for example if there is a fault in the cable then the whole network will be shutdown.
Advantages:
- Less expensive than a star topology due to less footage of cabling and no network hubs
- Simple for smaller networks not requiring higher speeds
Disadvantages:
- Limited in size and speed
- One bad connector can take down entire network
- Terminators are required at both ends of the backbone cable
RING
In a ring topology, the network signal is passed through each network card of each device and passed on to the next device. This is similar to a bus network with ends joined up. It may or may not depend on a central file server. All the computers and other devices in this network can communicate with each other but the transmission is one way only. This provides fast and simple transmission. A message token addressed to the destination is sent around the ring where it will go to each computer until it reaches the correct one with the same address. The MAU (Multistation Access Unit) makes a logical ring connection between the devices internally. When each device signs on or off, it sends an electrical signal which trips mechanical switches inside the MAU to either connect the device to the ring or drop it off the ring.
The Token Ring LAN is not as common as the Ethernet but it is a little more efficient. The Token Ring connects each station to its two neighbors no more or less. The way this type of LAN works is there is a special bit pattern called a token that circulates throughout the network as long as the network is idle. When a station wants to transmit something it grabs the token and begins its transmission. Because only one computer can send data at a time then it eliminates collisions all together but sacrifices its speed in the process. After the station is done transmitting then it puts the token back out into the ring. It only ran at 4 Mbps. The reason the Token Ring is a little bit better is in the fact it does not have to listen to the network for collisions so the hardware is completely digital.
Advantages:
- Very orderly network where every device has access to the token and the opportunity to transmit
- Can create much larger network using Token Ring
Disadvantages:
- One malfunctioning workstation or bad port in the MAU can create problems for the entire network
- Moves, adds and changes of devices can affect the network
STAR
The difference between the star network and the bus or ring network is that all the devices are connected to the hub individually. The hub will handle the signals from the workstations in turn.
Star topologies have a very good, reliable and accurate performance even when the network is busy. Any data from one computer doesn’t collide with the data from another computer because each computer has its own separate cable. If there is a fault in one of the cables then the whole network will not break down only the one with the faulty cable. Installing this network is expensive because an individual cable is needed for each computer.
Advantages:
- Easy to install and wire.
- No disruptions to the network then connecting or removing devices.
- Easy to detect faults and to remove parts.
Hubs work at the data link layer, acting as a breakout box for signals they receive. A hub is a common wiring point for a star topology.
Disadvantages:
- Requires more cable length
- If the hub or concentrator fails, nodes attached are disabled.
- More expensive than other topologies because of the cost of the concentrators.
LAN Interconnections
Network card. In order to access the network, each workstation must have a network card connected to it. This is very similar to a printed circuit board which is slotted into the CPU. It contains all the necessary electronics and connections to allow the workstation to be linked up to the network.
Repeaters work at the physical level. They are not intelligent work, which means that they do not perform switching or routing functions. A repeater's main task is to boost the signal it receives from one segment of a network for transmission on a different segment. Generally, a repeater is used to extend a segment of a network.
bridges can be used to connect the same or similar segments of a LAN or whole networks. Bridges can be used strategically to isolate traffic on a network. A bridge examines packet headers to determine whether the packet needs to be passed to other segments attached to it. If a packet does not need to be passed along, the bridge will drop the packet, thereby reducing network traffic.
transceiver performs the same function as a network card. Normally, a transceiver is external to the computer, while a NIC is internal. Transceivers isolate the host electrically from the network and detect collisions. Additionally, transceivers can act as adapters for devices that are designed to work with different network cabling.
router's job is to send packets created by higher layers of the network to their ultimate destination. Much of the work at this level involves discovering where
a packet's ultimate destination may one and sometimes many protocols.
Data flow under the OSI model is organized into the following seven layers:
The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.
The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. Ex: this layer defines the framing, addressing and check summing of Ethernet packets.
The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Ex: this layer defines the addressing and routing structure of the Internet.
The Transport Layer describes the quality and nature of the data delivery. Ex: this layer defines if and how retransmissions will be used to ensure data delivery.
The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call.
The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating point numbers can be exchanged between hosts with different math formats.
The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations
how is information sent through the OSI model.
Normally a communication request originates at the highest layer (Application Layer). The request is passed down through the lower layers in the form of a packet called a protocol data unit (PDU). Layers in the protocol stack communicate with their adjacent layers via one or more Service Access Points (SAP). Each succeeding layer in the stack adds its own information to the PDU that will be read by its counterpart (peer) layer on the receiving system. Once the data arrives at the lower layers, the PDU is encoded into data frames and placed onto the cable for transmission. The data frames make their way to the receiving system and the entire process is reversed as the PDU makes its way up the protocol stack. As it moves up the stack, each layer "unwraps" the PDU and receives the information from its peer layer on the sending system.