NIC

The NIC (Network Interface Card) is part of the host computer. It contains the MAC address, uniquely identifying the machine on the network. The NIC contains the data link network layer logic, and performs other basic functions of detecting and processing network communication at the lowest level.

Repeaters

Repeaters are devices which connect two physical wires. They are powered and boost the signal if it must travel over long physical distances. In the case of digital signals the repeater restores the signal, removing noise as well.

Hubs

Hubs are non-intelligent network devices which do little more than connect wires together.  They operate on the physical layer only, and thus do not stop broadcast storms or prevent network collisions. Powered hubs also function as repeaters, boosting signals and cleaning digital signals.

Bridges

Bridges work on layer one. They interpret data frames, recording the MAC addresses on outgoing packets so that they can build a bridging table that associates each MAC address to a port on the bridge. Bridges also read the MAC address on incoming data frames, then uses the table to send the frame through the correct port. If the incoming frame has an unknown MAC address then the bridge floods the frame to all ports.  Bridges are much slower than hubs because they store frames internally to read them before forwarding them.

Switches

Switches do everything that a bridge does, and more.  The main difference between a bridge and a switch is that switches use a variety of different higher level techniques to improve latency and reduce errors and collisions.

Store and Forward

This switching technique stores incoming frames in a buffer, and does not forward it until the entire frame has been received. It also performs a CRC error check to ensure that the frame is valid. If it is not then it drops it, saving bandwidth and further limiting collisions by not forwarding an invalid frame. Unfortunately this technique is the slowest, because the switch has to wait for the entire frame. This technique is adopted from the technique used by bridges to forward frames, but with an added error check.

Cut Through

This forwarding technique reads the incoming frame until it detects the destination MAC address and then immediately begins forwarding it. This is the fastest technique, but it does not allow error checking. In addition, this technique is more likely to cause network collisions.

Fragment Free

This frame forwarding technique reads the first 64 bytes of the frame and then forwards the frame. On modern contention based Ethernet networks most network collisions happen within the first 64 bytes of the frame. As with cut through forwarding there is no error detection on the switch.  This technique offers high latency and also significantly limits collisions.

Adaptive Cut Through

This hybrid technique detects the error rate on the connection.  If there are many errors and network collisions then it switches to store and forward technique.  If there are very few errors due to a high quality connection then it uses cut through technique.  This allows high latency on good connections, and fewer collisions and errors on bad connections.

Other Switch Functionality

More expensive switches may operate even higher up on the OSI model, even up to the session or application layer to perform advanced statistical analysis and security functions.  Switches can also be used to create VLANs (virtual local area networks).  This technique allows groups of ports to be treated like a single LAN. This creates separate broadcast domains, stopping broadcast storms and improving security by preventing different VLANs from interfering with each other. In addition VLANs can be configured to operate across multiple switches that communicate via higher level routers. In a static VLAN each port is manually mapped to a specific VLAN. In a dynamic VLAN the ports determine which VLAN they belong to using a manually created database defining which MAC addresses are in which VLAN. This allows devices to be moved between ports without having to reconfigure the VLAN manually.

Routers

Routers operate at the third layer of the OSI model.  They are used to connect multiple networks. Rather than using bridging tables of MAC addresses, they instead read the logical address from the packets. These logical addresses are stored in a routing table. The advantage of using a router to connect two switches is that the router separates the two switches into two logical networks. This does even more to prevent collisions, improve security, and stop broadcast storms.

Since the router is a level three device that communicates using packets it can be used to connect networks of different types, for example ethernet and token ring networks.

Interior routers operate within an office, directing data between separate LANs.  Border routers connect interior LANs to WANs. Exterior routers connect WANs together, and are typically owned by ISPs.

The most important functionality of routers is that they calculate what path network packets should take as they travel over the network. In static routing, paths are chosen be administrators manually. This only works in small offices and other interior routers because it does not keep track of changing conditions present in the exterior internet. However, static routing is extremely fast, so for a small interior network it makes a lot of sense. Dynamic routing is much better for large external networks, because it allows routers to communicate with each other to keep track of changing conditions on the network. This communication is designed to work automatically so that humans do not have to interfere.  However, this computation is vulnerable to much higher latency, however, so routers require much faster processors and better hardware than a switch or bridge.

Three different routing protocols are used: distance vector, link state, and hybrid.

Distance Vector Routing Protocol

This routing protocol tries to minimize the number of hops, or the number of routers to the destination. However it doesn't take into account the quality and speed of the connection. They are effective for small networks only. Routing Information Protocol (RIP) was an early distance vector routing protocol. It causes routers to communicate with each 30 seconds, and also limits the number of hops to 15, making RIP useless for modern internet. Border Gateway Protocol (BGB) has replaced RIP as the primary distance vector routing protocol used on border and exterior routers.

Link State Routing Protocol

In this technique routers communicate with each other when there are changes. Every router maps the entire network independently. This allows every router to be much more flexible and capable of dealing with problems. OSPF (Open Shortest Path First) was an early link state routing protocol. Under this protocol there is no hop limitation. The most commonly used link state routing protocol is EIGRP (Enhanced Interior Gateway Routing Protocol). This protocol is a hybrid which combines distance vector and link state routing depending on the situation. It is useful for interior and border networks.

Brouters

Brouters can operate as either a switch or a bridge. Unlike a bridge which can only accept frames and read MAC addresses, and a router which can only accept packets and read IP addresses, a brouter can read either frames or packets and forward them accordingly.

Gateways

Gateways translate between different protocols, for example allowing Linux, Mac OS X, and Windows networks to communicate and share files. Gateways can help different email systems communicate with each other, translate voice data to data packets and vice versa, or work as a firewall. Gateways are primarily software that can be run on computers, servers, or routers.

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