Lecture in Connecting LANs, Backbone Networks, and Virtual LANs

Definition of Terms

• A repeater is a connecting device that operates in the physical layer of the Internet model. A repeater regenerates a signal, connects segments of a LAN, and has no filtering capability.
• A passive hub is just a connector. It connects the wires coming from different branches.
• An active hub is actually a multipart repeater. It is normally used to create connections between stations in a physical star topology.
• A bridge is a connecting device that operates in the physical and data link layers of the Internet model.
• A bridge has filtering capability. It can check the destination address of a frame and decide if the frame should be forwarded or dropped.
• A bridge has a table used in filtering decisions. A bridge does not change the physical (MAC) addresses in a frame.
• A transparent bridge can forward and filter frames and automatically build its forwarding table.
• A transparent bridge is a bridge in which the stations are completely unaware of the bridge’s existence. If a bridge is added or deleted from the system, reconfiguration of the stations is unnecessary.
• A bridge can use the spanning tree algorithm to create a loopless topology.
• In graph theory, a spanning tree is a graph in which there is no loop.
• Note that there is only one single path from any LAN to any other LAN in the spanning tree system. This means there is only one single path from one LAN to any other LAN. No loops are created.
• The bridges send special messages to one another, called bridge protocol data units (BPDUs), to update the spanning tree.
• A hub is a multiport repeater.
• Another way to prevent loops in a system with redundant bridges is to use source routing bridges. A transparent bridge's duties include filtering frames, forwarding, and blocking.
• A three-layer switch is used at the network layer; it is a kind of router.
• The two-layer switch performs at the physical and data link layers. A two-layer switch is a bridge, a bridge with many ports and a design that allows better (faster) performance.
• Some new two-layer switches, called cut-through switches, have been designed to forward the frame as soon as they check the MAC addresses in the header of the frame.
• A router is a three-layer device that routes packets based on their logical addresses (host-to-host addressing).  A router normally connects LANs and WANs in the Internet and has a routing table that is used for making decisions about the route. 
• A three-layer switch is a router, but a faster and more sophisticated. The switching fabric in a three-layer switch allows faster table lookup and forwarding.
• A gateway is normally a computer that operates in all five layers of the Internet or seven layers of OSI model. A gateway takes an application message, reads it, and interprets it.
• A backbone LAN allows several LANs to be connected.
• A backbone is usually a bus or a star.
• In a bus backbone, the topology of the backbone is a bus; in a star backbone, the topology is a star.
• A point-to-point link acts as a LAN in a remote backbone connected by remote bridges.
• A virtual local area network (VLAN) is configured by software, not by physical wiring. 
• VLANs create broadcast domains.
• Membership in a VLAN can be based on port numbers, MAC addresses, IP addresses, IP multicast addresses, or a combination of these features.
• Members of a VLAN can send broadcast messages with the assurance those users in other groups will not receive these messages.
• VLANs are cost- and time-efficient, can reduce network traffic, and provide an extra measure of security.
• In 1996, the IEEE 802.1 subcommittee passed a standard called 802.1Q that defines the format for frame tagging. The standard also defines the format to be used in multiswitched backbones and enables the use of multivendor equipment in VLANs.
• An amplifieramplifies the signal, as well as noise that may come with the signal, whereas a repeater regenerates the signal, bit for bit, at the original strength.
• Stations can be grouped by port number, MAC address, IP address, or by a combination of these characteristics.

Five categories of connecting devices

The five categories contain devices which can be defined as:

1. Those which operate below the physical layer such as a passive hub.
2. Those which operate at the physical layer (a repeater or an active hub).
3. Those which operate at the physical and data link layers (a bridge or a two-layer
   switch).
4. Those which operate at the physical, data link, and network layers (a router or a
   three-layer switch).
5. Those which can operate at all five layers (a gateway).

Note: You can proceed to take the multiple choice exam regarding this topic. Connecting LANs, Backbone Networks, and Virtual LANs - Set 1 MCQs

List of Data Communications Lectures

Lectures in Data Communications

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