The present invention relates to data communications between network servers and peripheral device servers on a network. More particularly, the present invention relates to a system for low latency data transmissions over a Local Area Network from a network to a peripheral device server.
Generally, the networking and serial communications industries define the term “latency” as the time required for a packet of information to travel from a source to a destination.
Traditionally, internal PC serial cards enabled connection and integration of a wide variety of peripheral devices that provide Input/Output (I/O) support for applications (such as DNC, material handling, SCADA, and Point-of-Sale applications). However, PC serial cards require that the user locate the computer within close proximity of the connected serial-devices. Alternatively, the user could install expensive long-distance serial cabling to connect the computer to peripherals that were deployed remotely from the PC.
With the general adoption of 10Base-T Ethernet by the mid 1990s, serial-cards began to outgrow these proximity limitations. In the late 1990s, Comtrol Corporation of Minneapolis, Minn. and other serial connectivity companies released the first network-based serial concentrators that offered Ethernet based connections to serial devices. By allowing connections to serial devices over the Ethernet Local Area Networks (LANs), these network serial concentrators or “device servers” gained immediate industry attention. New driver software was developed to enable PC-based application software access to the remotely deployed (Network-attached) communication ports, apparently seamlessly. The device servers allowed integrators and system designers to eliminate serial connections at the individual computers and to provide serial connections over the LAN through a device server positioned at the serial peripheral location, regardless of how far they were from the application server. Since the device servers allowed users to maintain control from remotely located networked computers, integrators and systems designers were able to lower equipment and installation costs while enjoying a new level of design flexibility.
For the vast majority of existing serial port related applications, the transition from individual computers with serial cards connected directly to the device to a remotely located, network attached serial device server offered several advantages. First, serial device location was no longer constrained by distance from the PC hosting the Application software with which the device was working. This was significant due to environmental considerations that may have been acceptable for a serial device but not suitable for a PC. Second, existing Ethernet cabling replaced costly serial cabling. Third, Host Application PCs could be moved or changed with no impact to the serial devices.
For serial devices attached directly to a computer communications port (COM port), latency typically is measured in terms of milliseconds, which means that few (if any) users detect latency induced delays in the performance of the peripheral device attached to the COM port. However, with migration of serial ports onto the network, elements such as network traffic, poorly written COM port redirector software, and network hardware (including hubs and routers) each contributed to varying degrees of delay, which could result in a cumulative delay of up to hundreds of milliseconds.
For most applications, such network-induced transmission delays do not present an overriding concern. However, in markets such as material handling, satellite communications, and real-time device monitoring, a delay of even one hundred milliseconds can create major problems. For example, a company that operates an automated package handling system may connect various serial devices (scanners, scales, sorters) used along its conveyor system to a material control application program running on a PC in the shop supervisor's office. When a package passes the scanner, it reads a label, transmits the data over the network to the application which in turn sends a signal to the sorter which directs the package to the proper destination. A delay in receiving the scanner data or receiving the sorter control directive can result in the package having already passed the sorter before the proper routing directions were received. As you can imagine, such a situation would cause chaos and undermine the reliability of the entire package handling system.
The near real-time transmission requirements for such time-sensitive applications place constraints on system designers to include only the most efficiently designed device server. To meet the stringent requirements of these time sensitive applications, device server manufacturers must take great care to optimize their hardware and driver software for low-latency operation. If designed properly, the combination of low-latency device servers with proper network layout and bandwidth considerations will ensure that the benefits of device server technology can be realized even in the most time-critical installations.
Moreover, even with the optimum combination of hardware elements, network transmission protocols and routers introduce additional transmission delays. Furthermore, various interactions between the operating system of the application server, the serial driver software installed on the server for communicating with the serial device server, and the firmware operating in the serial device server can each contribute to the overall transmission latency, which are difficult to detect and to isolate.
Therefore, it is desirable to provide a serial device server communication capability that reduces transmission latencies between a PC server and a serial device server to a level that approximates the latency of a directly connected serial device.