Introduction
Until quite recently, there was a lack of sufficient standards for the interface between the hardware, software and communications channel of data communication networks. In response, computer manufactures have developed network architectures to support the development of advanced data communications networks.
The goal of network architectures is to promote an open, simple, flexible and efficient telecommunications environment. This is accomplished by the use of standard protocols, standard communications hardware and software interfaces and the design of a standard multi-level interface between end users and computer systems.
The International Standards Organisation (ISO) has developed a seven layer Open Systems Interconnection (OSI) model to serve as a standard model for network architectures. Examples of network architectures include IBM’s System Network Architecture (SNA) and DECnet by the Digital Equipment Corporation.
An important suite of protocols that has become so widely used that it is equivalent to a network architecture is the Internet’s Transmission Control Protocol/Internet Protocol also known as TCP/IP. Another example is the local area network architecture for automated factories sponsored by General Motors and other manufacturers called the Manufacturing Automation Protocol (MAP).
OSI Model
The function and operation of each layer of the OSI model is discussed hereunder:
Until quite recently, there was a lack of sufficient standards for the interface between the hardware, software and communications channel of data communication networks. In response, computer manufactures have developed network architectures to support the development of advanced data communications networks.
The goal of network architectures is to promote an open, simple, flexible and efficient telecommunications environment. This is accomplished by the use of standard protocols, standard communications hardware and software interfaces and the design of a standard multi-level interface between end users and computer systems.
The International Standards Organisation (ISO) has developed a seven layer Open Systems Interconnection (OSI) model to serve as a standard model for network architectures. Examples of network architectures include IBM’s System Network Architecture (SNA) and DECnet by the Digital Equipment Corporation.
An important suite of protocols that has become so widely used that it is equivalent to a network architecture is the Internet’s Transmission Control Protocol/Internet Protocol also known as TCP/IP. Another example is the local area network architecture for automated factories sponsored by General Motors and other manufacturers called the Manufacturing Automation Protocol (MAP).
OSI Model
The function and operation of each layer of the OSI model is discussed hereunder:
Layer 1: The Physical Layer
This layer is concerned with transmitting an electrical signal representation of data over a communication link. Typical conventions would be: voltage levels used to represent a “1” and a “0”, duration of each bit, transmission rate, mode of transmission, and functions of pins in a connector.
Layer 2: The Data Link Layer
This layer is concerned with error-free transmission of data units. The data unit is an abbreviation of the official name of data-link-service-data-units; it is sometimes called the data frame. The function of the data link layer is to break the input data stream into data frames, transmit the frames sequentially, and process the acknowledgement frame sent back by the receiver. Data frames from this level when transferred to layer 3 are assumed to be error free.
Layer 3: The Network Layer
This layer is the network control layer, and is sometimes called the communication subnet layer. It is concerned with intra-network operation such as addressing and routing within the subnet. Basically, messages from the source host are converted to packets. The packets are then routed to their proper destinations.
Layer 4: The Transport Layer
This layer is a transport end-to-end control layer (i.e. source-to-destination). A program on the source computer communicates with a similar program on the destination computer using the message headers and control messages, whereas all the lower layers are only concerned with communication between a computer and its immediate neighbours, not the ultimate source and destination computers. The transport layer is often implemented as part of the operating system. The data link and physical layers are normally implemented in hardware.
Layer 5: The Session Layer
The session layer is the user’s interface into the network. This layer supports the dialogue through session control, if services can be allocated. A connection between users is usually called a session. A session might be used to allow a user to log into a system or to transfer files between two computers. A session can only be established if the user provides the remote addresses to be connected. The difference between session addresses and transport addresses is that session addresses are intended for users and their programs, whereas transport addresses are intended for transport stations.
Layer 6: The Presentation Layer
This layer is concerned with transformation of transferred information. The controls include message compression, encryption, peripheral device coding and formatting.
Layer 7: The Application Layer
This layer is concerned with the application and system activities. The content of the application layer is up to the individual user.
Networking devices
Networking devices are used to connect the segments of a network together or to connect networks to create an internetwork. These devices are classified into five categories namely switches, repeaters, bridges, routers and gateways. Each of these devices except the first one (switches) interacts with protocols at different layers of the OSI model.
Switches
A switched network consists of a series of interlinked switches. Switches are hardware/software devices capable of creating temporary connections between two or more devices to the switch but not to each other. Switching mechanisms are generally classified into three methods: circuit switching, packet switching and message switching.
(a) Circuit switching creates a direct physical connection between two devices such as telephones or computers. Once a connection is made between two systems, circuit
switching creates a dedicated path between two end users. The end users can use the path for as long as they want.
(b) Packet switching is one way to provide a reasonable solution for data transmission.
In a packet-switched network, data are transmitted in discrete units of variable-length blocks called packets. Each packet contains not only data, but also a header with control information. The packets are sent over the network node to node. At each node, the packet is stored briefly before being routed according to the information in its header.
In the datagram approach to packet switching, each packet is treated independently of all others as though it exists alone. In the virtual circuit approach to packet switching, if a single route is chosen between sender and receiver at the beginning of the session, all packets travel one after another along that route. Although these two approaches seem the same, there exists a fundamental difference between them. In circuit switching, the path between the two end users consists of only one channel.
This layer is concerned with transmitting an electrical signal representation of data over a communication link. Typical conventions would be: voltage levels used to represent a “1” and a “0”, duration of each bit, transmission rate, mode of transmission, and functions of pins in a connector.
Layer 2: The Data Link Layer
This layer is concerned with error-free transmission of data units. The data unit is an abbreviation of the official name of data-link-service-data-units; it is sometimes called the data frame. The function of the data link layer is to break the input data stream into data frames, transmit the frames sequentially, and process the acknowledgement frame sent back by the receiver. Data frames from this level when transferred to layer 3 are assumed to be error free.
Layer 3: The Network Layer
This layer is the network control layer, and is sometimes called the communication subnet layer. It is concerned with intra-network operation such as addressing and routing within the subnet. Basically, messages from the source host are converted to packets. The packets are then routed to their proper destinations.
Layer 4: The Transport Layer
This layer is a transport end-to-end control layer (i.e. source-to-destination). A program on the source computer communicates with a similar program on the destination computer using the message headers and control messages, whereas all the lower layers are only concerned with communication between a computer and its immediate neighbours, not the ultimate source and destination computers. The transport layer is often implemented as part of the operating system. The data link and physical layers are normally implemented in hardware.
Layer 5: The Session Layer
The session layer is the user’s interface into the network. This layer supports the dialogue through session control, if services can be allocated. A connection between users is usually called a session. A session might be used to allow a user to log into a system or to transfer files between two computers. A session can only be established if the user provides the remote addresses to be connected. The difference between session addresses and transport addresses is that session addresses are intended for users and their programs, whereas transport addresses are intended for transport stations.
Layer 6: The Presentation Layer
This layer is concerned with transformation of transferred information. The controls include message compression, encryption, peripheral device coding and formatting.
Layer 7: The Application Layer
This layer is concerned with the application and system activities. The content of the application layer is up to the individual user.
Networking devices
Networking devices are used to connect the segments of a network together or to connect networks to create an internetwork. These devices are classified into five categories namely switches, repeaters, bridges, routers and gateways. Each of these devices except the first one (switches) interacts with protocols at different layers of the OSI model.
Switches
A switched network consists of a series of interlinked switches. Switches are hardware/software devices capable of creating temporary connections between two or more devices to the switch but not to each other. Switching mechanisms are generally classified into three methods: circuit switching, packet switching and message switching.
(a) Circuit switching creates a direct physical connection between two devices such as telephones or computers. Once a connection is made between two systems, circuit
switching creates a dedicated path between two end users. The end users can use the path for as long as they want.
(b) Packet switching is one way to provide a reasonable solution for data transmission.
In a packet-switched network, data are transmitted in discrete units of variable-length blocks called packets. Each packet contains not only data, but also a header with control information. The packets are sent over the network node to node. At each node, the packet is stored briefly before being routed according to the information in its header.
In the datagram approach to packet switching, each packet is treated independently of all others as though it exists alone. In the virtual circuit approach to packet switching, if a single route is chosen between sender and receiver at the beginning of the session, all packets travel one after another along that route. Although these two approaches seem the same, there exists a fundamental difference between them. In circuit switching, the path between the two end users consists of only one channel.
In the virtual circuit, the line is not dedicated to two users. The line is divided into channels and each channel can use one of the channels in a link.
(c) Message switching is known as the store and forwarding method. In this approach, a computer (or a node) receives a message, stores it until the appropriate route is free, and then sends it out. This method has now been phased out.
Repeaters
A repeater is an electronic device that operates on the physical layer only of the OSI model. A repeater boosts the transmission signal from one segment and continues the signal to another segment. Thus, a repeater allows us to extend the physical length of a network. Signals that carry information can travel a limited distance within a network before degradation of the data integrity due to noise. A repeater receives the signal before attenuation, regenerates the original bit pattern and puts the restored copy back on to the link.
Bridges
Bridges operate in both the physical and the data link layers of the OSI model. A single bridge connects different types of networks together and promotes interconnectivity between networks. Bridges divide a large network into smaller segments. Unlike repeaters, bridges contain logic that allows them to keep separate the traffic for each segment. Bridges are intelligent enough to relay a frame towards the intended recipient so that traffic can be filtered. In fact, the filtering operation makes bridges useful for controlling congestion, isolating problem links and promoting security through the partitioning of traffic.
A bridge can access the physical addresses of all stations connected to it. When a frame enters a bridge, the bridge not only regenerates the signal but also checks the address of the destination and forwards the new copy to the segment to which the address belongs. When a bridge encounters a packet, it reads the address contained in the frame and compares that address with a table of all the stations on both segments. When it finds a match, it discovers to which segment the station belongs and relays the packet to that segment only.
Bridges can be programmed to reject packets from particular networks. Bridges forward all broadcast messages. Bridges do not normally allow connection of networks with different architectures. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected.
Routers
Routers operate in the physical, data link and network layers of the OSI model. The
Internet is a combination of networks connected by routers. When a datagram (a TCP/IP packet containing data and a source and destination address) goes from a source to a destination, it passes through many routers until it reaches the router attached to the destination network. Routers determine the path a packet should take. Routers relay packets among multiple interconnected networks. In particular, an IP router forwards IP datagrams among the networks to which it connects.
Gateways
Gateways operate over the entire range in all seven layers of the OSI model. Internet routing devices have traditionally been called gateways. A gateway is a protocol converter, which connects two or more heterogeneous systems and translates among them. The gateway thus refers to a device that performs protocol translation between devices. A gateway can accept a packet formatted for one protocol and convert it to a packet formatted for another protocol before forwarding it. The gateway understands the protocol used by each network linked into the router and is therefore able to translate from one to another.
(c) Message switching is known as the store and forwarding method. In this approach, a computer (or a node) receives a message, stores it until the appropriate route is free, and then sends it out. This method has now been phased out.
Repeaters
A repeater is an electronic device that operates on the physical layer only of the OSI model. A repeater boosts the transmission signal from one segment and continues the signal to another segment. Thus, a repeater allows us to extend the physical length of a network. Signals that carry information can travel a limited distance within a network before degradation of the data integrity due to noise. A repeater receives the signal before attenuation, regenerates the original bit pattern and puts the restored copy back on to the link.
Bridges
Bridges operate in both the physical and the data link layers of the OSI model. A single bridge connects different types of networks together and promotes interconnectivity between networks. Bridges divide a large network into smaller segments. Unlike repeaters, bridges contain logic that allows them to keep separate the traffic for each segment. Bridges are intelligent enough to relay a frame towards the intended recipient so that traffic can be filtered. In fact, the filtering operation makes bridges useful for controlling congestion, isolating problem links and promoting security through the partitioning of traffic.
A bridge can access the physical addresses of all stations connected to it. When a frame enters a bridge, the bridge not only regenerates the signal but also checks the address of the destination and forwards the new copy to the segment to which the address belongs. When a bridge encounters a packet, it reads the address contained in the frame and compares that address with a table of all the stations on both segments. When it finds a match, it discovers to which segment the station belongs and relays the packet to that segment only.
Bridges can be programmed to reject packets from particular networks. Bridges forward all broadcast messages. Bridges do not normally allow connection of networks with different architectures. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected.
Routers
Routers operate in the physical, data link and network layers of the OSI model. The
Internet is a combination of networks connected by routers. When a datagram (a TCP/IP packet containing data and a source and destination address) goes from a source to a destination, it passes through many routers until it reaches the router attached to the destination network. Routers determine the path a packet should take. Routers relay packets among multiple interconnected networks. In particular, an IP router forwards IP datagrams among the networks to which it connects.
Gateways
Gateways operate over the entire range in all seven layers of the OSI model. Internet routing devices have traditionally been called gateways. A gateway is a protocol converter, which connects two or more heterogeneous systems and translates among them. The gateway thus refers to a device that performs protocol translation between devices. A gateway can accept a packet formatted for one protocol and convert it to a packet formatted for another protocol before forwarding it. The gateway understands the protocol used by each network linked into the router and is therefore able to translate from one to another.
