At a very high level, gigabit transceivers (GT) are I/O superhighways that pump data from one chip to another at extremely fast rates. The right GT can unclog bottlenecks and speed up a system, an important design consideration in communications and real-time processing in particular. Any number of applications are looking to leverage GTs, but a given market segment may have many standards or protocols and use models. Sometimes there may be several standards targeting one application, forcing the designer to figure out which one best fits the function you want your system to perform. To select the best GT, one needs to be sure of being up- to-date on the latest protocols.
Industry-standard connectivity protocols exist in a wide range of market segments, from wireless communications to consumer electronics. Many of these modern protocols are based on the Open System Interconnection (OSI) model, in which interoperability between network devices and software is broken down into layers. In the FPGA world, intellectual property (IP) in libraries such as the Xilinx LogiCORE and AllianceCORE often uses higher-level serial connectivity protocols such as PCI Express in addition to lower-level physical-layer (PHY) protocols like 1000BASE-X.
However, determining the correct PHY protocol template for your given design project is not always as straightforward as selecting a higher-level protocol. As with many industries, consolidation and design reuse can create a complex maze that you must navigate. Understanding the "higher" layer protocol and its relationship to the "lower" layer protocol specifications - and being mindful of how each industry defines the PHY - will help you choose the appropriate protocol template to achieve your design goals.
Let's review these protocols and then take a look at the best methods to help you select the right one for your design.
The OSI is an ISO standard for worldwide communications that defines a framework for implementing protocols in seven layers. Control passes from one layer to the next, starting at the application layer in one station and proceeding to the bottom, physical layer, over the channel to the next station and back up the hierarchy. From the highest to the lowest, the OSI ladder consists of the application, presentation, session, transport, network, data link and physical layers.
Protocols of the application layer directly serve the end user by distributing information services appropriate to an application, to its management and to system management. The next-highest rung in the OSI model, the presentation layer, provides a set of services in which the application layer may select to enable it to interpret the meaning of the data exchanged. These services are for the management of the entry exchange, display and control of structured data.
The session layer assists in supporting the interactions between cooperating presentation entities, while the transport layer provides a universal transport service in association with the underlying services that the lower layers supply. The network layer provides functional and procedural means to exchange network service data units between two transport entities over a network connection.
Finally, the data link layer provides the functionality and procedural means to establish, maintain and release data links between network entities, while the physical layer offers mechanical, electrical, functional and procedural characteristics to establish, maintain and release physical connections between data link entities.
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