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Telecommunications Concepts Chapter 1.2 Communications Systems Modeling 1 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 2 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 3 09-07-K.Steenhaut & J.Tiberghien - VUB Layered Network Models • The problem: Complexity of networked systems • The solution: Decomposition into independent subsystems • Layered network model – Introduced by IBM (SNA, 1971) – Standardized by ITU (X200) and ISO (7498) – Method universally adopted – Standards not really obeyed in practice 4 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 5 09-07-K.Steenhaut & J.Tiberghien - VUB A diplomatic negotiation Political arguments Service Access Points Translation Cryptography Transmission 6 09-07-K.Steenhaut & J.Tiberghien - VUB A diplomatic negotiation Political arguments Service Access Points Single language messages Cryptography Transmission 7 09-07-K.Steenhaut & J.Tiberghien - VUB A diplomatic negotiation Political arguments Service Access Points Single language messages Encrypted messages Transmission 8 09-07-K.Steenhaut & J.Tiberghien - VUB A diplomatic negotiation Political arguments Service Access Points Single language messages Encrypted messages Electromagnetic waves 9 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 10 09-07-K.Steenhaut & J.Tiberghien - VUB Layered Model Definitions Level (n+1) entity Virtual level n+1 communication path using a level n+1 protocol Level (n+1) entity Physical Communication Path Level n Service Access Points Level (n) entity Virtual level n communication path using a level n protocol Level (n) entity Level n Addresses Level (n-1) Service Provider 11 09-07-K.Steenhaut & J.Tiberghien - VUB Layered Model Definitions Example: The Telephone Service Virtual Interpersonal communication path Physical Communication Path Telephone Service Access Points Telephone Numbers Telephone Network 12 09-07-K.Steenhaut & J.Tiberghien - VUB Layered Model Definitions Protocol Data Units Level (n+1) entity (n+1)PDU Level (n+1) entity Level (n) entity (n)PDU Level (n) entity Level (n-1) Service Provider 13 09-07-K.Steenhaut & J.Tiberghien - VUB Layered Model Definitions Layer overhead Level (n+1) entity Level (n+1) entity Connection oriented protocol Level (n) entity Level (n) entity Level (n-1) Service Provider 14 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 15 09-07-K.Steenhaut & J.Tiberghien - VUB Connection oriented communication Typical example: A telephone conversation 3 step process: 1. Establish connection using the signaling protocol 2. Use connection using an informal protocol being billed for duration of usage 3. Terminate connection using the signaling protocol 16 09-07-K.Steenhaut & J.Tiberghien - VUB Connection oriented communication A temporary communication channel is build through the telephone network 17 09-07-K.Steenhaut & J.Tiberghien - VUB Connection oriented communication Main points • Network has a state : the list of active connections • Network can guarantee Quality Of Service – Reservation of transmission capacity at connection establishment – Monitoring of data transfer and correction of transmission errors by retransmission • Network is fragile : state can be lost. • Application domain : – – – – 18 Very efficient for transferring high volumes of data Mandatory if stringent Quality Of Service requirements Poor initial response times due to connection setup Inapplicable if nodes have high failure rate. 09-07-K.Steenhaut & J.Tiberghien - VUB Connectionless communication Typical example: The postal service Letters, with an address and a stamp, are individually carried from a Post office box to a personal mailbox 19 09-07-K.Steenhaut & J.Tiberghien - VUB Connectionless communication No connections are needed to carry mail 20 09-07-K.Steenhaut & J.Tiberghien - VUB Connectionless communication Main points • The network is stateless. • No guaranteed Quality Of Service. – No reservation of transmission capacity – No error correction by retransmission • Network is robust : – No state stored in any node – Each node operates almost independently from others • Application domain : – Whenever single message response times are critical – Whenever nodes are unreliable 21 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 22 09-07-K.Steenhaut & J.Tiberghien - VUB A two layers model. Networks Layer # Interfaces = # Applications * # Networks 23 Interoperability Connectivity Applications Layer 09-07-K.Steenhaut & J.Tiberghien - VUB A three layers model. Networks Layer # Interfaces = # Applications + # Networks 24 Connectivity Internet & Transport Layer Interoperability Applications Layer 09-07-K.Steenhaut & J.Tiberghien - VUB The Internet & Transport Layer Applications Layer Transport Layer Internet Layer Networks Layer 25 09-07-K.Steenhaut & J.Tiberghien - VUB The Internet & Transport Layers Example: www http TCP IP IP SKYNET UUNET Networks Layer 26 09-07-K.Steenhaut & J.Tiberghien - VUB Contents • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Communication networks according to a layered model • Side-tracks: –Connection oriented vs. Connectionless networks. –The seven layers of the OSI model 27 09-07-K.Steenhaut & J.Tiberghien - VUB The 7 OSI layers 28 Interoperability Connectivity Applications Layers Internet & Transport Layer 7 : Application 6 : Presentation 5 : Session 4 : Transport 3 : Network Networks Layers 2 : Data Link 1 : Physical 09-07-K.Steenhaut & J.Tiberghien - VUB Introduced concepts • Layered communications system – protocol = horizontal convention – Service access point & address • Connectivity : – service offered by lower layers • Interoperability : – full communications services • Connection Oriented Communications • Connectionless Communications 29 09-07-K.Steenhaut & J.Tiberghien - VUB Bibliography To know More about network modeling Fred Halsall Data Communications, Computer Networks and Open Systems Fourth Edition Addison-Wesley 1995. ISBN 0-201-42293-X Recommended for this chapter 30 09-07-K.Steenhaut & J.Tiberghien - VUB