Storage units are essential parts and/or a peripheral component of each video surveillance recording solution and are divided into two main categories: external and internal. Internal storage is usually a set of hard disks which are available in different capacities and technical specifications. Examples of external storages are removable flash disks, portable HDDs, etc. In all of the storage units the interface plays an essential role due to its functionality as the gateway of all of the data transferred between the storage unit and the recorder. The speed and performance of the data interface can be the performance bottleneck of the recording solution and may determine the maximum devices that the recorder can optimally support. The ATA interface has been originally developed for the internal HDDs of desktop PCs, while Small Computer System Interface (SCSI) has been developed for servers and large scale storage systems. Due to demand for high speed data transfer, hard disk interfaces are moving from parallel to serial transmission. For instance, Serial Attached SCSI (SAS) will replace Parallel SCSI and Fiber Channel (FC), Serial ATA (SATA) will replace Parallel ATA. This report describes the features of the main interfaces involved in storage systems. Also, the application of several interfaces in video surveillance is studied and the best choice for video recorders is discussed. At the end of the report, a comparative study is presented.
ATA (also known as IDE) stands for Advanced Technology Attachment which is used to connect hard disk drives, CD-ROM drives and similar peripherals to computers. The first ATA interface is now commonly referred as PATA (parallel ATA), working based on parallel transmission. It provides a transfer rate of 133 Mb/s.
SCSI, standing for Small Computer System Interface, is a parallel interface standard used primarily for high speed hard drives. It supports faster data transfer rates than the IDE storage interface. Since in this technology multiple SCSI hard drives can be connected to single SCSI interface, it is really an I/O bus rather than simply an interface. Although SCSI has been popular in the past, today many users are switching over to SATA drives.
SCSI is gradually evolved into SAS or serial attachment SCSI. SAS is a point-to-point serial interface in which controllers are linked directly to disk drives. Its full-duplex signal transmission supports 3.0Gb/s. Using SAS, multiple devices (up to 128) will be enable to connect simultaneously with thinner and longer cables, confirming its superiority over the traditional SCSI.
SATA (Serial Advanced Technology Attachment)
SATA or serial ATA is a standard hard disk interface invented to replace the parallel ATA (PATA). Since in parallel transmission, several bits are sent at the same time, it seems this method is better than serial transmission in terms of speed, but noise problem occurs in parallel transmission. Since many wires have to be used (one for each bit to be sent), one wire generates interference in another. This is the main drawback of parallel transmissions like PATA. On the other hand, serial transmission uses just one wire to transmit data, so it is more robustness against noise.
In fact, SATA was developed to serialize the parallel ATA interface which transmits data in a serial mode. It is a very simple interface which supports only point to point connection. Since in SATA at least eight bits per clock cycle will be transmitted, it is eight times faster than serial transmission where only one bit is transmitted per clock.
Actually, higher clock rate can be used on serial transmission, thus, transferring in very high clock rates leads to higher transfer rate compared to parallel transmission. SATA provides a transfer rate of 1.5 Gb/s.
Another advantage of serial transmission is fewer wires need to be used. Parallel ports use 40 pin connector and 80 wire cables, while SATA ports use seven pin connector and seven wire cables. So, using thinner cables makes airflow inside the PC case simply.
External Serial Advanced Technology Attachment or eSATA is a SATA interface variation that supports external storage devices. It competes with FireWire 400 and universal serial bus (USB) 2.0 to provide fast data transfer speeds. eSATA increases the speed of SATA in terms of exterior storage. Its transfer rate reaches at least to triple the rate of USB 2.0 and FireWire 400, although it has one drawback. eSATA requires its own power connector, unlike the other interfaces, meaning that an additional USB cable is needed to supply energy.
It uses the same pins and the same protocol as internal SATA, thus eSATA drives offers the same high speed data transfer rate. However, eSATA can support the cable length of two meters compared to the one meter supported by SATA.
It is worth mentioning that unlike USB and Firewire interfaces, eSATA does not have to translate data between the interface and the computer, this enhances data transfer speed, also you do not need additional chip. Since eSATA offers such fast transfer rates, it has been a popular external hard drive interface used by video editors, audio producers, and other media professionals.
While eSATA is one of the fastest interfaces, it is surpassed by both Thunderbolt (10 Gbps) and Thunderbolt 2.0 (20 Gbps), which are alternatives to eSATA.
Why SATA is superior in video surveillance?
In video surveillance, just achieving the required storage capacity is not enough. Maintaining data integrity during transmission as well as error correction features is also needed. Parallel ATA drives deliver admirable operation in these tasks and now SATA drives are raising surveillance storage performance even higher. The SATA interface has presented valuable advantages in very large scale technology (VLSI) and high-speed serial transmission, enabling SATA drives to deliver an unbeatable blend of performance, flexibility, data integrity and reliability.
In NVR or DVR hard disks are connected to SATA connector on motherboard via SATA interface cable. It should be mentioned that each SATA interface can support specified hard disk capacity.
There are two major advantages for SATA which encourage user to use SATA in video surveillance. SATA has shown its superiority in RAID operations because all connections are point-to-point and the design is not complicated. Also, since RAID controllers are designed for larger numbers of hard disk drives, they can only be connected using the small SATA seven-wire interface on a backplane. This would be impossible via parallel ATA by 80-wire busses. The quantity of SATA interface determines how many hard disks can be connected to NVR for video storage. By choosing the NVR with as many SATA interfaces as possible, you can connect many hard drives for video storage. If users have limited selection for this, extending the video storage via its eSATA interface is possible. The eSATA allows NVR to achieve unlimited video storage space.
According to the mentioned, using SATA drives in surveillance storage have been increased greatly and SATA interface is an excellent choice for high storage in video surveillance application, undoubtedly.
If multiple digital recorder systems are connected to one or more RAID systems, fiber channel optical cables are used as the interfaces. Since the connections become more flexible and connectivity is possible over a longer distance. On the other hand, if each digital recorder system has its own RAID system, SCSI cables can be used as well.
The advantages of SATA II
Due to the some features presented in this section, SATA II makes SATA more appropriate for long-term storage in video surveillance.
- Port multipliers
Port Multipliers increase the number of SATA devices which can be connected to a single controller, while reducing the amount of cables required to connect an array of SATA devices.
- Port selectors
Due to this feature two hosts can connect to one drive. Therefore, if one of the hosts fails, the second host will act as a spare, so that access to the storage is maintained. This type of redundancy is essential for video surveillance.
- Native command queuing
Native command queening is a feature to improve the hard disk performance by reordering the commands sent by the computer to the hard disk drive. Naturally Commands will arrive at a disk to read or write from different locations on the disk. By using SATA I, commands are executed in the order they arrived, while with SATA II drives, an algorithm is used to determine the most efficient order to execute commands. This feature improves systems performance in terms of data transfer speed.
Comparing eSATA with USB.3
Universal serial bus (USB), FireWire and eSATA are the most common methods to connect the external storage devices to your computer. USB 1 can only communicate with printers, mic and keyboards, while using USB 2, connecting to peripherals that stored data is possible like media players, USB flashes and external hard drives. Its transfer rate is 480 Mb/s.
The USB.3 port as the fastest port among Universal Serial Bus interfaces are usually colored blue in new PCs (not always) to differentiate them from older version USB 2.0 ports.
The transfer rate of USB 3.0 is 5 Gb/s, although this throughput is never exactly achieved. Since many devices themselves are not able to reach to that level of throughput, like spinning hard disks.
eSATA has also been considered as a professional interface, since it has been designed to work with hard drives primarily. This is a benefit of eSATA compared to USB, since USB has to be compatible with other functions like mouse, keyboard, charging, etc.
By using eSATA, PC’s motherboard just deals with one drive at a time, while with USB, it deals with multiple devices simultaneously.
eSATA was the fastest interface (1.5 to 6 Gb/s) on PCs, before the appearance of USB 3.0 and thunderbolt (10 Gb/s). It is worth mentioning that the read speed of eSATA is the half of USB 3.0, while write speed in eSATA is twice faster than USB 3.0.
Finally, if data is being transferred from PC to PC, USB 3.0 is more beneficial than eSATA undoubtedly, due to its high speed. If you deal with storage alone, especially for projects connected to a single PC, then eSATA is a winner. It might be faster, even in the cases that other multiple USB drives connected to the PC at the same time. So, eSATA interfaces are more common in TV video recorders.
The need for increased bandwidth, high speed data transfer and feasibility in storage systems, made interfaces to convert from parallel transmission method to serial transmission. Hence, the SATA and SAS interfaces was developed to achieve faster transfer rate. Due to RAID technology and multiple hard disk drives in video recorders, SATA interface is a valid choice for this industry. Since connecting several hard disks to backplane is possible easily.
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