1 Making Networks Work ITEC 370 George Vaughan Franklin University

2 Sources for Slides Material in these slides comes primarily from course text, Guide to Networking Essentials,Tomsho, Tittel, Johnson (2007). Other sources are cited in line and listed in reference section.

3 TCP/IP and OSI Models

4 The Network Communication Problem The problem of Network Communication is quite complex: –What medium should be used? –What voltage level, light level or wave amplitude represents a 1 versus 0? –What should be used for addressing the recipient? –How should the data be encoded? –How should errors be handled? –What path to choose?

5 Networking Models There are many issues to be worked out when attempting to establish digital communication. Just like programming, we decompose a difficult task into smaller (yet easier) tasks 2 dominate networking models are used to decompose the problem networking: –OSI: Open Systems Interconnection (OSI) networking suite –IEEE 802: family of standards

6 OSI and IEEE 802 Tomsho, Tittel, Johnson (2007) The IEEE defined a set of LAN standards to ensure network interface and cabling compatibility –Project 802 (inception on February (2) of 1980) Concentrates on standards that describe a network’s physical elements –NICs, cables, connectors, signaling technologies, media access control, and the like OSI model was not standardized until 1983–1984 –IEEE 802 standards predate the model –Both were developed in collaboration and are compatible with one another

7 OSI Reference Model OSI model divides the problem into 7different levels of complexity – called layers. Each layer ‘n’ is only concerned with how to communicate to its neighbor layers, ‘n+1’ and ‘n-1’. Layer ‘n’ could care less about layers ‘n+2’ and ‘n-2’.

8 Structure of the OSI Reference Model Tomsho, Tittel, Johnson (2007)

9 OSI Layers OSI defines the interface between layers. Each layer ‘n’ provides services to layer ‘n+1’. Communicating devices must have compatible protocol stacks. Layer ‘n’ in device ‘A’ has a virtual connection to layer ‘n’ in device ‘B’.

10 Virtual Communication Tomsho, Tittel, Johnson (2007)

11 Stack Connection Over a Network

12 Protocol Data Units (PDU) Each layer operates on a set of data called the PDU. The PDU in layer ‘n’ is not the same as the PDU in layer ‘n+1’ (PDU ‘n’) = (Header ‘n’) + (PDU ‘n+1’) The process of adding a header is called encapsulation. Similar to putting a letter in an envelope (the envelope is like a header – it wraps the data).

13 Layers and PDUs

14 Application Layer Tomsho, Tittel, Johnson (2007) Layer 7; PDU: data –Set of interfaces to access networked services E.g., networked file transfer, message handling, and database query processing –Handles network access, moving data from sender to receiver, and error recovery for applications –Components usually have a client and a server part E.g., HTTP, Client for Microsoft Networks, NFS –Possible problems: missing/misconfigured client or server SW, incompatible or obsolete commands used to communicate between client and server

15 Presentation Layer Tomsho, Tittel, Johnson (2007) Layer 6 –Data-formatting info for network communications –Handles: protocol conversion, character set issues, encryption/ decryption, and graphics commands –May compress data –A redirector operates at this layer Intercepts requests for service from the computer; those that can’t be handled locally are redirected to a networked resource that can handle the request –Usually built into the Application layer component E.g., FTP, HTTP

16 Session Layer Tomsho, Tittel, Johnson (2007) Layer 5 –Permits two parties to hold ongoing sessions –Handles session setup, data or message exchanges, and teardown when the session ends –Monitors session identification so that only designated parties can participate –Monitors security services for access control –Examples: name lookup and user logon and logoff E.g., DNS name resolution, FTP’s logon/logoff –End-to-end task synchronization services –Manages mechanics of any ongoing conversation

17 Transport Layer Tomsho, Tittel, Johnson (2007) Layer 4; PDU: segment –Manages end-to-end transfer of data –Segments long data streams into chunks Resequences chunks into original data on receipt –Includes error checks to ensure error-free delivery –Handles flow control –E.g., TCP (TCP/IP) and SPX (from IPX/SPX) –Layer 4 problems include a corrupt protocol stack and segments that are too large for the medium between the source and destination networks The latter forces Network layer to fragment segments, which causes performance degradation

18 Transport Layer (continued) Tomsho, Tittel, Johnson (2007)

19 Network Layer Tomsho, Tittel, Johnson (2007) Layer 3; PDU: packet –Handles addressing messages for delivery –Translates logical addresses into physical addresses –Determines how to route transmissions from sender to receiver (routing process) –Traffic cop for network activity and handles routing and access control (during routing process) –E.g., IP (from TCP/IP) and IPX (from SPX/IPX) –Possible problems: incorrect IP addresses or subnet masks, incorrect router configuration, and router operation errors

20 Network Layer (continued) Tomsho, Tittel, Johnson (2007)

21 Data Link Layer Tomsho, Tittel, Johnson (2007) Layer 2; PDU: frame (has header and trailer (FCS)) –Sends PDUs from/to Network to/from Physical layer –FCS contains Cyclical Redundancy Check (CRC) It’s the responsibility of the upper layers (e.g., Layer 4) to retransmit data discarded due to errors –Header contains source/destination MAC addresses Destination address is of final destination or intermediate device (e.g., router) –The SW component at this layer is the NIC driver –HW components include NIC and switches –Possible problems: collisions, invalid frames, trying to use incompatible network architectures

22 Data Link Layer (continued) Tomsho, Tittel, Johnson (2007)

23 Physical Layer Tomsho, Tittel, Johnson (2007) Layer 1 –Converts bits into signals and vice versa Signals generated depend on the medium –Details for creating network connection are specified –Governs the type of connector used –Regulates the transmission technique –Handles intricacies of transmitting bits Specifies encoding mechanism Tries guarantee that received bits match pattern sent –Problems: improper media termination, EMI, faulty or misconfigured NICs and hubs

24 Summary of the OSI Layers Tomsho, Tittel, Johnson (2007)

25 TCP/IP and OSI Models

26 IEEE 802 Standards Wikipedia (n.d.) Defines many types of protocols and services. Maps to the Layers 1 and 2 in the OSI Reference Model. Divides Layer 2 (Link Layer) into two sub- layers: –Logical Link Control (LLC) –Media Access Control (MAC)

27 IEEE 802 Extensions to the OSI Reference Model Tomsho, Tittel, Johnson (2007)

28 IEEE 802 Specifications Tomsho, Tittel, Johnson (2007)

29 IEEE 802 Specifications (continued) Tomsho, Tittel, Johnson (2007)

30 Protocols and Protocol Suites A protocol is a set of rules to manage the communication task of one or more layers. Many protocols only apply to a single layer: – IP (Internet Protocol) applies to the network layer (layer 3) –TCP (Transmission Control Protocol) applies to the transport layer (layer 4). A collection of protocols that work together to support multiple layers is called a protocol suite. TPC/IP is not a protocol – it is a protocol suite. TCP/IP dominates and is replacing other protocol suites.

31 Protocol Stack Software that implements a protocol suite is called a protocol stack. A protocol stack is composed of software elements, each element mapping to one (or more) OSI layers. Examples: –TCP/IP (Linux, Unix, Windows, Novell Netware 5.0 and later) –IPX/SPX (Novell Netware 4.x and earlier) –NetBEUI (IBM developed for PC – no longer supported in Windows). –AppleTalk – originally developed for Macs

32 Data Frame Types Unicast Frames –Only one destination. –NICs see all frames on shared medium. –NICS only forward unicast frames to Network layer that match NICs MAC address. Multicast Frames (Odom, 2006) –Frames that are sent to a range of MAC addresses (typically E or F) –Cheap switches treat multicast frames just like broadcast frames. –More expensive switches can detect which ports want multicast frames and which do not. –Often used for audio and video conferences. Broadcast Frames –Sent to all devices in LAN. –Sent to MAC address FFFF.FFFF.FFFF

33 References Tomsho, Tittel, Johnson (2007). Guide to Networking Essentials. Boston: Thompson Course Technology. Odom, Knott (2006). Networking Basics: CCNA 1 Companion Guide. Indianapolis: Cisco Press Wikipedia (n.d.). OSI Model. Retrieved 09/12/2006 from IEEE 802. Retrieved 01/21/ Wilson, G. (2001) OSI Model Layers. Retrieved 09/23/2006 from simodel.html