Storage over SONET


The ONS 15454 MSPP can support 1G and 2G line rate FC and FICON over SONET using Generic Framing Procedure (GFP). As with other services cards in the ONS 15454 MSPP, the FC-MR-4 card terminates the SONET signal on the cross-connect card, which switches the signal to the trunk optical card, such as the OC192 card.

Fibre Channel Multirate 4-Port (FC-MR-4) Card

The ONS 15454 MSPP uses the FC-MR-4 card to extend the storage networks over a SONET network. Service providers can offer their customers extended storage. For example, Figure 8-6 shows a customer's private SONET ring using existing services, point-to-point DS3, SONET, and Ethernet purchased from the service provider. You can simply add a four-port FC-MR-4 card into the existing 15454 MSPP chassis to the customer's data center MSPPs to extend the FC network between data centers.

Figure 8-6. Using ONS 15454 to Extend FC


1G and 2G FC

The ONS 15454 can support 40 subrate Fibre Channel terminations. Each FC-MR-4 series card supports four point-to-point SAN extension circuits. This is accomplished by using four pluggable optical interfaces on the front of the FC-MR-4 card. These pluggable interfaces, called gigabit interface converters (GBICs), are used to interface to the SAN device. This GBIC is dual rate and can be set to FC or FICON at either 1 Gbps or 2 Gbps using the Cisco Transport Controller (CTC), the graphical-based interface. Because the FC-MR-4 card has 48 STS-1s going out of the back of the card, the card supports two 1-Gbps FC or one 2-Gbps Fibre Channel.

Note

The GBIC comes in two varieties: 850-nm and 1310-nm wavelengths.


The distance of the FC-MR-4 card depends upon the speed of the interface and whether buffer-to-buffer credit spoofing is used. You can achieve long distances when using the buffer-to-buffer credit spoofing on the FC-MR-4 card. For example, you can achieve a distance of 2300 km at 1 Gbps and 1150 km at 2 Gbps.

Overcoming the Round-Trip Delay Limitation in SAN Networks

FC uses flow control to avoid data loss. The transmitting device waits for an acknowledgment (called R_RDY signal) for every data frame that is sent. The number of frames that the device can send cannot exceed the buffer capacity of the receiving device. The sending device, therefore, has a certain number of buffer credits, with each buffer credit allowing the transmitting node to send one data frame. The amount of time that the acknowledgment takes to return to the sending device increases as the end-to-end delay increases. Increasing the distance is the major contributor to end-to-end delay. The buffer capacity dictates the amount of distance that you can go while still sending at line rate. You can estimate the approximate distance by knowing that one buffer credit is needed for every 2 km for a 1-Gbps FC link.

You can achieve much longer distances between the data centers with buffer-to-buffer credit spoofing when using FC. Buffer-to-buffer credit spoofing is supported in the FC-MR-4 card. This means that the FC-MR-4 card terminates the "receiver readies" messages from the attached SAN device. This is commonly referred to as spoofing. In addition, the idle frames are terminated at the local FC-MR-4 card and regenerated at the other FC-MR-4 card. This results in increased distance and bandwidth between the data centers.

Using VCAT and LCAS

The FC-MR-4 card supports subrates. In other words, you are not restricted to set up point-to-point 1 Gbps (STS24c) or 2 Gbps (STS48c); you can set up a variety of other SONET contiguous concatenated (CCAT) rates and high-order virtual concatenated (VCAT) rates. With high-order VCAT, you can group individual STS1s and STS3cs to better match the bandwidth required between the data centersor perhaps you need to capture stranded bandwidth on the SONET ring. The Synchronous Transfer Signals (STSs) that are grouped together are referred to as a VCAT Group (VCG). It is important to understand that the VCG does not have to contain STSs that are contiguous within the SONET signal. You can select from the rates in Table 8-2.

Table 8-2. FC-MR-4 Series Available CCAT and VCAT Rates

CCAT Rates

1-Gbps FC/FICON

2-Gbps FC/FICON

STS-1

STS-3c

STS-6c

STS-9c

STS-12c

STS-18c

STS-24c

STS-36c

N/A

STS-48c

VCAT Rates

  

STS-1-Xv (X = 1 to 24)

STS-3c-Xv (X = 1 to 8)

STS-1-Xv (X = 1 to 48)

STS-3c-Xv (X = 1 to 16)


Subrate support on the FC-MR-4 card allows the service provider to offer multiple tiers of SAN extension service. This enables smaller businesses that don't require full-line rate or that can't afford this service level to use SAN extension service from their service provider.

The VCG is reconfigurable with the use of the Link Capacity Adjustment Scheme (LCAS). LCAS is a capacity-control mechanism that adds and removes circuits into and out of a VCG in a hitless manner. This means that STS1 or STS3c circuits can be added as part of an STS1-based VCG or STS3c-based VCG, respectively, without affecting the SAN extension service. LCAS also can dynamically add a member back to the VCG when that member becomes available after a failure occurred. The individual circuits are referred to as members of the VCG. For example, an STS1 within the VCG can be removed if the circuit has an operational state of Out-of-service (OOS).

It is important to understand that when using VCAT, different path lengths among the individual member circuits of a VCG can exist. This can make the traffic arrive at the destination out of phase with other members of the VCG. The FC-MR-4 card solves this problem by using a buffer to compensate for the differential delay among members.




Building Multiservice Transport Networks
Building Multiservice Transport Networks
ISBN: 1587052202
EAN: 2147483647
Year: 2004
Pages: 140

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