Classes of Networks

1. Classes of Networks • LAN (Local Area Network) • Maximum distance not more than a few kms • Ownership by a single organization • Transmission speed of at least several Mbps (tens to hundreds are economical) • Often broadcast, shared media based

2. Classes of Networks - LANS • Some widely used standards include: • IEEE 803.3 - Ethernet • IEEE 803.5 - Token ring • FDDI • ATM • An important issue in broadcast LANs is the allocation of the shared channel (media access control) • Control may be static (time division multiplexing) or dynamic (contention or arbitration)

3. Classes of Networks - MANS • MAN (Metropolitan Area Network) • Distances between 5 and 50 kms • Data rate above 1 Mbps • Standards: IEEE 802.6 DQDB, FDDI, and ATM

4. Classes of Networks - WANS • WAN (Wide Area Network) • Spans entire states or countries • Data rate of 1.544 (T1), and 45 (T3) Mbps common • Higher data rates are available with the wide deployment of ATM backbone networks • Often owned by multiple organizations

5. Classes of Networks - WANS • Usually separate communications functions from application functions • Transmission lines: circuits, channels or trunks • Switching elements: • Specialized computers connecting two or more circuits • These elements receive data on an incoming circuit and transmit it on an outgoing circuit

6. Classes of Networks - WANS • Intermediate Systems, Packet SwitchingNode, Data Switching Exchange, Router, etc. • Intermediate systems store a complete packet before forwarding it • store-and-forward; packet switched; point-to-point network

7. Classes of Networks - Internetworks • Internetworks • Unlike WANs they often interconnect different, incompatible networks • Use special types of intermediate systems called Gateways • Gateways translate between different types of physical media and network software

8. Network Software • Network software is highlystructured • This technique has been immensely successful • The key is Layered design • Each layer provides a service to the layer above • Each layer hides details of how the service is provided to the layer above • The Nth layer on one machine “talks to” or interacts with the Nth layer on another machine

9. Network Software - Protocols • Conventions and rules governing this interaction are specified by the Layer N Protocol • A protocol is an agreement about how communications are to proceed • Without a protocol, communication can be difficult or even impossible • E.g. Telephone conversation, Postal addresses

10. Layer 5 Protocol Layer 5 Layer 5 Layer 4 Layer 4 Protocol Layer 4 Layer 3 Layer 3 Protocol Layer 3 Layer 2 Layer 2 Protocol Layer 2 Layer 1 Layer 1 Protocol Layer 1 Network Software - Protocols Peer “entities” Interface Interface Interface Interface Physical Medium

11. Network Software - Protocols • Information is not actually transferred directly between peer layer N entities • Peer layer N entities carry on a virtual communication using the services of the layers below • Layer N passes data and control information down to (or receives data and control from) Layer N-1 until the physical medium is reached

12. Network Software - Interfaces & Services • Interfaces exist between each layer • Interface defines which primitive functions and services layer N-1 provides to layer N • Want layers to: • Perform a well defined, logically related set of functions • Minimize the amount of information needed to pass between layers • Keep interfaces “clean” to allow easy and transparent replacement of layers

13. Network Software - Protocols • The set of protocols and layers together make up the Network Architecture • A Network Architecture Specification must provide enough information to allow implementation in hardware/software • Implementation specific details are not part of the architecture and should be irrelevant for inter-operation • With one protocol per layer we have a Protocol Stack

14. The benefits of Layered Protocols • Network Architectures, Protocols and Protocol stacks are the Fundamentals of Computer Networks • They form the foundation for the very considerable success of computer networks in the real world • Multilayer communications protocols allow • ready adaptation of successful protocols to new technology (prevent obsolescence) • migration of protocols from software implementation (slow) to hardware (fast) as they evolve

15. More Benefits of Layered Protocols • Separate data and control information • Support differing levels of abstraction (message, packet, frame) with different sizes • Allow segmentation of large messages • Peer process abstraction facilitates reduction of difficult design task (a network architecture) into smaller manageable tasks (protocol layer architecture) • Typically lower layer protocols of “network software” are implemented in silicon (hardware)

16. Understanding Services and Protocols • Protocol is set of rules about the format and meaning of data units exchanged by peers • Protocol is used by entities to implement services • Protocol and/or its implementation can change and as long as the Service (interface) remains unchanged, higher layers are happy and continue to work • Like in abstract data types or object orientation, we decouple interface and implementation

17. Layering concept elsewhere • Layering is used in other software (e.g. Operating systems) - UNIX • For Network Software the important difference is that we are not allowed to violate layering (Layer 5 cannot directly access Layer 1) • For Network Software, its important Layers don’t “peek” into headers of other layers and rely on protocol data of other layers