Networking Essentials Notes - Section 2

How a Network Functions

The OSI Model

• International Standards Organization (ISO) specifications for network architecture.
• Each layer communicates logically with its associated layer on the other computer.
• Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.
• Called the Open Systems Interconnect or OSI model.
• Seven layered model, higher layers have more complex tasks.
• Each layer provides services for the next higher layer.

OSI Layers

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

Application Layer

• Handles general network access, flow control and error recovery.
• Serves as a window for applications to access network services.

Presentation Layer

• Determines the format used to exchange data among the networked computers.
• Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
• Translates data from a format from the Application layer into an intermediate format.
• Redirector operates at this level.

Session Layer

• Performs name recognition and security.
• Allows two applications running on different computers to establish use and end a connection called a Session.
• Provides synchronization by placing checkpoints in the data stream.
• Implements dialog control between communicating processes.

Transport Layer

• Responsible for packet creation.
• Unpacks, reassembles and sends receipt of messages at the receiving end.
• Provides flow control, error handling, and solves transmission problems.
• Provides an additional connection level beneath the Session layer.
• Ensures that packets are delivered error free, in sequence with no losses or duplications.

Network Layer

• Determines the route from the source to the destination computer.
• Responsible for addressing messages and translating logical addresses and names into physical addresses.
• Manages traffic such as packet switching, routing and controlling the congestion of data.

Data Link Layer

• Sends data frames from the Network layer to the Physical layer.
• Responsible for providing error free transmission of frames through the Physical layer.
• Packages raw bits into frames for the Network layer at the receiving end.

Physical Layer

• Transmits the unstructured raw bit stream over a physical medium.
• Defines data encoding and bit synchronization.
• Relates the electrical, optical mechanical and functional interfaces to the cable.
• Defines how the cable is attached to the network adapter card.

The 802 Project Model

• WAN components.
• Network Adapter Cards.
• Components used to create twisted-pair and coaxial cable networks.
• Defines Standards for the Data Link and Physical Layers.
• A crazy mnemonic for this table, but it works :-)

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802.1	Internet working
802.2	Division of Data Link Layer into sub layers LLC (Logical Link Control) Media Access Control (MAC)
802.3	CSMA/CD - Ethernet
802.4	Token Bus LAN (ARC net)
802.5	Token Ring LAN
802.6	MAN (Metropolitan Area Network)
802.7	Broadband Technical Advisory Group
802.8	Fiber-Optic Technical Advisory Group
802.9	Integrated Voice/Data Networks
802.10	Network Security
802.11	Wireless Networks
802.12	Demand Priority Access LAN, 100 Base VG - Any LAN

OSI Model Enhancements

The bottom two layers - Data Link and Physical - define how multiple computers can simultaneously use the network without interfering with each other.

• Divides the Data-link layer in to the Logical Link Control and Media Access Control sub layers.
• Logical Link Control
• Defines logical interface points called Service Access Points (Sap’s). This Sap’s are used to transfer information to upper layers.
• Manages error and flow control.
• 
  
• Media Access Control
• Is responsible for delivering error-free data between two computers.
• Communicates directly with the network adapter card and
• Categories
• 802.12 define standards for both this sub layer and the Physical layer
• 802.3
• 802.4
• 802.5 and

Drivers

• A device driver is software that tells the computer how to drive or work with the device so that the device performs the job it's supposed to.
• Provide communication between a network adapter card and the redirector in the computer.
• the Media Access Control driver is another name for the network card device driver
• When installing a driver, you need to know these things
• IRQ
• I/O Port Address
• Memory Mapped (Base Memory Address)
• Transceiver Type
• Drivers are called

Network Drivers,

MAC drivers,

NIC drivers.

• 
  
• Resides in the Media Access Control sub layer of the Data Link layer. Therefore, the NIC driver ensures direct communication between the computer and the NIC.

Packets

• Special control information is added in order to:
• disassemble packets
• reassemble packets
• check for errors
• Data is broken down into smaller more manageable pieces called packets.

Types of data sent includes

• Computer control data and commands and requests.
• Original block of data is converted to a packet at the Transport layer.
• Can contain information such as messages or files.
• Session control codes such as error correction and retransmission requests.

Packet Components

• Header
1. Alert signal to indicate packet is being transmitted
2. Source address.
3. Clock synchronization information.
4. Destination address.
• Data
1. Varies from 512 to 4096 bytes (4K), depending on the network
2. Contains actual data being sent.
• Trailer
1. Usually contains a CRC.
2. Content varies by protocol.

Packet Creation

• Look at the example on pp. 201 - 204
• Begins at the Application layer where data is generated.
• Transport layer breaks the data into packets and adds sequencing information needed to reassemble data at the other end => the structure of the packets is defined by the common protocol being used between the two computers.
• Data is passed through the Physical layer to the cable.
• Each layer subsequently adds information to the packet; the corresponding layer on the receiving machine reads the information.

Packet Addressing

• A broadcast type address gets attention of all computers on the network
• Every NIC sees all packets sent on its cable segment but only interrupts the computer if the packet address matches the computer's address

Protocols

• Protocols are rules and procedures for communication.

How Protocols Work

The Sending Computer

• Breaks data into packets.
• Prepares the data for transmission.
• Adds addressing information to the packet

The Receiving Computer (same steps in reverse)

• Strips the data from the packet.
• Copies the data to a buffer for reassembly.
• Takes the packet off the cable.
• Passes the reassembled data to the application.

Protocol Stacks (or Suites)

• Ensure that data is prepared, transferred, received and acted upon.
• A combination of protocols, each layer performing a function of the communication process.

The Binding Process

• Binding order dictates which protocol the operating systems uses first.
• Binding also happens with the Operating System architecture: for example, TCP/IP may be bound to the NetBIOS session layer above and network card driver below it. The NIC device driver is in turn bound to the NIC.
• Allows more than one protocol to function on a single network adapter card. (e.g. both TCP/IP and IPX/SPX can be bound to the came card

Standard Stacks

• Novell NetWare
• Apple AppleTalk
• TCP/IP
• ISO/OSI
• IBM SNA (Systems Network Architecture)
• Digital DECnet

Protocol types map roughly to the OSI Model into three layers:

Application Level Service Users

Application Layer

Session Layer
Presentation Layer

Transport Services

Transport Layer

Network Services

Network Layer
Data Link Layer
Physical Layer

Application Protocols

Work at the upper layer of the OSI model and provide application to application interaction and data exchange.

Examples:

• APPC-IBM's peer to peer SNA protocol used on AS400's
• FTAM: an OSI file access protocol.
• SMTP: Internet e-mail.
• FTP: Internet file transfer
• SNMP: Internet network management protocol.
• X.400: international e-mail transmissions.
• NCP: Novell client shells or redirectors.
• AppleTalk and AppleShare: Apple's protocol suite.
• AFP: Apple's protocol for remote file access.
• DAP (data access protocol): DECnet file access protocol.
• X.500: file and directory services across systems.
• Telnet: Internet protocol for logging on to remote hosts.
• Microsoft SMB: client shells and redirectors.

Transport Protocols

These protocols provide communication sessions between computers and ensure data is moved reliably between computers.

Examples:

• SPX (sequenced packet exchange): Novell protocol suite.
• NWLink: Microsoft implementation of IPX/SPX.
• NetBEUI: establishes communications sessions between computers and provides the underlying data transport services.
• TCP (transmission control protocol): internet protocol for guaranteed delivery of sequenced data.
• ATP, NBP: Apple's communication session and transport protocols.

Network Protocols

These provide link services

They also

• handle

• addressing and routing,
• error checking and
• Retransmission requests.

• Define rules for Ethernet or Token Ring.

Examples:

• IP (Internet Protocol): packet forwarding and routing.
• NWLink: Microsoft implementation of IPX/SPX.
• NetBEUI: Transport for NetBIOS sessions and applications.
• IPX: (Internet work Packet Exchange): Novell's protocol for packet forwarding and routing.
• DDP (datagram delivery protocol): An AppleTalk data transport protocol.

The IEEE protocols at the Physical Layer

802.3 (CSMA /CD - Ethernet)

• Logical bus network.
• Data is transmitted on the wire to every computer but only those meant to receive respond.
• can transmit at 10 Mbps
• CSMA /CD protocol listens and allows transmission when the wire is clear

802.4 (Token Passing)

• bus layout that used token passing
• token determines which computer can send
• every computer receives all of the data but only the addressed computers responds

802.5 (Token Ring)

• transmits at 4 Mbps or 16 Mbps
• logical ring network; physical set up as star network
• token determines which computer can send

Important Protocols

TCP/IP

• Routable, defector standard for internetworking.
• SMTP, FTP, SNMP are protocols written for TCP/IP
• Provides communications in a heterogeneous environment.
• Disadvantages are size and speed.

NetBEUI

• Originally, NetBIOS and NetBEUI were tightly tied together but, NetBIOS has been separated out to be used with other routable protocols. NetBIOS acts as a tool to allow applications to interface with the network; by establishing a session with another program over the network.
• NetBIOS extended user interface.
• Compatible with most Microsoft networks.
• NetBIOS operates at the Session layer.
• Small, fast and efficient.
• Not routable and compatible only with Microsoft networks.

X.25

• Originally established to connect remote terminals to mainframe hosts.
• Protocols incorporated in a packet switching network of switching services.

XNS

• Xerox Network System.
• Large, slow and produces a lot of broadcasts.
• Developed for Ethernet LANs but has been replaced by TCP/IP.

IPX/SPX and NWLink

• Small and fast.
• Used for Novell networks.
• Routable.

APPC

• Designed to enable application programs running on different computers to communicate and exchange data directly.
• Advanced Program to Program Communication
• Developed by IBM to support SNA.

AppleTalk

• Apple's proprietary protocol stack for Macintosh networks.

OSI Protocol Suite

• each protocol maps directly to a single layer of the OSI model

DECnet

• Defines communications over Ethernet, FDDI MAN's and WAN's.
• DECnet can also use TCP/IP and OSI protocols as well as its own protocols.
• Digital Equipment's proprietary protocol stack
• Routable.

Putting data on the Cable

Access Methods

The 4 major methods

• Carrier Sense Multiple Access Methods
1. With collision detection (CSMA/CD)
2. With collision avoidance (CSMA/CA).

• Carrier Sense Multiple Access with Collision Detection. (CSMA/CD)
1. Computer senses that the cable is free.
2. Data is sent.
3. If data is on the cable, no other computer can transmit until the cable is free again.
4. If a collision occurs, the computers wait a random period of time and retransmit.

• Known as a contention method because computers compete for the opportunity to send data. (Database apps cause more traffic than other apps)
• This can be a slow method
• More computers cause the network traffic to increase and performance to degrade.
• The ability to "listen" extends to a 2,500 meter cable length => segments can't sense signals beyond that distance.
• Token passing that allows only a singe opportunity to send data

• A Demand Priority method

• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
• All other computers wait until the data is sent.
• The major drawback of trying to avoid network collisions is that the network traffic is high due to the broadcasting of the intent to send a message.
• In CSMA/CA, the computer actually broadcasts a warning packet before it begins transmitting on the wire. This packet eliminates almost all collisions on the network because each computer on the network does not attempt to broadcast when another computer sends the warning packet.

• Token Passing
• Special packet is passed from computer to computer.
• Original sending computer receives the acknowledgment and sends the token on.
• NO collisions.
• The token comes from the Nearest Active Upstream Neighbor and when the computer is finished, it goes to the Nearest Active Downstream Neighbor.
• A computer that wants to transmit must wait for a free token.
• Computer takes control of the token and transmits data. Only this computer is allowed to transmit; others must wait for control of the token.
• Receiving computer strips the data from the token and sends an acknowledgment.
• uses "beaconing" to detect faults => this method is fault tolerant
• NO contention => equal access to all computers on the network.

• Demand Priority

0. 100 Mbps standard called 100VG-AnyLAN. "Hub- based".
1. Certain types of data are given priority if data reaches the repeater simultaneously. If two have the same priority, BOTH are serviced by alternating between the two.
2. Repeaters manage network access by performing cyclical searches for requests to send from all nodes on the network. The repeater or HUB is responsible for noting all addresses, links and end nodes and verifying if they are all functioning. An "end node" can be a computer, bridge, router or switch.

Advantages over CSMA/CD

1. Transmissions are through the HUB and are not broadcast to all other computers on the network.
2. There is only communication between the sending computer, the hub and the destination computer.
3. Computers Uses four pairs of wires which can send and receive simultaneously.

Other Methods

AppleTalk

• The cabling system for an AppleTalk network is called Local Talk.
• AppleTalk has a dynamic network addressing scheme.
• During boot up, the AppleTalk card broadcasts a random number on the network as its card address. If no other computer has claimed that address, the broadcasting computer configures the address as its own. If there is a conflict with another computer, the computer will try to use different IP combinations until it finds a working configuration.
• Local Talk uses CSMA/CA

ARC Net

• ARC Net uses a token passing method in a logical ring similar to Token Ring networks.
• However, a token is used to allow computers to speak in turn.
• The token is not passed in a logical ring order because ARC Net does not use the ring topology; instead the token is passed to the next highest numerical station
• Use DIP switches to set the number (the Station Identifier) of the workstations, which you want to be beside each other so the token is passed to the next computer efficiently.
• However, the computers in an ARC Net network do not have to be connected in any particular fashion.
• ARC Net can utilize a star, bus, or star bus topology.
• Data transmissions are broadcast throughout the entire network, which is similar to Ethernet.
• ARC Net isn't popular anymore => ARC Net speeds are a mere 2.5 Mbps.

Most important ARC Net facts for you to know:

• It uses a logical-ring media access method.
• ARC Net uses RG-62 (93 ohms) cabling;
• It can be wired as a star, bus, or star bus; and

Summary Chart

Feature or Function	CSMA/CD	CSMA/CA	Token Passing	Demand Priority
Type of Communication	Broadcast-based	Broadcast-based	Token-based	Hub-based
Type of Access Method	Contention	Contention	Non-contention	Contention
Type of Network	Ethernet	Local Talk	Token Ring ARC net	100VG-AnyLAN