While 1/2Gbit Fibre Channels have foundapplications in SAN, the Fibre Channel industryis turning its eyes to 10Gbit networks. What will this evolution belike and how will it affect the type of evaluation tests required for10Gbit Fibre Channel products?
To fully understand how the move to 10Gbit Fibre Channel will affectthe optical industry, requirements for 10Gbit Fibre Channeltechnologies and Fibre Channel standards changes should beconsidered.The 10Gbit Fibre Channel protocol runs at a signal rate of10.518Gbps and follows the same structure defined for all Fibre Channelrates.
The Fibre Channel protocol has defined five levels of functions.FC-0, FC-1, FC-2, FC-3 and FC-4, each containing functions as describedin Figure 1 below.
|Figure1: The Fibre Channel protocol has defined five levels of functions—theFC-0, FC-1, FC-2, FC-3 and FC-4.|
The ANSI INCITS 373-2003 (FC-FS)standard specifies the functions for FC-1, FC-2 and FC-3 levels.For10Gbit Fibre Channel, minor changes are made to FC-1 functions asdefined in FC-FS.
The major change in 10Gbit Fibre Channel is the creation of the ANSIINCITS 364-2003 (10GFC) standard, which specifies the physical layerrequirements for 10Gbit Fibre Channel interfaces. In addition, thearbitrated loops topology defined for lower rates is not supported bythe 10Gbit Fibre Channel protocol.
Figure 2 below describes theprotocol components within a 10Gbit Fibre Channel port as defined inthe 10GFC standard. Besides ANSI, ISO/IEC also has documents thatdefine the Fibre Channel protocol.
The 10GFC standard describes the signaling and physical interfacerequirement to transport data at a rate in excess of 10Gbps over afamily of FC-0 physical variants. Optional port management functionsare introduced at the FC-3 level as well.
The standard has defined two formats of the four quarter-speed lanesoptical physical variants and the one full-speed lane over one fibervariant. This article focuses on the one full-speed lane over one fibervariant, because it is the most popular among the three formats.
Statements made hereafter may not apply to the first twovariants.Aside from the higher rate, the physical layer design for10Gbit Ethernet (10GE) is adopted into the 10GFC standard.
|Figure2: A 10GFC level is created to provide translations between FC-1functions defined in FC-FS and XGMII functions defined in 10GE|
The 64B/66B transmission code is used in place of the FC-1 8B/10Btransmission code described in FC-FS for 1Gbit and 2Gbit FibreChannels. Although the 8B/10B code seems a straighter evolution to manyexisting Fibre Channel users, the 64B/66B code has higher bandwidthefficiency and leverages existing 10G technologies.
The 10Gbit Media Independent Interface (XGMII), the physical codingsublayer (PCS) where the 64B/66B coding/ decoding functions reside, thephysical medium attachment (PMA), and the physical medium dependent(PMD) layers as shown in Figure 2above , are defined in IEEE Standard 802.3ae -2002 for 10GE andexpanded in 10GFC – such that they are capable of operating at10.518Gbps.
It should be noted that a special jittery signal is introduced inthe 10GE standard to evaluate the performance of the receiver in aworst case scenario.
Within the 10GFC standard, a 10GFC level is created to adapt theFC-1 information defined in FC-FS to the XGMII. This permits standardoperations of the FC-1 functions, as defined in FC-FS and XGMIIfunctions defined in 10GE, to remain unchanged.
With single-lane 10Gbit Fibre Channel, the 8B/10B transmissioncoding is no longer part of FC-1 functions as defined in FC-FS.Primitive Signals, Primitive Sequences and port state machines remainwithin the FC-1 functions.
10GFC level's functions
The 10GFC level provides the necessary translations between the FC-1and the XGMII. There is no need to translate user data that come fromFC-2, as XGMII will pass them on unchanged. However, FC-1 ordered sets-such as frame delimiters, primitive signals and primitive sequences -are defined differently in FC-FS and XGMII.
For example, all FC-FS ordered_sets start with a leading K28.5special character followed by 3bytes that determine the meanings of theordered_sets. However, in 10GE, there is one control code defined foreach ordered_set. Therefore, an ordered_set from FC-1 must betranslated into the format that is recognized and supported at theXGMII for transmission.
Likewise, an ordered_set received from the XGMII needs to betranslated into the format that can be delivered to the FC-1 functions.The 10GFC level also qualifies primitive sequences received from XGMIIbefore delivering to the FC-1 functions.
The NOS ordered_set defined in FC-FS does not appear on XGMII; itis mapped by the 10GFC level to the RF ordered_set. Qualified LFreceived from XGMII is converted to the out-of-band signal”loss_of_sync” to the FC-1 level.
Although 10GFC uses the XGMII defined in 10GE, rules governing theinformation flow that can appear on XGMII for 10GFC and 10GE aredifferent. For example, the two technologies have different rules onthe generation of interframe gaps (IFG), primitive sequences andprimitive signals. Detailed requirements on XGMII, PCS, PMA and PMD canbe found in IEEE Standard 802.3ae—2002.
Testing the channel
Testing of 10Gbit Fibre Channel includes physical layer tests andprotocol tests (FC-2 and above). PHY layer tests evaluate the abilityof a DUT to carry information error-free from one place to another.
Protocol tests evaluate the DUT's ability to exchange information toestablish and release a connection, and its ability to forward andswitch data frames in accordance with given recommendations,specifications or standard.
Before protocol testing can be performed, PHY layer performancerequirements must be satisfied. The 10Gbit Fibre Channel PHY layerincludes the FC-1, XGMII, PCS, PMA and PMD. Evaluation of the PMDrequires the use of optical instruments for the measurement oftransceiver characteristics such as waveform, clock and sensitivity.
It is more difficult to find instruments for receiver testing thanfor transmitter testing; as special requirements for the input opticalsignal are not easily satisfied by most products in the market.
Testing the PCS and the XGMII require specialized tools that provideanalysis of the 64B/66B code transmitted by the DUT. The idealinstrument should be able to report PCS errors and statistics, andcapture and display the received 64B/66B code in a readable format.
This allows for examination of compliance with the rules of IFG,primitive signals, primitive sequences and link fault signaling asspecified in the standard, since this information is embedded in the64B/66B code stream.
The instrument should also be able to generate the appropriate64B/66B code, allow injection of error conditions, and allow editing oftransmitted bits to force the DUT receiver into or out of specificstates, to verify implementation in accordance with the standard.Client data performance can be determined by evaluating the BER of thepayload once the FC-1 level and below are tested.
Once the transport capability of the DUT is validated, a 10GFCprotocol tester is used to evaluate its ability to establish andrelease a connection, handle traffic and map user data to the FibreChannel signal in accordance with the standard. What must be testeddepends largely upon the nature of the DUT and its expected functions.
The major change to the Fibre Channel protocol is the adoption ofthe 64B/66B transmission code defined in 10GE and the consequentcreation of the 10GFC level.
Therefore, specialized tools that can perform PCS analysis areessential in examining compliance with the standard. Moreover, theintroduction of stressed receiver conformance testing in the 10GEstandard also constitutes a challenge to the testing of the 10GbitFibre Channel receivers.
Gaoyao Tang is an applicationsengineer at Innocor Ltd.