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

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

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

4 Internetting Station A Station B 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 internet address of the other station

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

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

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

8 Physical Layer (OSI Layer 1) b Physical transmission standards b Physical ConnectorConnector Transmission mediaTransmission media b Electrical Voltage levelsVoltage levels Meanings of various voltage levelsMeanings of various voltage levels

9 The Data Link Layer (OSI Layer 2)

10 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)Data packages at various layers are called protocol data units (PDUs) The Data Link Layer PDU is called a frameThe Data Link Layer PDU is called a frame Organization allows addressing on shared data linksOrganization allows addressing on shared data links Organization allows error checking and other meta delivery processesOrganization 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 linksControls when stations may transmit in shared media LANs and other shared data links

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

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

13 IPX/SPX b Novell Corporation b De facto (proprietary/closed) b Used only in Novell NetWare PC networks But NetWare dominates PC networkingBut NetWare dominates PC networking Second most widely used architecture at transport layerSecond most widely used architecture at transport layer

14 SNA b Systems Network Architecture b IBM Corporation b De facto (proprietary/closed) b Dominates in mainframe communication On IBM mainframe systemsOn IBM mainframe systems On mainframe systems of other vendorsOn mainframe systems of other vendors

15 Transport-Layer Traffic b TCP/IP38%Rising b IPX/SPX (NetWare)34%Falling b SNA (Mainframes)18% b Other (including OSI) 10% b Total100% Source: Molloy (1994), 1993 data

16 Transmission Speeds b Bit: single 1 or 0 b Transmission speed is measured in bits per second (bps) Duration of a single bitDuration of a single bit Not velocity of propagationNot velocity of propagation

17 Notations for Transmission Speed PrefixUseMetric MeaningExampleInterpretation kilokbps1,00064 kbps64,000 bps MegaMbps1 million1.544 Mbps1,544,000 bps GigaGbps1billion2.4 Gbps2,400,000,000 bps Tera Tbps1 trillion2 Tbps2,000,000,000,000 Note upper case T

18 LANs, MANs, & WANs b LAN Local area networkLocal area network Single office, building, campusSingle office, building, campus 10 Mbps to 100 Mbps to the desktop common10 Mbps to 100 Mbps to the desktop common 1 Gbps coming1 Gbps coming b Will carry most traffic, because most traffic is local

19 PC Networking on a small LAN Network Interface Card (NIC) In each PC Each client and server needs a NIC rather than a modem

20 PC Networking on a small LAN Wiring Hub 15 to 50 cm (6 to 19 inches) Usually, a wiring hub connects the computers together Telephone wiring and RJ-45 plugs

21 LANs, MANs, & WANs b WAN Wide area networkWide area network Intercity, internationalIntercity, international 9,600 bps to 1 Mbps common to the desktop9,600 bps to 1 Mbps common to the desktop Links with higher speed are usually shared (multiplexed) by several desktopsLinks with higher speed are usually shared (multiplexed) by several desktops b Emerged before LANs, due to high cost of long-distance telephone charges