Automation Systems Discrete Event Control Systems and Networked Automation Systems Additional Slides (Basics) Intermediate Systems
Outline Building Larger (and more Reliable) Networks: Intermediate Systems Repeater/Hub Bridge/Switch Router Gateway 355
Intermediate Systems Needed to build larger networks physical reasons repeaters and hubs (layer 1) reliability reasons switches and bridges (normally layer 2; however layer 3 and layer 4 switches are also in use) Needed to interconnect different network Router Gateway 356
Repeater (an aside on bit errors first) Attenuation Some of the signal traveling along the cable fades away Solution: Amplification Distortion Attenuation may be non-linear. Different parts of the frequency spectrum are attenuated differently Solution: Non-linear amplification (equalization) Noise With attenuation the signal approaches the level of random background noise (i.e. it becomes hard to detect the signal) Solution: Signal regeneration (sampling the signal, decode it to bits and then send it as a new fresh message) Repeaters regenerate signals after amplifying and equalizing them 357
Repeaters and Hubs Used between a pair of segments to provide signal amplification and regeneration to restore a good signal level before sending it from one cable segment to another (Additionally the preamble is reconstructed). Allow the network to span a larger distance. Provide electrical isolation from failures in the cable or attached systems. Network of repeaters and hubs is called a "Shared Ethernet" or a "Collision Domain". The various systems sharing the Ethernet all compete for access using the CSMA/CD access protocol. If a repeater sees a collision on a cable segment, the repeater detects this and then generates a JAM signal to all connected output ports. Since a repeater is responsible for changing the bit timing, and may introduce additional bits (preamble reconstruction) it can not guarantee that the Inter- Frame Gap (IFG) is preserved. Hubs are a form of repeater with multiple ports (they are sometimes also known as "multi-port repeaters" or "active star networks"). Every incoming frame is sent to all output ports (a hub is not able to recognize the addresses in the header of a frame, and therefore is unable to identify which port to send to) 358
Bridges and Switches Like repeaters/hubs bridges/switches allow to enlarge the network Main difference: Filtering A bridge connects two networks: only messages with destination in the other network are repeated A switch is a multi-port bridge and again transfers messages only on those segments (ports) where the receiver is connected Most important: with a bridge or a switch a collision domain ends! To do the filtering Bridges and Switches maintain address tables describing the connected networks. Based on the used address information we have Layer-2 Switching (MAC-Address) Original Idea Layer-3 Switching (IP-Address) Quite common today Layer-4 Switching (TCP-Port) May allow further optimization (priorities based on the applications involved) 359
Operation of Switches Store-and-Forward vs. Cut-Through Store-and-Forward: The complete packet is read in, analyzed and sent out Cut-Through: After enough bytes are received to decide on the output port, sending at the output starts Cut Through: Less Delay, Less Jitter Store-and-Forward: Detection of Errors in Packets, Different data-rates at input and output are possible Blocking vs. Non-Blocking A non-blocking switch can handle all connections at the full data-rate at the same time Limitations blocking switch Managed vs. Un-managed un-managed: all packets are transferred based on the address-table managed: additional rules to control traffic (IP-filtering, security) 360
Router A router is an Intermediate System (IS) which operates at the network layer of the OSI reference model. Routers are used to connect two or more IP networks, or an IP network to an internet connection. A router consists of a computer with at least two network interface cards supporting the IP protocol. The router receives packets from each interface and forwards the received packets to the appropriate output interface. The router uses the information held in the network layer header (i.e. IP header) to decide whether to forward each received packet, and which network interface to use to send the packet. Decision is based on Routing Table (where is the destination) and Filter Table (what is allowed) Example: Firewall 361
Gateways Allow the connection of completely different networks (Layer 7) Every PC is a gateway Input port: USB Camera Output Port: Internet Gateway from CAN to LON (developed at TU KL) A station (computer) connecting industrial networks to the internet 362
Summary This lecture gave an introduction to Intermediate Systems What you should know now: Repeater/Hub Bridge/Switch, Types of Switches Router Gateway Difference between Hub and Switche 363