ATA-over-Ethernet vs iSCSI

Every so often someone voices interest in ATAoE support for Solaris or tries to engage in an ATAoE versus iSCSI discussion. There isn't much out there in the way of information on the topic so I'll add some to the pot...

If you look just at the names of these two technologies you can easily start to equate them in your mind and start a running mental dialog reguarding which is better. But, most folks make a very common mistake.. ATA-over-Ethernet is exactly that, over ethernet. Whereas iSCSI is Internet SCSI, or as some people prefer to think SCSI over IP. So we've got two differentiators just given the names of these technologies alone: ATA vs SCSI command set, and Ethernet vs IP stack. The interesting thing is the latter discussion.

There is a natural give and take here. The advantage of ATAoE is that you don't have the overhead of translating ATA to SCSI then back to ATA if your using ATA drives, so there is a performance pickup there. Furthermore, because we don't have the girth fo the TCP/IP stack underneight we don't burden the system with all that processing, which adds even more performance. In this sense, ATAoE strips away all the stuff that gets in the way of fast storage over ethernet. But, naturally, there is a catch. You can't route ethernet, thats what TCP/IP is for. That means that with ATAoE your going to be building very small and localized storage networks on a single segment. Think of a boot server which operates without TCP/IP, you've got to have one per subnet so that it see's the requests.

iSCSI on the otherhand might be burdened by the bulk of the TCP/IP stack, however it has the ability to span the internet because of it. You can have an iSCSI target (server) in New York and an iSCSI initiator (client) in London connected across a VPN and its not a problem. Plus, iSCSI is an open and accepted standard. ATAoE on the otherhand is open but it was created and developed by Coraid who also happens to be the only supplier of ATAoE enclosures. That may change, but we'll see how well it catches on.

ATAoE promises to be smaller and faster than the industry standard iSCSI, and it is, but unless you are using a very local application your going to be in trouble. Not to mention the lack of enclosure and driver support for non-Linux systems.

The question then becomes: Should OpenSolaris support ATAoE? Personally, I don't think we should ever be against the idea of anything new, if someone wants to do it, we should all get behind it. But looking at Solaris I doubt the idea would stick. First and foremost Solaris is an OS that adheres to the standards and plays by the rules, even when its painful. Linux doesn't always play by those rules and often it gains from breaking them. Linux is a great experimental platform, no doubt, but I just don't think the ideals of ATAoE mesh well with the goals of Solaris. Furthermore, ATAoE doesn't offer the level of scalability, flexablilty, and managability that we get with iSCSI. The performance hit of TCP/IP is definately a downside, but the advantages it brings to the table far out weight the downsides I think.

Here are some links to help you explore the subject more on your own:

• Coraid
• ATAoE Wikipedia Entry
• Coraid ATAoE Linux drivers
• Linux Journal Article: 'Kernel Korner - ATA Over Ethernet: Putting Hard Drives on the LAN'

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ATA over Ethernet a ‘strict no’ in Data Center Networks

While exploring for storage networking technologies, there are chances that one can come across ATA over Ethernet (ATAoE). It is nothing but, ATA command set transported directly within Ethernet Frames. ATA over Ethernet approach is similar to that of a Fibre Channel over Ethernet (FCoE), but in reality the former has gained fewer acceptances from the industry.

As a matter of fact, ATAoE is limited to a single vendor (Vendor lock-in) and its specifications reveal that its protocol length is limited to 12 pages when compared with iSCSI, which has a 257 pages length of protocol.

Although, ATA over Ethernet was considered as an unsighted fast technology, it got overshadowed by the virtues of the iSCSI in the long run.

Storage networking specialists go with the opinion that ATAoE protocol is broken and so it is not a good recommendation to be deployed in the data centers. In order to further cement this statement, let us go into further details.

• ATA over Ethernet has no sequencing- This protocol doesn’t support single sequence of numbers, which allow the storage arrays and servers to differentiate between requests or split a single request into numerous Ethernet frames. As a result of no sequencing, ATAoE offers its server the facility to go for a single request with a particular storage array.
• ATAoE offers zero transmission- This protocol has no packet loss detection or recovery mechanism.
• No fragmentation- ATA over Ethernet requests fit directly into Ethernet frames and so the fragmentation of a single request into multiple frames is not possible. As a result the achievement of data flow is almost zero. With the use of jumbo frames, the transfer of only two sectors is possible via each request.
• Authentication is nil- This protocol if proposed for use in data centers, will not have authentication. So, as a result there is no network security in this protocol and so non-routability of AoE is a source of inherent security.
• Asynchronous writes have weak support- Due to the absence of retransmissions and sequencing, asynchronous writes are handled in an in-considerate fashion.

The final word is that this protocol would have worked almost 30 years ago, when TFTP-Trivial file transfer protocol was designed. But now, in the present world, it will simply be treated as a broken protocol design class.

According to analysis of industry specialists, just go on with an ATAoE protocol to build a home network. For mission critical data center applications, ATA over Ethernet is a ‘strict no’.

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Understanding ADSL Technology

An acronym for Asymmetric Digital Subscriber Line, ADSL is the technology that allows high-speed data to be sent over existing POTS (Plain Old Telephone Service) twisted-pair copper telephone lines. It provides a continuously available data connection whilst simultaneously providing a continuously available voice-grade telephony circuit on the same pair of wires.

ADSL technology was specifically designed to exploit the "one-way" nature of most internet communications where large amounts of data flow downstream towards the user and only a comparatively small amount of control/request data is sent by the user upstream. As an example, MPEG movies require 1.5 or 3.0 Mbps down stream but need only between 16kbps and 64kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth. The ADSL specification supports data rates of 0.8 to 3.5 Mbit/s when sending data (the upstream rate) and 1.5 to 24 Mbit/s when receiving data (the downstream rate). The different upstream and downstream speeds is the reason for including "asymmetric" in the technology's name.

ADSL Standard	Common Name	Downstream rate	Upstream rate
ANSI T1.413-1998 Issue 2	ADSL	8 Mbit/s	1.0 Mbit/s
ITU G.992.1	ADSL (G.DMT)	8 Mbit/s	1.0 Mbit/s
ITU G.992.1 Annex A	ADSL over POTS	8 Mbit/s	1.0 MBit/s
ITU G.992.1 Annex B	ADSL over ISDN	8 Mbit/s	1.0 MBit/s
ITU G.992.2	ADSL Lite G.Lite)	1.5 Mbit/s	0.5 Mbit/s
ITU G.992.3/4	ADSL2	12 Mbit/s	1.0 Mbit/s
ITU G.992.3/4 Annex J	ADSL2	12 Mbit/s	3.5 Mbit/s
ITU G.992.3/4 Annex L	RE-ADSL2	5 Mbit/s	0.8 Mbit/s
ITU G.992.5	ADSL2+	24 Mbit/s	1.0 Mbit/s
ITU G.992.5 Annex L	RE-ADSL2+	24 Mbit/s	1.0 Mbit/s
ITU G.992.5 Annex M	ADSL2+	24 Mbit/s	3.5 Mbit/s

The downstream and upstream rates displayed in the above table are theoretical maximums. The actual data rates achieved in practice depend on the distance between the DSLAM (in the telephone exchange) and the customer's premises, the gauge of the POTS cabling and the presence of induced noise or interference.

Broadband is generally defined as a connection which is greater than 128kbs (kilo-bits per second).

Voice-grade telephony uses a bandwidth of 300Hz to 3.4kHz. The sub 300Hz bandwidth can be used for alarm-system data-transfer/monitoring. Bandwidth above 3.4kHz can be used to carry ADSL traffic.

Analogue voice circuits have a nominal 600 ohms impedance at the VF frequency range but exhibit an impedance of around 100 ohms at the frequency range used by ADSL.

DMT Discrete MultiTone modulation technology is used to superimpose the ADSL bandwidth on top of the telephony bandwidth.ADSL typically uses frequencies between 25 kHz and around 1.1 MHz. The lower part of the ADSL spectrum is for upstream tansmission (from the customer) and the upper part of the spectrum is for downstream (towards the customer) transmission.

The ADSL standard allows for several spectra divisions but the upstream band is typically from 25 to 200 kHz and the downstream band is typically 200kHz to 1.1MHz. in a FDM Frequency Division Multiplexed system, different frequency ranges are used for upstream and downstream traffic. Echo-cancelled ADSL allows the downstream band to overlap the upstream band, significantly extending the available downstream bandwidth and extends the upstream bandwidth to provide faster upstream data rates.

POTS/ADSL spectrum allocation is represented in the following diagram.

A DSLAM Digital Subscriber Line Access Multiplexer is installed at the telephone exchange. and has a modem for each customer and network interface equipment. A POTs Splitter Rack is used to separate voice traffic and data traffic on the customers telephone line.

ADSL filters and filter/splitters are used in the customer's premises to separate ADSL data from analogue speech signals and prevent interference between the two types of service. It's important that the specifications of the filters and filter/splitter you use are checked to ensure that effective filtering and equipment isolation and protection are achieved.

The ADSL standard (G.99x.x series) covers several xDSL systems, protocols and tests. They encompass a framework for operation with individual networks and providers free to adapt their system within the framework guidelines. The standards provide the boundaries for equipment manufacturers.

ADSL Physical (PHY) Layer Parameters

Downstream
Overall symbol rate	4kHz
Number of carriers per DMT	symbol 256
Subcarrier spacing	4.3125kHz
Cyclic prefix length	32 samples
Operational modes	FDM or Echo Cancelled
FDM Mode frequency range	64 to 1100kHz
Echo Cancelled Mode frequency range	13 to 1100kHz
Number of bits assigned per subcarrier	0 to 15 (no bits assigned to 64k QAM)*
Synchronisation	Pilot tone at subcarrier 64, f = 276kHz
Upstream
Number of subcarriers per DMT symbol	32
Cyclic prefix length	4 samples
FDM Mode frequency range	11 to 43 kHz
Echo Cancelled Mode frequency range	11 to 275 kHz
Synchronisation Pilot Tone at subcarrier	16, f = 69kHz
Handshake/initialisation	Per G.994.1

* The lower three to six subcarriers are set to a gain of "0" (turned off) to permit the simultaneous operation of a POTS service provided that a filter/splitter is installed at the customer's premises telephone line entry point.

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WDS Overview : Wireless Distribution System

A wireless distribution system (WDS) is a system enabling the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the traditional requirement for a wired backbone to link them. The notable advantage of WDS over other solutions is it preserves the MAC addresses of client frames across links between access points.

An access point can be either a main, relay, or remote base station.

• A main base station is typically connected to the (wired) Ethernet.
• A relay base station relays data between remote base stations, wireless clients, or other relay stations; to either a main, or another relay base station.
• A remote base station accepts connections from wireless clients and passes them on to relay stations or to main stations. Connections between "clients" are made using MAC addresses.

All base stations in a wireless distribution system must be configured to use the same radio channel, method of encryption (none, WEP, WPA or WPA2) and the same encryption keys. They may be configured to different service set identifiers. WDS also requires every base station to be configured to forward to others in the system.

WDS may also be considered a repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). However, with the repeater method, throughput is halved for all clients connected wirelessly. This is because wifi is an inherently half duplex medium and therefore any wifi device functioning as a repeater must use the Store and forward method of communication.

WDS may be incompatible between different products (even occasionally from the same vendor) since the IEEE 802.11-1999 standard does not define how to construct any such implementations or how stations interact to arrange for exchanging frames of this format. The IEEE 802.11-1999 standard merely defines the 4-address frame format that makes it possible.

Technical

WDS may provide two modes of access point-to-access point (AP-to-AP) connectivity:

• Wireless bridging, in which WDS APs (AP-to-AP on sitecom routers AP) communicate only with each other and don't allow wireless stations (STA) (also known as wireless clients) to access them
• Wireless repeating, in which APs (WDS on sitecom routers) communicate with each other and with wireless STAs

Two disadvantages to using WDS are:

• The maximum wireless effective throughput may be halved after the first retransmission (hop) being made. For example, in the case of two APs connected via WDS, and communication is made between a computer which is plugged into the Ethernet port of AP A and a laptop which is connected wirelessly to AP B. The throughput is halved, because AP B has to retransmit the information during the communication of the two sides. However, in the case of communications between a computer which is plugged into the Ethernet port of AP A and a computer which is plugged into the Ethernet port of AP B, the throughput is not halved since there is no need to retransmit the information. Dual band/radio APs may avoid this problem, by connecting to clients on one band/radio, and making a WDS network link with the other.
• Dynamically assigned and rotated encryption keys are usually not supported in a WDS connection. This means that dynamic Wi-Fi Protected Access (WPA) and other dynamic key assignment technology in most cases cannot be used, though WPA using pre-shared keys is possible. This is due to the lack of standardization in this field, which may be resolved with the upcoming 802.11s standard. As a result only static WEP or WPA keys may be used in a WDS connection, including any STAs that associate to a WDS repeating AP.

OpenWRT, a universal third party router firmware, supports WDS with WPA-PSK, WPA2-PSK, WPA-PSK/WPA2-PSK Mixed-Mode encryption modes. Recent Apple base stations allow WDS with WPA, though in some cases firmware updates are required. Firmware for the Renasis SAP36g super access point and most third party firmware for the Linksys WRT54G(S)/GL support AES encryption using WPA2-PSK mixed-mode security, and TKIP encryption using WPA-PSK, while operating in WDS mode. However, this mode may not be compatible with other units running stock or alternate firmware.

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