Advances in multi-gigabit data transfer have been dominated by serial data technologies like USB (Universal Serial Bus) and PCI Ex (Peripheral Component Interface Express), as well as technologies that have converted from parallel to serial, like SAS (Serial Attached SCSI).
Now, as we leap another order of magnitude in data rate from the 3rd generations of serial technologies like USB3 at 5 Gb/s and PCI Ex Gen 3 at 8 Gb/s to 40 and 100 Gb/s Ethernet (40 GbE and 100 GbE), parallel architectures are coming back. The most ambitious is 100 GbE’s four lanes at 25 Gb/s each.
In this paper we start with a quick, high level view of the emerging multi-Gb/s architectures and then delve into the tricks that make them work and how to analyze them.
High-speed serial systems are analyzed with at least one of three goals: diagnostics, compliance, or functional test. Compliance testing is an exhaustive checklist of performance benchmarks designed to assure the interoperability of components made by different vendors. Diagnostics, or hardware debug, involves providing well-understood conditions so that problems can be traced to their causes. Functional test, usually associated with manufacturing, employs a limited number of fast tests to determine if a product works.
Figure 1 shows the essential components of HSS (High Speed Serial) technology. The transmitter serializes a parallel data stream and transmits it through a channel that typically includes conducting cables and backplanes, and/or optic fibers. The most challenging technology is at the receiver because ones and zeros can’t be distinguished at these data rates with a simple slicer. Eye diagrams of signals at several Gb/s are more often than not closed. Tricks at the transmitter, like preemphasis and de-emphasis, and equalization at the receiver effectively reopen the eye so that symbols can be accurately decoded.
Fig 1: Principal components of a high speed serial system.
The paper goes on to examine the transmitter, transmitter and channel testing, the receiver, closed eye analysis, equalization of the transmitter and receiver, receiver and functional testing. To download the full article click here