CSCI 4550/8556 Computer Networks

1. CSCI 4550/8556Computer Networks Comer, Chapter 15: Networking Ownership, Service Paradigm

2. Network Ownership • Private network - owned by a single organization or company. • Public network - owned by a common carrier (e.g., phone company).

3. Private Networks • Often use LAN technology • Multiple LANs in a building or campus, linked together • Sometimes called an intranet .

4. Private Network Architecture • Operates autonomously from other networks (e.g., Internet) • Usually includes one or few closely managed external connections • May restrict access at connections • Example - UNO

5. Managing Private Networks • The organization • buys its own equipment • hires its own staff to design, implement, maintain and upgrade the network • is responsible for all network management

6. Extending Private Networks • Large organizations may have multiple buildings or campuses • The may only install cables on their own property • They may contract for leased lines from a common carrier • Example – United Parcel Service (UPS)

7. Public Networks • Operated by common carrier • May be telephone company or other organization that builds network out of leased lines • Sometimes called a Public Switched Network (PSN). • Multiple organizations subscribe and connect • Data transits public network to other organizations • Example - AT&T

8. Virtual Private Network (VPN) • A VPN (Virtual Private Network) combines features of private and public networks • It is limited to a single organization • It uses a public network for connectivity • Connections, sometimes called tunnels, connect sites • Each site sees the tunnel as a point-to-point link direct to another site • It cannot be accessed by other users of the public network

9. The Service Paradigm • Connection-oriented • similar to the telephone system. • the endpoints establish and maintain a connection as long as they have data to exchange. • Connectionless • similar to the postal system • the endpoint puts the data to send into a packet and hands it to the network for delivery.

10. Connection-Oriented Service • One endpoint requests a connection from the network. • The other endpoint agrees to establish the connection. • The computers exchange data through the connection. • The API typically provides a stream interface . • A source (either endpoint) delivers a stream of data to the network. • The network breaks the stream into packets for delivery. • The data transmission is not necessarily continuous; like the telephone system, the connection remains in place even when no data is being transmitted. • Eventually, one endpoint request the network to break the connection when the data exchange is complete.

11. Connectionless Service • No connection is established prior to sending or receiving data. • The source of the data adds destination information to the data and delivers it to the network. • The network delivers each data item individually (note the absence of the stream API).

12. Comparison • Connection-oriented • accounting is easier • the application can learn of network problems immediately • fits the stream concept of input/output better than connectionless service • Connectionless • less overhead than connection-oriented • the network is easier to implement

13. Connection Duration and Persistence • Connections can be made on-demand or set up permanently. • Switched connections or switched virtual circuits • Permanent connection or provisioned virtual circuit • Permanent connections • Originally these were “hard-wired.” • They are now configured at system initialization time. • Switched connections • The computer maintains a permanent connection to the network • The network makes each connection on demand. • Internal components are switches; network is a switched data network

14. Examples of Service Paradigms

15. Address and Connection Identifiers • An address is a complete, unique identifier • Connectionless delivery requires an address on each packet • Connection-oriented delivery can use a shorthand that identifies the connection rather than the destination • ATM - 160-bit address, 28-bit connection identifier • Connection identifier includes: • 12-bit virtual path identifier (VPI) • 16-bit virtual circuit identifier (VCI) • Connection identifier local to each computer • May be different in different parts of the ATM switch

16. Network Performance Characteristics • Delay – how long does it take a frame to go from the source to the destination? • Throughput – how many frames can be sent to the network for delivery in a given time interval? • Delay  Throughput – how much data can be present (stored) in the network?

17. Network Delay • Delay is a measure of the time required for data to transit a network. • It is the sum of the following individual delays: • Propagation delay – the time to travel across the medium • Switching delay – the time required for network components (hub, bridge, packet switch) to forward data • Access delay – the time required to get control of the medium (CSMA/CD, token) • Queuing delay – the time a frame is queued in packet switches

18. Throughput • Throughput measures the rate at which data can be transmitted in a network • 300 bps (bits per second) – old modems • 1 Gbps (gigabit [109] bits per second) – typical of the fastest modern networks • Bandwidth – the limit of the underlying hardware • Effective throughput – the real data delivery rate

19. The Relationship Between Delay and Throughput • As throughput approaches the theoretical maximum, delay usually increases. • Excessive traffic causes a condition known as congestion . • The effective delay, D, can often be approximated as

20. The Delay – Throughput Product • This is computed as the delay time multiplied by the effective throughput. • It measures the amount of data that can be present, or effectively stored, in the network. • In fast networks with long delay times, a sending computer can generate large amounts of data before the destination receives the first bit

21. Summary • Public vs. private networks • Connection-oriented vs. connectionless service paradigms • A connection identifier is a shorthand for a destination address • Networks can be measured to determine • Delay • Throughput • Delay-throughput product