Basic Communications Model Standards are needed at all

Basic Communications Model Standards are needed at all Layers User Layer Application Layer Computer (Transport) Layer Transmission Layer 1

Subnet Layer Station B Station A 2 2 1 1 4 3 Transmission of Messages 1. Within a Single Subnet, or 2. in Point-to-Point Links Between Subnets 2

Internet Layer Station B Station A 2 1 3 4 Routing of Messages Across multiple subnets in an internet 3

Internetting Station B Station A 2 1 4 3 internet: collection of subnets such that any station on any subnet can communicate with any other station on any other subnet simply by giving the 4 internet address of the other station.

Layering in Major Architectures Basic Model TCP/IP IPX/SPX Application Computer (Transport) Transport: TCP, etc. Complex: SPX, etc. Internet Subnet (OSI) OSI Application (7) Presentation (6) Session (5) Transport (4) Network (3): Internet & Subnet Data Link (2) Physical (1) 5

OSI Architecture b b Reference Model for Open Systems Interconnection (OSI) Standards Agencies • • • b ISO (computers) Don’t confuse OSI and ISO! ITU-T (telecommunications) De jure standards (official/open) 6

OSI Architecture b b Standards are sophisticated Sophistication brings complexity • Slow standards development b • Products are late to market and expensive • The result: low acceptance of OSI standards in general But OSI dominates at the single network (subnet) transmission layer 7

Physical Layer (OSI Layer 1) b b Physical transmission standards Physical • • b Connector Transmission media Electrical • Voltage levels • Meanings of various voltage levels 8

The Data Link Layer (OSI Layer 2) 9

Data Link Layer (OSI Layer 2) b Organize data for transmission over a data link • Data packages at various layers are called protocol data units (PDUs) • The Data Link Layer PDU is called a frame • Organization allows addressing on shared data links • Organization allows error checking and other meta delivery processes b Media Access Control • Controls when stations may transmit in shared media LANs and other shared data links 10

TCP/IP Architecture b Internet Engineering Task Force (IETF) • Of the Internet Society b De jure (official/open) 11

TCP/IP Architecture b Standards are simple • Often have “Simple” in the name • Standards and products are fast to market • Products are inexpensive and fast to market b Widely Used • On the Internet • In many corporate networks • Dominates at internet and transport layers 12