DATA TRANSMISSION

Synchronous transmission

The term synchronous is used to describe a continuous and consistent timed transfer of data blocks.
These types of connections are used when large amounts of data must be transferred very quickly from one location to the other. The speed of the synchronous connection is attained by transferring data in large blocks instead of individual characters.
The data blocks are grouped and spaced in regular intervals and are preceded by special characters called syn or synchronous idle characters. See the following illustration.
Figure 1. Synchronous transmission

After the syn characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin.
An analogy of this type of connection would be the transmission of a large text document. Before the document is transferred across the synchronous line, it is first broken into blocks of sentences or paragraphs. The blocks are then sent over the communication link to the remote site. With other transmission modes, the text is organized into long strings of letters (or characters) that make up the words within the sentences and paragraphs. These characters are sent over the communication link one at a time and reassembled at the remote location.
The timing needed for synchronous connections is obtained from the devices located on the communication link. All devices on the synchronous link must be set to the same clocking.
The following is a list of characteristics specific to synchronous communication:
• There are no gaps between characters being transmitted.
• Timing is supplied by modems or other devices at each end of the connection.
• Special syn characters precede the data being transmitted.
• The syn characters are used between blocks of data for timing purposes.


Asynchronous transmission

The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character.
An example of synchronous transmission is shown in the following figure.
Figure 1. Asynchronous transmission

These additional bits provide the timing or synchronization for the connection by indicating when a complete character has been sent or received; thus, timing for each character begins with the start bit and ends with the stop bit.
When gaps appear between character transmissions, the asynchronous line is said to be in a mark state. A mark is a binary 1 (or negative voltage) that is sent during periods of inactivity on the line as shown in the following figure.
Figure 2. Mark (idle) bits in the data stream


When the mark state is interrupted by a positive voltage (a binary 0), the receiving system knows that data characters are going to follow. It is for this reason that the start bit, which precedes the data character, is always a space bit (binary 0) and that the stop bit, which signals the end of a character, is always a mark bit (binary 1).
The following is a list of characteristics specific to asynchronous communication:
• Each character is preceded by a start bit and followed by one or more stop bits.
• Gaps or spaces between characters may exist.

TOPOLOGY:

Tree topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 4). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.
Fig. 4. Tree topology
Advantages of a Tree Topology
· Point-to-point wiring for individual segments.
· Supported by several hardware and software venders.
Disadvantages of a Tree Topology
· Overall length of each segment is limited by the type of cabling used.
· If the backbone line breaks, the entire segment goes down.
· More difficult to configure and wire than other topologies.
5-4-3 Rule
A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A populated segment is one which has one or more nodes attached to it . In Figure 4, the 5-4-3 rule is adhered to. The furthest two nodes on the network have 4 segments and 3 repeaters/concentrators between them.
This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling or a combination of a fiber backbone with UTP cabling is used. If there is a combination of fiber optic backbone and UTP cabling, the rule is simply translated to 7-6-5 rule.

Mesh topology

A type of network setup where each of the computers and network devices are interconnected with one another, allowing for most transmissions to be distributed, even if one of the connections go down. This type of topology is not commonly used for most computer networks as it is difficult and expensive to have redundant connection to every computer. However, this type of topology is commonly used for wireless networks. Below is a visual example of a simple computer setup on a network using a mesh topology.

B2C- BUSINESS TO CONSUMER

Definition:

Business that sells products or provides services to end-user consumers.

Information:
While business-to-consumer activity exists both online and offline, the acronym B2C has primarily been used to describe the online variety.
B2C businesses played a large role in the rapid development of the commercial Internet in the late 20th century. Large sums of venture capital flowed to consumers in the form of free online services and discounted shopping, spurring adoption of the new medium.
When the capital markets turned sour, however, the B2C companies were among the first to fall, and they fell fast. Many companies tried to follow the herd of investors by undergoing a B2C to B2B makeover.

The term B2C, once popular, is now less frequently used, and the infrequent references are often followed by "...is dead." However, some analysts still predict that consumer businesses will thrive online, just not as big and fast as initially predicted. This is not news, of course, to the many niche businesses that are already enjoying success online.

B2B- BUSINESS TO BUSINESS

Business-to-business (B2B) describes commerce transactions between businesses, such as between a manufacturer and a wholesaler, or between a wholesaler and a retailer. Contrasting terms are business-to-consumer (B2C) and business-to-government (B2G).

The volume of B2B transactions is much higher than the volume of B2C transactions. The primary reason for this is that in a typical supply chain there will be many B2B transactions involving subcomponent or raw materials, and only one B2C transaction, specifically sale of the finished product to the end customer. For example, an automobile manufacturer makes several B2B transactions such as buying tires, glass for windshields, and rubber hoses for its vehicles. The final transaction, a finished vehicle sold to the consumer, is a single (B2C) transaction.
The term "business-to-business" was originally coined to describe the electronic communications between businesses or enterprises in order to distinguish it from the communications between businesses and consumers (B2C). It eventually came to be used in marketing as well, initially describing only industrial or capital goods marketing. Today it is widely used to describe all products and services used by enterprises. Many professional institutions and the trade publications focus much more on B2C than B2B. This is a strange development as most sales and marketing people work in B2B.

C2C- CONSUMER TO CONSUMER

Consumer-to-consumer (C2C) (or citizen-to-citizen) electronic commerce involves the electronically-facilitated transactions between consumers through some third party. A common example is the online auction, in which a consumer posts an item for sale and other consumers bid to purchase it; the third party generally charges a flat fee or commission. The sites are only intermediaries, just there to match consumers. They do not have to check quality of the products being offered.

Example:
Consumer-to-consumer (C2C) (or citizen-to-citizen) electronic commerce involves the electronically-facilitated transactions between consumers through some third party. A common example is the online auction, in which a consumer posts an item for sale and other consumers bid to purchase it; the third party generally charges a flat fee or commission. The sites are only intermediaries, just there to match consumers. They do not have to check quality of the products being offered.

Universities:
C2C are becoming more popular amongst students in universities because these are large communities in the same geographical region that are low on money. So they are looking for deals very often and these kinds of websites offer this. Universities themselves set up places for students to sell textbooks and other stuff to other students, you can even advertise that you are subletting your apartment. An example of this from above is Tiger books and Dalhousie University Classifieds, both of these are put together by the school itself for the students.