What is meant by data encapsulation?

Data encapsulation is the process of adding a header to wrap data as it flows down the OSI model, this is done to add additional information required to process the data as it moves from source to destination.

Encapsulation begins at layer 4 of the OSI as the Transport layer converts the data into segments. Here the header contains port numbers of source and destination.

When the segments are converted into packets, at the network layer, another header is added containing the IP addresses of both source and destination.

The data link layer adds another header containing the source and destination MAC addresses. It also adds a trailer which contains a frame check sequence that is used to verify the data on receipt.

Why is a data stream broken into data packets for transmission over a network?

Dividing a data stream into smaller data packets avoids transmissions consuming large amounts of bandwidth for long periods of time. The smaller packets means the bandwidth can be shared and the length of time each opportunity for a device to transmit data is reduced.

In addition to this, transmitting data in packets reduces the effect of data corruption or data loss between devices as each individual packet can be retransmitted instead of having to send the entire data stream.

RING / FDDI / STAR topologies

RING topology

The RING network consists of a closed loop of networked devices. Each device is directly connected to another device in the loop. The RING network can cover greater distances than some other types of networks (e.g. BUS networks) due to each host’s ability to regenerate the message as it passes through it and the fact that there is no central hub requiring each device to connect to.


The most common implementation of the RING topology is the Token Ring network. These networks use a 'Token' that is passed along the RING to prevent data collision. Data can only be sent by the computer (host) with the Token, and the Token can only be held for a set period of time before being passed on to the next computer, if when the Token is received no data is required to be sent, then the Token is passed on immediately.

Original RING networks were implemented using coaxial cable limiting the maximum bandwidth achievable by the network to either 4Mbps or 16Mbps.

RING networks can be expensive to implement and costly to run – any work on the structure of the ring, or fault resulting in an effective break in the ring, results in a period of downtime (all ends of the RING network must be terminated in order for the RING to function). Also, if there is a break in any of the cables, the whole network is down. On the upside, they have the potential to deliver fantastic bandwidth over large distances.


FDDI topology

The Fiber Distributed Data Interface (FDDI) network builds on the basis of the single RING network, but consists of two RING networks running side by side. This architecture is know as 'dual-ring', and the data on each RING flows in an opposite direction (counter-rotating).


The two rings are known as ‘Primary’ and ‘Secondary’. The Secondary ring is generally used as a back-up RING, with the Primary delivering up to 100Mbps capacity. In some circumstances where the Secondary ring is not being used for backup, the capacity can be doubled and therefore effectively deliver 200Mbps of bandwidth.

An advantage of FDDI over a standard RING network is the fact that FDDI uses fibre optics, and so is capable of far higher bandwidth and further distances between hosts (and so overall distance).


Also, by using two rings instead of one, FDDI allows for an element of redundancy that a RING network cannot. If a computer is taken out of the network, the secondary ring forms a loop using the remaining computers and thus maintains the network between the remaining computers.


STAR topology

The STAR network is built using a central 'connection' known as the hub, off which all other network devices are connected. This 'hub' can be a hub, switch or router, and depending on the device, ranges from a simple data link to actively processing which messages are sent to which machine.

The basic hub design consists of messages being sent from a single computer to the hub, and the hub then forward the message onto all other computers connected to the hub.


STAR networks have a typical bandwidth of 100Mbps, though this does vary, and use Ethernet cabling (CAT 5).

Data collision can be high on a STAR network, and this increases as the network grows. Whenever there is a data collision, the whole network is 'frozen' for an instance and then effectively 'reset'. As data collision increases as more and more devices are added to the network, this can have a major impact on performance.

Devices such as switches and routers reduce data collision, as these actively manage data direction, effectively reducing the traffic on the network and therefore reducing the possibility of data collision.


Comparison of STAR and RING topologies

Below is a table showing comparison of RING and STAR networks:

	RING	STAR
Reliability	The RING network is limited by design, as the whole network is down whenever there is a fault in the line	Good, the network remains up, even if there is a failure taking out a computer.
Cost	RING networks are reasonably easy to implement, as there is no central hub.	A central hub is require and cabling needs to be routed from all attached devices to this hub.
Speed	The RING network can offer high speeds on reasonable size networks as data only goes in one direction. Though there on larger networks speed becomes an issue do to data having to pass through every computer between the sender and receiver.	Can offer excellent speed, even on larger networks if combined will switches and routers. Data is only sent from host to receiver, with few other devices involved.
Maintainability	Network has to be down to add or remove a computer from the network. Though only cabling is required between to a computer on the network to the new computer	Very good as new computers can be added to the network without any down time. Though if new cabling is required this can prove to be an issue, as it has to be routed to the nearest hub.
Expandability	There is a limit to the number of computers that can be included on a RING network before there is a drop in performance.	There is good potential for expansion on a STAR network, as with the use of switches and routers, additional network activity is minimal.
Data Collision	Data Collision is not an issue on a RING network, as only the computer that currently holds the token can transmit data.	Can become an issue when there is a large amount of network activity either on large networks or those that the functionality of switches and routers.

The CSMA/CD Protocol

Each PC connected to an Ethernet network connects using a Network Interface Card (NIC), which controls data connectivity between the host PC and the network. Note: Each NIC has a unique identifier called the MAC address (which can be seen as the ‘Physical Address’ when using the ‘IPCONFIG /ALL command in MSDOS).

The Carrier Sense Multiple Access / Collision Detection protocol (CSMA/CD) means that all NICs connected to the network must listen and wait until there is no traffic on the network.

At this point each NIC has an equal chance to transmit data across the network. If any of the other NICs are transmitting data across the network, there will be a signal called a 'carrier' on the network that informs other NICs that the network is busy. This is defined as 'Carrier Sense'.

When there is no traffic on the network, each NICs has an equal chance of transmitting data, and this is defined as 'Multiple Access'.

As data can taken a defined period of time to travel across the network, and isn't instant (though to us it may appear to be so), there is a chance that two NICs may transmit data at the same time.

When this is detected by the nearest NIC at the point where the two transmissions meet, a signal is sent across the network that causes all transmissions to cease for a period of time before transmissions resume.

This is defined as 'Collision Detection', and when this happens each NIC resets an internal random delay that avoids the same collision occurring when transmissions can resume.

The CSMA/CD protocol by design allows an equal chance for each NIC to transmit across the network allowing the network to be shared by several NIC connected devices.

What model was developed to describe networking?

To address the problem, the International Organization for Standardization (ISO) researched networks schemes like DECNET, SNA, and TCP/IP.As a result of this research, the ISO recognized there was a need to create a network model that would help vendors create networks that would work compatibly and interoperably with other networks. The OSI Reference Model, released in 1984, was the descriptive scheme they created. By creating the OSI model, the ISO was providing vendors with a set of standards thus ensuring greater compatibility and interoperability between the various types of network technologies that were being produced by many companies around the world.

DECNET - Group of communications products (including a protocol suite) developed and supported by Digital Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is the most recent iteration and supports both OSI protocols and proprietary Digital protocols. Phase IV Prime supports inherent MAC addresses that allow DECnet nodes to coexist with systems running other protocols that have MAC address restrictions.

SNA - Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences. SNA is essentially composed of seven layers.

TCP/IP - Transmission Control Protocol/Internet Protocol. Common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.

Interoperability - Ability of computing equipment manufactured by different vendors to communicate with one another successfully over a network.

OSI Reference Model - Open System Interconnection reference model. Network architectural model developed by ISO and ITUT-T The model consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The highest layer (the application layer) is closest to the user; the lowest layer (the physical layer) is closest to the media technology. The lowest layer is implemented in hardware. The next to lowest layer is implemented in hardware and software, while the upper five layers are implemented only in software. The OSI reference model is used universally as a method for teaching and understanding network functionality.

ITUT-T - International Telecommunication Union Telecommunication Standardization Sector (ITU-T) (formerly the Committee for International Telegraph and Telephone [CCITT])---An international organization that develops communication standards.

Standards - Set of rules or procedures that are either widely used or officially specified. See also defacto standard.

Why were networking standards needed?

Early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous expansion in networking. As companies realized how much money could be saved and how much they could gain in productivity by using network technology, they began adding networks and expanding existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s, growing pains from this expansion were being felt.

Because many of the emerging network technologies were built using different hardware and software implementations, one problem that soon surfaced was that many of the new network technologies were incompatible. Increasingly, it became difficult for networks using different specifications to communicate with each other.

Why and how did networking start?

Applications written for business helped create the PC industry. Early computers were standalone devices. In other words, each computer operated on its own, independently from other computers. It soon became apparent that this was not an efficient or cost effective way for businesses to operate. A solution was needed that would successfully address three problems: duplication of equipment and resources, inability to communicate efficiently, and the lack of network management.

One early solution to these problems was the creation of local area networks, or LANs.

Because they connected workstations, peripherals, terminals, and other devices in a single building, LANs made it possible for businesses using computer technology to efficiently share such things as files and printers.

As the use of computers by businesses grew, however, it soon became apparent that even LANs were not sufficient. In a LAN system, each department or business was an electronic island.

What was needed was a way to move information efficiently and quickly from one business to another. The solution was the creation of metropolitan area networks, or MANs, and wide area networks, or WANs. Because WANs connected networks that served users across a large geographic area, they made it possible for businesses to communicate with each other even though they were geographically distant from each other.

Local-area network (LAN) - High-speed, low-error data network covering a relatively small geographic area. LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LAN technologies. Compare with MAN and WAN.

Metropolitan-area network (MAN) - Network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN. Compare with LAN and WAN.

Wide-area network (WAN) - Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Frame Relay, SMDS, and X.25 are examples of WANs. Compare with LAN and MAN.