Commercial PV System Data Monitoring, Part One: Page 4 of 11
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By Bill Reaugh, VP of project development, Draker Laboratories
A TCP/IP (transmission control protocol/Internet protocol) network is created in several layers, or programming abstractions: the link layer, the Internet layer, the transport layer and the application layer. However, TCP/IP is only one type of network that can be used to transfer information from one place to another. Open data protocol (ODP) is another type of network architecture; Modbus and CANbus are others. Each network type has varying numbers of layers, but all are built up in a similar way.
Each layer of abstraction allows the layers around it to function without having to know the specific programming needs of the others. Information is passed through each layer, from the source to the destination, based on the needs of the network.
Physical layer. A data monitoring system’s physical layer is the network hardware. This includes the cables, jacks, connectors, computers and other physical devices that are connected together. While this is not technically a layer of programming abstraction, it is required for the other layers to exist. For example, CAT 5 cable, RJ-45 connectors and jacks and Ethernet cards are all physical components in a TCP/IP network.
Link layer. This is the basic structure used to connect one device to another. The link layer is where individual devices are addressed, generally by a media access control (MAC) address in a local area network (LAN) or wide area network (WAN). Note that MAC addresses are specific to the hardware and generally permanent. This is also the level where things like virtual private network (VPN ) connections are created.
Internet layer. Message routing takes place at the Internet layer. Devices are assigned an IP (Internet protocol) address that in a manner of speaking tells physical layer devices, like routers and switches, who they are. These IP addresses act as a proxy for the MAC addresses used in the link layer.
IP addresses can be dynamic, meaning that the device or MAC address currently using an IP address may be different today than it was yesterday or even 5 minutes ago. Devices called dynamic name servers (DNS) keep track of the MAC and IP address associations as they change and also allow for domain names to be used in lieu of IP addresses. For example, you probably do not recognize IP address 18.104.22.168—but because of DNS it has a recognizable domain name: google.com.
Transport layer. The protocol used to send IP data packets is assigned at the transport layer. The most common is TCP (transmission control protocol), but UDP (user datagram protocol) is also widespread. TCP and UDP provide the structure and error checking required for a packetized data transmission system.
In a packetized system, a message is broken into small fragments. Each fragment is then sent across the network in a “best effort” system, meaning each finds its own way from source to destination. TCP and UDP rebuild the message from all of the bits, make sure they have all arrived and are in the correct order and request that missing ones be resent.
Application layer. The main user interaction with the system occurs at the application layer. HTTP (hypertext transfer protocol), FTP (file transfer protocol), SMTP (simple mail transport protocol) and many other protocols exist at this level.