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Learn about essential topics in networking theory, including what a network is, how networks communicate, addressing schemes, data transmission using packets, and the OSI model. Understand the layers of networking operations and the responsibilities of each in communication tasks.
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Introduction • Overview of the basic concepts of networking • Also discusses essential topics of networking theory
What is a Network? • A network is a collection of devices that share a common communication protocol and a common communication medium. • Devices - computers, printers, telephones, televisions, coke machines, etc.
What is a Network? • Computing-centric model - services and devices bound to individual machines • Network-centric model - services and devices are distributed across a network • Network and software standards (e.g. Jini) exist to allow devices and hardware talk to each other over networks and to allow instant plug-and-play functionality
What is a Network? • Besides devices providing services, there are also devices that keep the network going, for example, • Network cards - to allow a computer to talk to a network. E.g. ethernet card. • Routers - machines that direct data to the next "hop" in the network • Hubs - allow multiple computers to access a network • Gateways - connect one network to another. E.g. a LAN to the Internet.
How do Networks Communicate? • Networks consist of connections between computers and devices. • Connections: • Wires and cables - use electricity for transmitting data • Wireless - use infrared / radio • Fiber-optic cables - use light
How do Networks Communicate? • Connections carry data (bits - 0's and 1's) between one point (node) in the network and another. • For data to be successfully delivered to individual nodes, these nodes must be clearly identifiable.
Addressing • Each node in a network is typically represented by an address. • The manufacturer of the network interface card (NIC) is responsible for ensuring that no two card addresses are alike, and chooses a suitable addressing scheme. • Each card will have this address stored permanently, so that it remains fixed.
Addressing • There are many addressing schemes available. E.g. Ethernet network cards are assigned a unique 48-bit number. • This physical address is referred to by many names, such as: • Hardware address • Ethernet address • Media Access Control (MAC) address • NIC address
Addressing • Often, machines are known by more than one type of address. E.g. a network server may have a physical Ethernet address as well as an Internet Protocol (IP) address, or it may have more than one network card. • For inter-network communications, the IP address is used.
Data Transmission Using Packets • Sending individual bits of data from node to node is not very cost effective. • Overhead involved - e.g address of destination node. • Most networks group data into packets.
Data Transmission Using Packets • A packet consists of a header and data segment. • The header contains: • Addressing information (e.g sender & recipient) • Checksums to ensure packet has not been corrupted • Other info needed for transmission across network Header fields Data 1101000111010100001
Data Transmission Using Packets • To transmit data, a direct connection is usually not available. • So packets are sent to their destination nodes via intermediary nodes in the network. • Due to network conditions (such as congestion or network failures), packets may take arbitrary routes, and sometimes may be lost or arrive out of sequence.
Data Transmission Using Packets • Packet transmission and transmission of raw bits are low-level processes. • Most network programming deals with high-level transmission of data.
Communication Across Layers • The concept of layers was introduced to acknowledge and address the complexity of networking theory. • The most popular approach to network layering is the Open Systems Interconnection (OSI) model created by the International Standards Organization (ISO)
Communication Across Layers • The OSI model groups network operations into seven layers.
Communication Across Layers • Each layer is responsible for some form of communication task, but each task is narrowly defined and usually relies on the services of one or more layers beneath it. • Generally, programmers work with one layer at a time; details of the layers below are hidden from view.
Layer 1 - Physical Layer • This layer is network communication at its most basic level. • At this level, networking hardware transmit sequence of bits between two nodes. • Java programmers do not work at this layer - it is the domain of hardware driver developers and electrical engineers. • No real attempt is made to ensure error-free data transmission
Layer 2 - Data Link Layer • This layer is responsible for providing a more reliable transfer of data, and for grouping data together into frames. • Frames are similar to data packets but are blocks of data specific to a single type of hardware architecture. • Frames have checksums to detect errors in transmission. • Corrupted frames are discarded so that they will not be passed to higher layers.
Layer 3 - Network Layer • The network layer deals with data packets which are sent across the network. • Communication at this level is still very low-level; network programmers are rarely required to write software services for this layer.
Layer 4 - Transport Layer • This layer is concerned with controlling how data is transmitted. • It deals with issues such as automatic error detection and correction, and flow control (limiting the amount of data sent to prevent overload).
Layer 5 - Session Layer • The purpose of this layer is to facilitate application-to-application data exchange, and the establishment and termination of communication sessions. • Connection-oriented communication can increase network delays and bandwidth consumption. Some applications choose to use a connectionless form of communication.
Layer 6 - Presentation Layer • This layer deals with data representation and data conversion. • Different machines use different types of data representation (e.g. 8-bit integers on one system and 16-bit integers on another). • Data compression • Data encryption
Layer 7 - Application Layer • This layer is where the vast majority of programmers write code. • Protocols for this layer dictate the semantics of how requests for services are made (e.g requesting a file). • In Java, almost all network software written will be for this layer.
Advantages of Layering • Helps simplify networking protocols. • Protocols can be designed for interoperability • Software that uses Layer n can talk to software running on another machine that supports Layer n, regardless details of the lower layers. Example: a network layer protocol can work with an Ethernet network and a token ring network.
