There are two basic types of Wavelength Division Multiplexing (WDM) solutions—both are available for CWDM and DWDM implementations, depending on customer requirements:
• Transponder-based solutions allow connectivity to switches with standard 850 or 1310 nm optical SFP transceivers. A transponder is used to convert these signals using Optical-to-Electrical-to-Optical (O-E-O) conversion to WDM frequencies for transport across a single fiber. By converting each input to a different frequency, multiple signals can be carried over the same fiber.
• SFP-based solutions eliminate the need for transponders by requiring switch equipment to utilize special WDM transceivers (also known as “colored optics”), reducing the overall cost. Coarse or Dense WDM SFPs are similar to any standard transceiver used in FC switches, except they transmit on a particular frequency within a WDM band. Each wavelength is then placed onto a single fiber through the use of a passive multiplexer.
Traditionally, SFP-based solutions were used as low-cost solutions and were mostly CWDM- based. Due to compliance requirements, some customers are using these solutions to minimize the number of active, or powered, components in the infrastructure. Along with the need for increasing bandwidth and the use of such solutions to support Ethernet as well as FC connectivity, some customers are now using DWDM SFP-based implementations and require DWDM colored optics rather than CWDM colored optics to allow sufficient connections through a single fiber.
Time Division Multiplexing
Time Division Multiplexing (TDM) takes multiple client-side data channels, such as FC, and maps them onto a single higher-bit-rate channel for transmission on a single wavelength. TDM can be used in conjunction with a WDM solution to provide additional scalability and bandwidth utilization. Because TDM sometimes relies on certain FC primitives to maintain synchronization, it may require special configuration on SAN switches. Most TDM devices require IDLE primitives as fill words. Specific configuration modes are used on SAN switches to support the use of IDLE as fill words.
Additionally, it should be noted that TDM-based systems can result in a level of jitter or variable latency. As such, it isn’t possible to make broad statements about the ability to use frame-based trunking and, in general, best practice is to avoid frame-based trunking on a TDM-based configuration. The need to use IDLE primitives may impact the availability of other vendor-specific features. The specific details often depend on switch firmware levels and which configuration mode is used for compatibility.
Synchronous Optical NETwork (SONET) is a standard for optical telecommunications transport, developed by the Exchange Carriers Standards Association for ANSI. SONET defines a technology for carrying different capacity signals through a synchronous optical network. The standard defines a byte-interleaved multiplexed transport occupying the physical layer of the OSI model.
SONET and Synchronous Digital Hierarchy (SDH) are standards for transmission of digital information over optical networks and are often the underlying transport protocols that carry enterprise voice, video, data and storage traffic across Metropolitan and Wide Area Networks (MANs and WANs). SONET/SDH is particularly well-suited to carrying enterprise, mission-critical storage traffic, because it’s connection-oriented and latency is deterministic and consistent. FC-SONET/SDH is the protocol that provides the means for transporting FC frames over SONET/SDH networks. FC frames are commonly mapped onto a SONET or SDH payload using an International Telecommunications Union (ITU) standard called Generic Framing Procedure (GFP).
SONET is useful in a SAN for consolidating multiple low-frequency channels (such as ESCON and 1, 2 Gb FC) into a single higher-speed connection. This can reduce DWDM wavelength requirements in an existing SAN infrastructure. It can also allow a distance solution to be provided from any SONET service carrier, saving the expense of running private optical cable over long distances.
The basic SONET building block is an STS-1 (Synchronous Transport Signal), composed of the transport overhead plus a Synchronous Payload Envelope (SPE), totaling 810 bytes. The 27-byte transport overhead is used for operations, administration, maintenance and provisioning. The remaining bytes make up the SPE, of which an additional 9 bytes are path overhead.
An STS-1 operates at 51.84 Mb/sec, so multiple STS-1s are required to provide the necessary bandwidth for ESCON, FC, and Ethernet, as shown in Figure 3. Multiply the rate by 95 percent to obtain the usable bandwidth in an STS-1 (reduction due to overhead bytes). One OC-48 can carry approximately 2.5 channels of 1 Gb/sec traffic, as shown in Figure 3. To achieve higher data rates for client connections, multiple STS-1s are byte-interleaved to create an STS-N. SONET defines this as byte-interleaving three STS-1s into an STS-3, and subsequently interleaving STS-3s. By definition, each STS is still visible and available for ADD/DROP multiplexing in SONET, although most SAN requirements can be met with less complex point-to-point connections. The addition of DWDM can even further consolidate multiple SONET connections (OC-48), while also providing distance extension.
Like TDM, FC-SONET devices typically require special switch configuration to ensure the use of IDLE rather than ARB primitives for compatibility.
Note again that using FC-SONET/SDH-based systems can result in a level of jitter, or variable latency. As a result, it’s impossible to make broad statements about the ability to use frame-based trunking and, in general, it’s best practice to avoid frame-based trunking on an FC-SONET/SDH-based configuration.
TCP/IP and Gigabit Ethernet (GbE) and 10GbE
Gigabit Ethernet (GbE) is a terminology describing a family of technologies involved in the transmission of Ethernet packets at the rate of 1024 megabits (Mb/sec or 1 gigabit per second) or multiples thereof such as 10GbE and 100GbE. GbE is defined by the IEEE publication 802.3z, which was standardized in June 1998. This is a physical layer standard following elements of the ANSI Fibre Channel’s physical layer. This standard is one of many additions to the original Ethernet standard (802.3 – Ethernet Frame) published in 1985 by the IEEE organization. GbE/10GbE is mainly used in distance extension products as the transport layer for protocol such as TCP/IP. However, in some cases, the product is based on a vendor-unique protocol. Distance products using GbE/10GbE may offer features such as compression, write acceleration and buffer credit spoofing.
The Transmission Control Protocol (TCP) is a connection-oriented transport protocol that guarantees reliable in-order delivery of a stream of bytes between the endpoints of a connection. TCP achieves this by assigning each byte of data a unique sequence number, maintaining timers, acknowledging received data through the use of acknowledgements (ACKs) and retransmission of data, if necessary. Once a connection is established between the endpoints, data can be transferred. The datastream that passes across the connection is considered a single sequence of 8-bit bytes, each of which is given a sequence number.
This article has discussed fiber optic characteristics and the different technologies used in long-distance storage network extension. A follow-up article will delve into network design for high availability, best practices and FCIP trunking technology.