6.6. Newer SCSI Technologies

SCSI technology is continuing to evolve. Two important new SCSI technologies are iSCSI and Serial SCSI.

6.6.1 iSCSI

Internet SCSI (iSCSI) is a new storage protocol for native Ethernet/TCP/IP storage networks. iSCSI transports storage blocks across networks and the Internet.

iSCSI is similar to Fibre Channel, but the data transmission differs. The SCSI storage protocol is layered on top of the IP protocol, which allows data to be sent over a network or the Internet.

HP uses iSCSI to extend a consolidated storage area network (SAN) and to enable IP tunneling to and from remote sites. This iSCSI tunneling will be an alternative to the remote site replication protocols currently being used.

During iSCSI tunneling, the following process occurs:

1. User or an application requests data, a file, or an application.

2. The SCSI commands and data request are processed by the operating system.

3. The commands are encapsulated or encrypted, and a packet header is added. These packets are then transmitted over an Ethernet connection.

4. The packet is received and disassembled or decrypted. The SCSI commands and requests are separated.

5. The SCSI commands and data are sent to the SCSI controller, then to the SCSI storage device.

Note

Data can be returned in response to the request. iSCSI is bidirectional.

6.6.2 Serial SCSI

Serial SCSI is also known as IEEE 1394, the name of the standard, or as the Apple brand name FireWire. It is a high-performance serial bus and provides a versatile, high-speed, low-cost method of interconnecting a variety of computer peripherals, storage systems, and consumer electronics devices. Serial SCSI characteristics include the following:

• Fast 100, 200, or 400Mb/s; or 12.5, 25, or 50MB/s

• Guaranteed Both isochronous and asynchronous transfers

• Consumer-friendly Small connectors, hot-pluggable, auto ID, and auto termination

• Expandable 63 devices (16 daisy chained), 4.5m flexible cable

The IEEE 1394 serial bus is also

• A digital interface There is no need to convert digital data into analog and tolerate a loss of data integrity.

• Physically small The thin serial cable can replace larger and more expensive interfaces.

• Hot-pluggable You can add or remove 1,394 devices with the bus active.

• Inexpensive It is priced for consumer products.

• Scalable Currently you can mix 100, 200, and 400Mb/s data rates on one bus, with an 800Mb/s and multi-GB/s upgrade path.

• Flexible The topology supports daisy chaining and branching for true peer-to-peer communication.

6.6.2.1 SERIAL SCSI CABLES

Instead of a 68-wire cable, Serial SCSI uses a 6-wire cable, as shown in Figure 6-12.

Figure 6-12. IEEE 1394 cable cross-section.

Because signal management using IEEE 1394 technology is limited to a single data line, it supports isochronous data transfer, which provides guaranteed data transport at a predetermined rate. Serial SCSI is ideal for multimedia I/O delivery, such as streaming video and audio.

6.6.3 Parallel/Serial SCSI Comparison

In a parallel SCSI interface, data is transmitted 8 or 16 bits at a time, in parallel. Serial SCSI transmits one bit at a time. The distinction between parallel and serial SCSI is similar to the difference between the serial and parallel ports.

The SCSI bus bandwidth is directly proportional to its width. The change from 8- or 16-bit data transfer widths to one bit at a time might seem like a step backward, but the dramatic increases in speed allowed by serial communication reverses this bandwidth loss.

Improved technology increases the demand for bandwidth. The demand for increased speed has driven the SCSI bus from the original, to Fast, then Ultra SCSI. However, each time a faster bus is created, it becomes more difficult to manage the complex signaling and prevent data corruption.

When you send signals across a parallel bus, there is no guarantee that they will all arrive at their destination at the same time. They might become scrambled on different signals because data transfer is asynchronous. Serial SCSI supports both asynchronous and isochronous data transfers.

Note

Isochronous (guaranteed bandwidth) data transfer ensures that data flows at a preset rate so that an application can handle it in a timed way.

Single-ended parallel SCSI has certain restrictions that do not apply to serial SCSI:

• To ensure data integrity, the maximum cable length for single-ended SCSI decreases by half each time the signal speed doubles.

• The 20MHz of Ultra SCSI is the highest achievable transmission frequency using the older, single-ended bus.