An architectural model developed by the International Standards Organization (ISO) is frequently used to describe the structure and function of data communication protocols. This architectural model, called the Open Systems Interconnect (OSI) Reference Model, contains seven layers that define the functions of data communications protocols. Each layer represents a function performed when data is transferred between cooperating applications across an intervening network. A layer does not define a single protocol, it defines a data communications function that may be performed by any number of protocols. Therefore, each layer may contain multiple protocols, each providing a service suitable to the function of that layer. Every protocol communicates with its peer. A peer is an implementation of the same protocol in the equivalent layer on a remote system. Each protocol is only concerned with communicating to its peer, it does not care about the layer above or below it. However, there must also be agreement on how to pass data between the layers on a single computer, because every layer is involved in sending data from a local application to an equivalent remote application. The individual layers do not need to know how the layers above and below them function, they only need to know how to pass data to them. Isolating network communications functions in different layers minimizes the impact of technological change on the entire protocol suite. New applications can be added without changing the physical network, and new network hardware can be installed without rewriting the application software. Although the OSI model is useful, the TCP/IP protocols don't match its structure exactly.
The approach used to designing a communication system is known as a layered architecture. Each layer has specific responsibilities and specific rules for carrying out those responsibilities, and knows nothing about the procedures the other layers follow. The layer carries out its task and delivers the message to the next layer in the process, and that is enough.
The Open Systems Interconnect (OSI) reference model is the ISO (International Standards Organization) structure for the "ideal" network architecture. This Model outlines seven areas, or layers, for the network. These layers are (from highest to lowest):
Data can be routed through an internetwork using the following three types of information:
A repeater acts on a purely electrical level to connect to segments. All it does is amplify and reshape (and, depending on the type, possibly re-time) the analog waveform to extend network segment distances. It does not know anything about addresses or forwarding, thus it cannot be used to reduce traffic as a bridge can in the example above.
A hub is a common wiring point for star-topology networks, and is a common synonym for concentrator (though the latter generally has additional features or capabilities). Arcnet, 10Base-T Ethernet and 10Base-F Ethernet and many proprietary network topologies use hubs to connect multiple cable runs in a star-wired network topology into a single network. Token-Ring MSAUs (Multi-Station Access Units) can also be considered a type of hub, but don't let a token-ring bigot hear that. Hubs have multiple ports to attach the different cable runs. Some hubs (such as 10Base-T and active Arcnet) include electronics to regenerate and re-time the signal between each hub port. Others (such as 10Base-F or passive Arcnet) simply act as signal splitters, similar to the multi-tap cable-TV splitters you might use on your home antenna coax (of course, 10Base-F uses mirrors to split the signals between cables). Token- Ring MSAUs use relays (mechanical or electronic) to reroute the network signals to each active device in series, while all other hubs redistribute received signals out all ports simultaneously, just as a 10Base2 multi-port repeater would.
A bridge will connect to distinct segments (usually referring to a physical length of wire) and transmit traffic between them. This allows you to extend the maximum size of the network while still not breaking the maximum wire length, attached device count, or number of repeaters for a network segment.
A bridge must implement both the physical and data link layers of the protocol stack. Bridges are fairly simple devices. They receive frames from one connection and forward them to another connection known to be en route to the destination. When more than one route is possible, bridges ordinarily can't determine which route is most efficient. In fact, when multiple routes are available, bridging can result in frames simply traveling in circles. Having multiple paths available on the network is desirable, however, so that a failure of one path does not stop the network. With Ethernet, a technique called the spanning-tree algorithm enables bridged networks to contain redundant paths.
Token Ring uses a different approach to bridging. When a device needs to send to another device, it goes through a discovery process to determine a route to the destination. The routing information is stored in each frame transmitted and is used by bridges to forward the frames to the appropriate networks. Although this actually is a data link layer function, the technique Token Ring uses is called source routing.
The bridge must implement two protocol stacks, one for each connection. Theoretically, these stacks could belong to different protocols, enabling a bridge to connect different types of networks. However, each type of network, such as Ethernet and Token Ring, has its own protocols at the data link layer. Translating data from the data link layer of an Ethernet to the data link layer of a Token Ring is difficult, but not impossible. Bridges, which operate at the data link layer, therefore, generally can join only networks of the same type. You see bridges employed most often in networks that are all Ethernet or all Token Ring. A few bridges have been marketed that can bridge networks that have different data link layers.
A learning bridge monitors MAC (OSI layer 2) addresses on both sides of its connection and attempts to learn which addresses are on which side. It can then decide when it receives a packet whether it should cross the bridge or stay local (some packets may not need to cross the bridge because the source and destination addresses are both on one side). If the bridge receives a packet that it doesn't know the addresses of, it will forward it by default.
A bridge as described above that has an Ethernet interface on one side and a serial interface on the other. It would connect to a similar device on the other side of the serial line. Most commonly used in WAN links where it is impossible or impractical to install network cables. A high-speed modem (or T1 DSU/CSU's, X.25 PAD's, etc) and intervening telephone lines or public data network would be used to connect the two remote bridges together.
Routers work much like bridges, but they pay attention to the upper network layer protocols (OSI layer 3) rather than physical and data link layer (OSI layer 1 & 2) protocols. A router will decide whether to forward a packet by looking at the protocol level addresses (for instance, TCP/IP addresses) rather than the MAC address. Because routers work at layer 3 of the OSI stack, it is possible for them to transfer packets between different media types (i.e., leased lines, Ethernet, token ring, X.25, Frame Relay and FDDI). Many routers can also function as bridges.
A different method of path determination can be employed using data found at the network layer. At that layer, networks are identified by logical network identifiers. This information can be used to build a picture of the network. This picture can be used to improve the efficiency of the paths that are chosen. Devices that forward data units based on network addresses are called routers.
With TCP/IP, routing is a function of the network layer. By convention, the network on which the data unit originates counts as one hop. Each time a data unit crosses a router, the hop count increases by one. This assumes that all of the paths between the routers provide the same rate of service. A simple hop-count algorithm would be misleading if some lines have greater capacity than others. Apart from such extreme cases, however, hop-count routing is a definite improvement over no routing planning at all.
Routing operates at the network layer. By the time data reach that layer, all evidence of the physical network has been shorn away. Both protocol stacks in the router can share a common network layer protocol. The network layer does not know or care if the network is Ethernet or Token Ring. Therefore, each stack can support different data link and physical layers. Consequently, routers posses a capability, fairly rare in bridges, to forward traffic between dissimilar types of networks. Owing to that capability, routers often are used to connect LANs to WANs.
Building routers around the same protocol stack as are used on the end-nodes is possible. TCP/IP networks can use routers based on the same IP protocol employed at the workstation. However, it is not required that routers and end-nodes use the same routing protocol. Because network layers need not communicate with upper-layer protocols, different protocols may be used in routers than are used in the end-nodes. Commercial routers employ proprietary network layer protocols to perform routing. These custom protocols are among the keys to the improved routing performance provided by the bets routers.
When deciding whether to use a bridge or router in a particular situation your network layout, type and amount of hosts and traffic, and other issues (both technical and non-technical) must be considered. Routing would always be preferable to bridging except that routers are slower and usually more expensive (due to the amount of processing required to look inside the physical packet and determine which interface that packet needs to get sent out), and that many applications use non-routeable protocols (i.e., NetBIOS, DEC LAT, etc.).
Rules of thumb:
On a shared Ethernet network, data is sent to a hub which then rebroadcasts this data to all ports on the network until it gets to its proper destination. When more users are added to the network, more data signals are broadcast, and consequently, more signals can collide with each other, causing the network to slow down.
Switched Ethernet, on the other hand, provides dedicated bandwidth through a "private" connection between two devices on a network. There are no collisions because every user is essentially on a private line. Switched Ethernet, which is based on standard Ethernet and uses the same wiring types, is able to detect the destination of data being sent along the network, and forwards that data directly to the place it's going rather than rebroadcasting it to every port on the hub. Switched Ethernet is like having your own private phone line instead of sharing phone privileges on a party line.
The use of switches has a number of advantages: