DownloaD LecturE NoteS

Access Methods

The 4 major methods

• Carrier Sense Multiple Access Methods
  1. with collision detection (CSMA/CD)
  2. with collision avoidance (CSMA/CA)
• Token passing that allows only a singe opportunity to send data
• A Demand Priority method

• 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.
• Carrier Sense Multiple Access with Collision Avoidance  (CSMA/CA)
  • 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.
  • 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.
• Token Passing
  • Special packet is passed from computer to computer.
  • 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.
  • Original sending computer receives the acknowledgment and sends the token on.
  • the token comes from the Nearest Active Upstream Neighbor  and when the computer is finished, it goes to the Nearest Active Downstream Neighbor
  • uses “beaconing” to detect faults => this method is fault tolerant
  • NO contention => equal access to all computers on the network
  • NO collisions
• Demand Priority
  1. 100 Mbps standard called 100VG-AnyLAN. “Hub- based”.
  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.
  3. 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.

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

  Advantages over CSMA/CD

Other methods
Appletalk

• The cabling system for an AppleTalk network is called LocalTalk.
• LocalTalk uses  CSMA/CD
• AppleTalk has a dynamic network addressing scheme.
  • During bootup, 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.

ARCNet

• ARCNet uses a token passing method in a logical ring similar to Token Ring networks.
• However, the computers in an ARCNet network do not have to be connected in any particular fashion.
  • ARCNet can utilize a star, bus, or star bus topology.
• Data transmissions are broadcast throughout the entire network, which is similar to Ethernet.
• However, a token is used to allow computers to speak in turn.
  • The token is not passed in a logical ring order because ARCNet 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.
• ARCNet isn’t popular anymore => ARCNet speeds are a mere 2.5 Mbps.

Most important ARCNet facts for you to know:

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

802.3 (CSMA /CD – Ethernet)

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

802.4 (Token Passing)

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

802.5 (Token Ring)

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

TCP/IP

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

NetBEUI

• NetBIOS extended user interface.
• 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 operates at the Session layer.
• Small, fast and efficient.
• Compatible with most Microsoft networks.
• Not routable and compatible only with Microsoft networks.

X.25

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

XNS

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

IPX/SPX and NWLink

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

APPC

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

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

• Digital Equipment’s proprietary protocol stack
• 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
• Routable.

Protocols

• Protocols are rules and procedures for communication.

How Protocols Work

The Sending Computer

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

The Receiving Computer (same steps in reverse)

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

Protocol Stacks (or Suites)

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

The Binding Process

• 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
• 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.

Standard Stacks

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

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

Application Level Service Users

Application Layer
Presentation Layer
Session 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.
• X.400: international e-mail transmissions.
• X.500: file and directory services across systems.
• SMTP: Internet e-mail.
• FTP: Internet file transfer
• SNMP: Internet network management protocol.
• Telnet: Internet protocol for logging on to remote hosts.
• Microsoft SMB: client shells and redirectors.
• 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.

Transport Protocols

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

Examples:

• TCP (transmission control protocol): internet protocol for guaranteed delivery of sequenced data.
• 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.
• 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.
• IPX: (Internetwork Packet Exchange): Novell’s protocol for packet forwarding and routing.
• NWLink: Microsoft implementation of IPX/SPX.
• NetBEUI: Transport for NetBIOS sessions and applications.
• DDP (datagram delivery protocol): An AppleTalk data transport protocol.

Packets

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

Types of data sent includes

• Can contain information such as messages or files.
• Computer control data and commands and requests.
• Session control codes such as error correction and retransmission requests.

• Original block of data is converted to a packet at the Transport layer.

Packet Components

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

Packet Creation

• Look at the example on pp. 201 – 204
• Begins at the Application layer where data is generated.
• Each layer subsequently adds information to the packet; the corresponding layer on the receiving machine reads the information.
• 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.

Packet Addressing

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