Building a Network with OSI Chapter 3

Contents Explain the major functions of network hardware Describe the functions of network software Define each of these functions as part of the OSI seven-layer model

Overview

The Big Job Whenever you have a big job to do, it helps to break it down into discrete chunks or functions For example, moving your family from one city to another could be broken down into: What to move How to pack How to load the van How to unload the van The job of moving data from one place to another is also a big job

OSI Seven-Layer Model The big job of moving data from one place to another is broken down into functions defined by the OSI Seven-Layer Model

Case Study Example Let’s look at the process of networking from a conceptual viewpoint Assume we are just trying to move a file from one PC to another in a small office One of the workers has just completed a new employee handbook She needs to transfer the Word document to the other worker for review

Case Study Example The file could be copied to a diskette, USB, or CD/DVD and handed over to the other worker – called Sneakernet – but that’s not necessary today The document may be transferred using the network The next section examines the various hardware required

Let’s Get Physical

Cables Most networks use a cable like that shown as a physical channel to move the bits of data Unshielded Twisted Pair (UTP) cable Uses 4 wires: 2 for sending and 2 for receiving data

Hubs Each computer system has a cable leading to a device called a hub Usually located in a closet The hub sends the data received from one system to all the other systems attached to it

Network Interface Card Network Interface Cards (NICs – pronounced “Nicks”) are installed in PCs Network cables are attached to the NICs

NIC to Hub Connections Cables run from the NIC in the PC to a jack on the wall Cables run through the walls to the closet where they connect to a hub

Network Cabling System

Network Interface Cards Since all the networked systems are connected to the same hub, each system must have a unique identifier Media Access Control (MAC) address A unique address burned into a ROM chip on the network card Each MAC address is 12 hex characters or 48 bits in length MAC address printed on surface of chip – it’s burned inside the chip.

MAC Addresses MAC addresses are 48 bits long Usually represented using hexadecimal characters (12 hex digits = 48 bits) Here’s a typical MAC address: 004005-607D49 Identifies the manufacturer Unique serial number determined by the manufacturer No two MAC addresses are ever the same!

WINIPCFG Used on Windows 98/Me systems to view network configuration MAC address

ipconfig /all Used on Windows NT/2000/XP systems to view network configuration Determine your own MAC address using either WINIPCFG or ipconfig/all. MAC address

Bits A MAC address is a series of ones and zeros called bits Data is sent using pulses of electricity, light, or radio waves

Frames Data is sent across the network in frames Frames are discrete chunks of data There are a number of different frame types used in different networks. All NICs on the same network must use the same frame type or they will not be able to communicate with other NICs.

Fields Frames are made up of fields that contain information Frames typically contain the recipient’s MAC address, the sender’s MAC address, the data itself, and a cyclic redundancy check (CRC) for error checking CRCs (Cyclic Redundancy Check) are used to check the frame for accuracy Frames are sometimes corrupted while in transit from one device to another The data in the frame is divided by a number called the key, producing a four byte CRC value that is placed in the frame at the sending station The receiving station does the same calculation on the received frame If the value matches the CRC, the frame is good

Data What is inside the data part of the frame? It could be part of a file, a print job, a web page, anything NICs do not care what the data, or payload, is

Frame Size Different networks use different sizes of frames Many frames hold about 1500 bytes of data The sending software breaks up large amounts of data into smaller chunks The receiving station must then put the chunks back together in the proper order

Processing Framesq All devices on the network see the frame, but only the device that it is addressed to will process it Every frame is received by every NIC The MAC address is used to decide if the frame belongs to a given device

Getting Data on the Cable Only one system may speak at a time since the cable is shared Processes are used to keep two NICs from talking at the same time

Getting To Know You How does the sending NIC know the MAC address of the NIC to which it is sending data? Most of the time the two devices have talked before, so the destination MAC address is already known If the MAC address is not known, then a broadcast message is sent over the network The destination device will respond by sending its MAC address A broadcast MAC address is FF-FF-FF-FF-FF-FF How does the destination device know that it is the intended destination device? Remember, the MAC address is still unknown. On most networks today, another address called an IP address is used to identify the destination device.

Moving Frames – Step 1 The sending system’s network software hands some data to the NIC The NIC begins building a frame

Moving Frames – Step 2 After the NIC creates the frame, it adds the CRC and data to it It also puts its MAC address and the destination’s MAC address in the frame

Moving Frames – Step 3 When no other NIC is using the cable, it sends the frame through the cable

Moving Frames – Step 4 The frame propagates down the wire to the hub The hub creates a copy of the frame to send to every other system The receiving device processes the frame

Moving Frames – Step 5 The receiving station checks the CRC value in the frame If the value matches what it should, then the NIC sends the data portion to the network operating system for processing If the value does not match, the frame has errors and must be resent

Beyond the Single Wire – Network Software

Beyond the Single Wire What if one system is using a modem to dial into the network? What if one of the systems is a Macintosh?

Routers A single network with single hub can only support up to 1,024 computers before the network becomes too slow Routers are used to chop large networks up into smaller ones

Network Protocols Network protocols define rules for how systems are addressed, how to chop data up into chunks, how to deal with routers, and so on As a network grows a more universal addressing method than MAC addresses is needed TCP/IP are the most popular universal addressing protocols

TCP/IP Transmission Control Protocol (TCP) Internet Protocol (IP) Gives each device on the network a unique numeric identifier IP addresses consist of four 8-bit numbers Each 8-bit number ranges from 0 to 255 192.168.4.232 No two systems on the same network share the same IP address

DHCP IP addresses must be manually configured on each device Or they may be automatically configured using Dynamic Host Configuration Protocol (DHCP) Each system in a network has two addresses MAC address burned into a chip on the NIC IP address configured through software

Sample Network with Addresses

IP packet with frame added Packets The network software creates a packet that contains the sending and receiving IP addresses along with the data The packet is enclosed within a frame that contains the sending and receiving MAC addresses IP packet in a frame The diagram above does not include all the fields in an IP packet – it is a highly simplified IP packet. IP packet IP packet with frame added

Connecting to the Internet To connect the local network to the Internet a router is needed The local hub is connected to the router The router is connected to the Internet through a phone line The phone line uses a different kind of frame, so the router strips the frame and creates a new one

Connecting to the Internet Router removing network frame and adding one for telephone line Adding a router to the network

Moving through the Internet The router strips off the MAC addresses and uses the type of addressing the phone company uses instead The frame uses the IP address to guide it to the receiving system The receiving router strips off the telephone frame and adds the MAC address for the receiving system The NIC strips off the MAC header and hands the frame off to the NOS

Assembly and Disassembly Most data is much larger than a single frame Network protocols chop up the data into smaller packets and gives each one a sequence number The sequence numbers are used by the receiving system to put the packets back in order and to assemble them Transmission Control Protocol (TCP) is one such protocol

Sessions If the receiving system also has a printer to share on the network, then it may receive packets for print jobs as well as packets for files The sending system must also contact the receiving system to make sure it is ready to handle a print job as well The software that handles these processes is called session software

Multiple Sessions Single session Multiple sessions How many sessions does a PC have running at a time? If your network uses TCP/IP, use the following command to list each session line by line: Netstat –a

Standardized Formats Macintoshes and PCs use very different formats Standardized formats have been created that allow very different operating systems to exchange data

VPNs Virtual Private Networks (VPNs) enable a user away from the office to connect to the corporate network via the Internet

Encryption Many networks encrypt data to prevent unauthorized access Both the sending and receiving system must know the encryption method used

Network Applications Users use network applications to exchange data on a network My Network Places in Windows for files Internet Explorer or Netscape Navigator for web pages Outlook Express for e-mail

Copy a File Across a Network The next few slides illustrate a typical process that takes place to copy a file from one machine to another over the network

My Network Places The receiving station finds the remote file using My Network Places

Moving the Word Document Drag and drop the Word document from My Network Places to the Desktop

Disassembly The sending system chops the packet into segments and assigns sequence numbers

Addressing Each packet is assigned addresses

Assembling the Frame The NIC adds a frame around each packet containing the MAC addresses

Grabbing the Frame When the network cable is not busy, the frame is sent down the wire Every NIC looks at the frame. Only the receiving device grabs it

Checking the CRC The receiving station checks the CRC to make sure the frame doesn’t have an error It then strips off the frame header and passes it up to the next layer

Missing Packets If one of the packets is missing, the receiving device asks for it to be resent

It’s Amazing! Once the document has been reassembled, it is sent to the proper application It’s amazing but almost all of this process is not seen by the user

The OSI Seven-Layer Model

Early Networking In the early days of networking each company designed the networking process from the ground up These proprietary systems could not talk to each other For networking to grow a model needed to be created to standardize the process

OSI Model The International Organization for Standardization (ISO) proposed the Open System Interconnection (OSI) Model The OSI Model is a seven-layer model that describes the networking process ISO is derived from the Greek word isos, which means equal.

OSI’s Seven Layers Layer 7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 2 Data Link Layer 1 Physical Exam Tip Memorize both the layer number and the name of each OSI layer.

Modular Design Each protocol is designed to deal with a specific layer Each protocol needs to know how to interface with the layer immediately above and below it These layers are not laws of physics – anybody that designs a network can do it any way they want. While many protocols fit neatly into one of the seven layers, others do not.

Layer 7 - Application Defines a set of tools that programs may use to access the network HTTP FTP Microsoft APIs

Layer 6 - Presentation Presents data from the sending system in a form that applications on the receiving system can understand ASCII and Unicode text formats Encryption Compression Translates between different machine languages

Layer 5 - Session Manages the connections between machines on the network Connections may be for file transfers, for print jobs, for e-mail, or many other possibilities Allows machines to keep track of who they are talking to

Layer 4 - Transport Breaks up data it receives from the upper layers into smaller pieces for transport On the receiving side the packets are reassembled from lower layers Provides for error checking Layer 4 is a pivotal layer in the process Lower layers are concerned with moving data from point A to point B Upper layers deal with the data

Layer 3 - Network Helps to get packets from network to network by adding unique identifiers (like IP addresses) to the packets

Layer 2 – Data Link Defines the rules for accessing and using the Physical layer Specifies the rules for identifying devices Determines which machine may use the network at a given time Checks for errors The Data Link layer is divided into two sub-layers: Media Access Control (MAC) Logical Link Control (LLC) The LLC layer sits above the MAC layer and under the Network layer. The MAC sub-layer controls access to the Physical layer. It encapsulates frames, adds source and destination MAC addresses and error checking, and decapsulates data at the receiving station. The LLC sub-layer provides an interface with Network layer protocols. It provides for flow control and retransmission of corrupt packets.

Layer 1 - Physical Defines the physical form taken by data when it travels across a cable Defines how ones and zeros are turned into actual electrical signals on a wire, light pulses on a fiber optic cable, or radio waves in a wireless network

NICs and Layers Network Interface Cards work at both layer 2 and layer 1 When it is said that a NIC is a layer 2 device, keep in mind that it also functions as a layer 1 device