iSCSI vs Fibre Channel

Though iSCS1 (Internet Small Computer System Interface) is

gaining popularity because of its advantage in terms of low cost over Fibre

channel, Fibre channel is deep rooted into the system and will not disappear

soon. Most organizations storing huge amounts of data on their SAN (Storage Area

Network) still use the Fibre-channel proto- col. 

iSCSI is a TCPIIP-based protocol, which is used to

establish a connection between the host and the clients. It is a transport

protocol for SCSI and operates on top of thc TCP layer. It is used for

encapsulating SCSI commands so that they can be sent over an IP net- work. There

is a need for encapsulating the SCSI data packets, as the packet-delivery rules

are different in IP and SCSI. To send the SCSI packets directly on the network,

it requires special SCSl connectors and channels. This problem of communication

between the SCSI and IP network is solved by iSCSI. Here's what happens in a

typical iSCSI network. 

The client sends are quest to the server for obtaining some

data. The server receives the request, processes it and stores it in the CDB

(Command Descriptor Block). Now iSCSI encapsulates this datain side the CDB and

makes it fit to be transferred over the IP network. It ensures that the trans-

action between the two clients goes on safely.

Now let's look at Fibre channel. Fibre channel is an effort

to combine the better of the two basic ways of communicating between two

computers or devices. There are two ways in which devices or PCs can be

connected together-channel and network. A channel offers point-to- point

connectivity, which means that it connects device A to device B. Since it is

simple point-to- point connectivity, the data-transfer speed is much higher as

compared to a network. On the other hand, a net- work is a collection of a

number of PCs, servers, devices etc located in the same or different

geographical locations.

These devices or PCs communicate with each other using a

particular protocol. Though a network is much slower than a channel, it is

capable of performing many more tasks as there are a number of different PCs and

devices involved. This means that a network can have a number of channels in it.

Fibre channel tries to combine the best of both.

The Iscsi PROTOCOL stack. Note that Iscsi sites on top of the TCP layer

The switch in a Fibre-channel network is referred to as a

Fabric, which intelligently manages the interconnectivity among the various

devices and nodes. Now the only task left for the Fibre-channel port is to

maintain a simple connection between its device and the Fabric. Two Fibre

cables, each of which is connected to a transmitter at one end and a receiver at

the other, establish the connection between the Fibre port and the Fabric. The

two Fibre cables transmit data in just one direction. The media used for setting

up a Fibre- channel network can be either optical or copper. The distance that

can be covered using Fibre channel depends on the type of media being used and

the desired speed. The medium having the highest performance and one that can

cover 10 Kms at a speed of 1GbitIs is the single mode Fibre-optic medium that

uses long wave laser light source.

iSCSl or Fibre channel?



Since iSCSI works over existing Ethernet networks it has higher latency.

This is a major problem when hundreds of files are to be accessed at the same

time, as the amount of time taken to do so be- comes considerably more. That's

why iSCSl is still a mid-level solution, considered optimal for mid- sized

businesses, as they do not re- quire such high-speed access. The advantages of

Fibre channel over iSCSl are that the former offers faster connectivity and

there are no special adapters required for providing Fibre-channel connectivity.

The data integrity is also better in a Fibre-channel network over long

distances. Fibre channel performs better than iSCSl when more number of drives

or devices are connected. If the number of drives or devices to be connected is

less, both show similar performance. An iSCSI' based SAN costs lesser than one

with Fibre channel. This is because iSCSl can be set up using low-cost Ethernet

NlCs (Network Interface Cards). But then it is slower than Fibre channel. iSCSl

uses the TCPIIP protocol, which makes it unsafe because compared with Fibre

channel it is a relatively hacking prone.

Various connectivity option available in Fiber channel

Other advantages of Fibre channel are: Fibre channel uses

lesser CPU power; it can travel greater distances and that too at greater

speeds. Fibre channel can attain speeds of up to 2 Gbps. On the other hand,

iSCSl can go up to only about 1 Gbps. Efforts are on to increase the Fibre-channel

speeds up to4 GR. Since iSCSI is based on Ethernet, its speeds are also fast

catching up and will soon touch about 10GB.

Looking at the other side of the coin, Fibre channel is

still a very expensive solution for a small business while iSCSl answers all its

needs. To counter that, though iSCSI does provide a low-cost solution, it is not

as safe and as fast as Fibre channel. The extra cost involved in setting up a

Fibre-channel based SAN comes from the separate Fibre-optic network that it

requires. On the other hand, an iSCS1-based SAN does not need a special network

to he laid: it can function on the existing Ethernet LAN infrastructure. As

mentioned earlier, iSCSl can work with the normal Ethernet NICs but that is not

all; it also re- quires special Host-bus adapters dedicated to the SAN. Many ven-

dors now come up with support for iSCSI-IBM (IBM Total Storage IP Storage 2Ooi

has implemented iSCSI), HP, Intel. Cisco and Microsoft (it has come up with driv-

ers for iSCSI-based products). Experts arespeculating that Fi- bre channel will

slowly fade out and iSCSI will completely take over, but it doesn't appear that

it will happen any time soon.

Ankil Kawatra