From bits to bytes: Decoding Encoding

March 1, 2011 – 1:16 am

With networking, care should be taken to understand if a given speed or performance capacity is being specified in bits or bytes as well as in base 2 (binary) or base 10 (decimal).

Another consideration and potential point of confusion are line rates (GBaud) and link speed which can vary based on encoding and low level frame or packet size. For example 1GbE along with 1, 2, 4 and 8Gb Fibre Channel along with Serial Attached SCSI (SAS) use an 8b/10b encoding scheme. This means that at the lowest physical layer 8bits of data are placed into 10bits for transmission with 2 bits being for data integrity.

With an 8Gb link using 8b/10b encoding, 2 out of every 10 bits are overhead. To determine the actual data throughput for bandwidth, or, number of IOPS, frames or packets per second is a function of the link speed, encoding and baud rate. For example, 1Gb FC has a 1.0625 Gb per second speed which is multiple by the current generation so 8Gb FC or 8GFC would be 8 x 1.0625 = 8.5Gb per second.

Remember to factor in that encoding overhead (e.g. 8 of 10 bits are for data with 8b/10b) and usable bandwidth on the 8GFC link is about 6.8Gb per second or about 850Mbytes (6.8Gb / 8 bits) per second. 10GbE uses 64b/66b encoding which means that for every 64 bits of data, only 2 bits are used for data integrity checks thus less overhead.

What do all of this bits and bytes have to do with clouds and virtual data storage network?

Quite a bit when you consider what we have talked about the need to support more information processing, moving as well as storing in a denser footprint.

In order to support higher densities faster servers, storage and networks are not enough and require various approaches to reducing the data footprint impact.

What this means is for fast networks to be effective they also have to have lower overhead to avoid moving more extra data in the same amount of time instead using that capacity for productive work and data.

PCIe leverages multiple serial unidirectional point to point links, known as lanes, compared to traditional PCI that used a parallel bus based design. With traditional PCI, the bus width varied from 32 to 64 bits while with PCIe, the number of lanes combined with PCIe version and signaling rate determines performance. PCIe interfaces can have one, two, four, eight, sixteen or thirty two lanes for data movement depending on card or adapter format and form factor.  For example, PCI and PCIx performance can be up to 528 MByte per second with 64 bit 66 MHz signaling rate.

 

PCIe Gen 1

PCIe Gen 2

PCIe Gen 3

Giga transfers per second

2.5

5

8

Encoding scheme

8b/10b

8b/10b

128b/130b

Data rate per lane per second

250MB

500MB

1GB

x32 lanes

8GB

16GBs

32GB

Table 1: PCIe generation comparisons

Table 1 shows performance characteristics of PCIe various generations. With PCIe Gen 3, the effective performance essentially doubles however the actual underlying transfer speed does not double as it has in the past. Instead the improved performance is a combination of about 60 percent link speed and 40 percent efficiency improvements by switching from an 8b/10b to 128b/130b encoding scheme among other optimizations.

Serial interface

Encoding

PCIe Gen 1

8b/10b

PCIe Gen 2

8b/10b

PCIe Gen 3

128b/120b

Ethernet 1Gb

8b/10b

Ethernet 10Gb

64b/66b

Fibre Channel 1/2/4/8 Gb

8b/10b

SAS 6Gb

8b/10b

Table 2: Common encoding

Bringing this all together is that in order to support cloud and virtual computing environments, data networks need to become faster as well as more efficient otherwise you will be paying for more overhead per second vs. productive work being done. For example, with 64b/66b encoding on a 10GbE or FCoE link, 96.96% of the overall bandwidth or about 9.7Gb per second are available for useful work.

By comparison if an 8b/10b encoding were used, the result would be only 80% of available bandwidth for useful data movement. For environments or applications this means better throughput or bandwidth while for applications that require lower response time or latency it means more IOPS, frames or packets per second.

The above is an example of where a small change such as the encoding scheme can have large benefit when applied to high volume or large environments.

Learn more in The Green and Virtual Data Center (CRC), Resilient Storage Networks (Elsevier) and coming summer 2011 Cloud and Virtual Data Storage Networking (CRC) at http://storageio.com/books

Ok, nuff said.

Cheers gs

Greg Schulz – Author Cloud and Virtual Data Storage Networking (CRC Press), The Green and Virtual Data Center (CRC Press) and Resilient Storage Networks (Elsevier)

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