the PCIe interconnect, which reaches 32 GT / s in each direction - 64 GT / s in total - has a bandwidth of 64 * 128/130, or about 63 Gbps - almost 7.88 gigabytes per second. With 16 interface channels, this means that it is possible to send via this interconnect 126 gigabytes of data per second .
Source: https://www.edn.com/electronics-news/4380071/What-does-GT-s-mean-anyway-
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Comments
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