Physical And Data Link Layers


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Physical and Data Link Layers

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Physical And Data Link Layers

  1. 1. Physical & Data Link Layers Layers that TCP/IP rely on February 4,2001 Professor Tom Mavroidis
  2. 2. TCP/IP daemons <ul><li>TCP/IP services are controlled by daemons </li></ul><ul><li>The most important of these is the Internet super server, inedt. </li></ul><ul><li>It is used to dispatch other server daemons to handle communications. </li></ul><ul><li>/etc/rc.local file runs inetd at boot time </li></ul><ul><li>inetd listens for socket connections and decides on what service to provide </li></ul>
  3. 3. Physical Layer <ul><li>Is the actual physical medium. </li></ul><ul><li>May be copper wire or fiber optic cable or the carrier wave in a wireless connection </li></ul>
  4. 4. Repeaters <ul><li>Transmit messages between networks boosting the signal in the process thereby increasing the distance between nodes. </li></ul><ul><li>Maximum # of repeaters in a segment = 4 </li></ul><ul><li>Maximum nodes in a segment for Thick coax is 100 and for thin 30 </li></ul><ul><li>Twisted pair supports 1024 nodes </li></ul>
  5. 5. Bridges <ul><li>More intelligent than repeaters </li></ul><ul><li>Operate at the Data Link layer. </li></ul><ul><li>Deal with frames rather than signals </li></ul><ul><li>Whole frame is read and retransmitted </li></ul><ul><li>examines the address to determine if on the same segment thereby reducing network traffic by not needlessly transmitting to another segment. </li></ul>
  6. 6. Switches <ul><li>Modern replacement for bridges </li></ul><ul><li>Perform better than bridges </li></ul><ul><li>Operates at Data Link layer </li></ul><ul><li>Accepts many different types of input </li></ul><ul><li>Routes traffic between and broadcasts between hosts in that group only </li></ul><ul><li>Use where LAN performance is critical </li></ul>
  7. 7. Routers <ul><li>Operates at the network layer </li></ul><ul><li>Frames are passed up from the Data Link layer </li></ul><ul><li>Can route between different types of networks I.E. Ethernet to token ring. </li></ul><ul><li>When a router also acts as a bridge it is known as a brouter </li></ul>
  8. 8. inetd <ul><li>Referred to as the network super daemon </li></ul><ul><li>Listens to several ports and dispatches other server daemons to handle incoming service requests for other protocols </li></ul><ul><li>example of inetd.conf </li></ul>
  9. 9. Inetd - continued <ul><li>If changes are made to this file you can have it reread by sending the daemon a HUP signal </li></ul><ul><li># killall -HUP inetd </li></ul><ul><li>inetd doesn’t work on all services such as HTTP, SQL, and LDAP. </li></ul><ul><li>Advantage is only one service has to be running for services that are used infrequently </li></ul>
  10. 10. Data Link Layer <ul><li>Addresses, controls and synchronizes data flow </li></ul><ul><li>Handles Data Addressing associated with each host </li></ul><ul><li>Controls data flow over the physical network </li></ul><ul><li>Data integrity, ensuring data arrives at the destination intact and in the right order </li></ul>
  11. 11. Data Addressing <ul><li>Addresses must be unique across the Physical Layer otherwise traffic will not be directed to the correct host </li></ul><ul><li>Duplication of addresses can cause network slowdown and may even cause a network segment to be inoperative. </li></ul>
  12. 12. Flow Control <ul><li>Ensures source host does not send faster than the destination host can receive otherwise the destination host will be overloaded. </li></ul><ul><li>Sets the maximum number of frames the source can send at once before waiting for the destination to let it continue </li></ul>
  13. 13. Flow Control - continued <ul><li>Often implemented in the upper protocol layers but can also be implemented at the Data Link layer. </li></ul><ul><li>The mechanism for handling overload is called Source Quench. </li></ul><ul><li>Source Quench lies within the ICMP protocol which lives in the Network layer. </li></ul>
  14. 14. Data Integrity <ul><li>Provided by dividing data into frames and adding a Cyclic Redundancy Check (CRC) </li></ul><ul><li>Electronic signature (mathematically computed) for the frame. </li></ul><ul><li>Computed at both ends, if differs retransmission is requested by the receiving end </li></ul>
  15. 15. Timers and Time Outs <ul><li>Timer is started when frame is sent </li></ul><ul><li>If no ack received in time frame is resent </li></ul><ul><li>Each frame is numbered so acknowledging frame can be identified </li></ul><ul><li>Usually handled by higher layers although can be implemented in the Data Link layer </li></ul>
  16. 16. Frames <ul><li>A packet of information </li></ul><ul><li>Many formats, sizes and encoding </li></ul><ul><li>Three ways to delineate frames </li></ul><ul><li>1)Mark the end of the frame with a special delimiter </li></ul><ul><li>2)Put the length in the frame header </li></ul><ul><li>3)Use fixed length packets </li></ul>
  17. 17. Frame Sizes <ul><li>Maximum frame size determines maximum data payload </li></ul><ul><li>Known as Maximum Transmission Unit (MTU) </li></ul><ul><li>If data is larger than MTU it must be spread across more than one frame </li></ul>
  18. 18. Manchester encoding <ul><li>Popular way to encode a string of data </li></ul><ul><li>Surround each packet byte with start, stop, and parity bits </li></ul><ul><li>Based on transitions rather than simple voltage </li></ul><ul><li>Another method is differential Manchester encoding - lack of transition denotes 1 </li></ul>
  19. 19. CSMA/CD - Physical layer <ul><li>CSMA/CD Carrier Sense Multiple Access with Collision Detect </li></ul><ul><li>host listens for a quiet period before sending frame (CS) </li></ul><ul><li>CD - host detects when a collision occurred </li></ul><ul><li>waits a random amount of time and resends </li></ul><ul><li>further retries use an exponential back-off </li></ul>
  20. 20. Variation of CSMA/CD <ul><li>1-persistent - always sends a frame in response to a clear line (Ethernet uses this) </li></ul><ul><li>non-persistent - each station waits a random interval before checking the line </li></ul><ul><li>p-persistent - uses a variable probability scheme </li></ul>
  21. 21. Strength of CSMA/CD <ul><li>Good at handling variable loads </li></ul><ul><li>Minimizes collisions at low to medium loads </li></ul><ul><li>Easy to plug in or remove hosts </li></ul><ul><li>Requires little admin </li></ul><ul><li>Simple concept - easy to implement </li></ul>
  22. 22. Token Passing <ul><li>Logical token is passed between hosts in a ring </li></ul><ul><li>Host can only send frames while it holds on to the token. </li></ul><ul><li>No Collisions </li></ul><ul><li>Much more complex than CSMA/CD </li></ul>
  23. 23. Ethernet <ul><li>Most common LAN in the TCP/IP environment </li></ul><ul><li>Connected in bus or star topology </li></ul><ul><li>Uses CSMA/CD </li></ul><ul><li>Three types of cable can be used </li></ul><ul><li>10Base5, 10Base2, 10BaseT </li></ul>
  24. 24. Token Ring <ul><li>Designed by IBM standardized by IEEE (referred to as IEEE 802.5) </li></ul><ul><li>Uses a logical and physical ring topology </li></ul><ul><li>Network is physically constructed into a star shape connected by MSAUs Multistation Access Units. </li></ul><ul><li>Speed are 4 and 16 Mbps </li></ul><ul><li>Uses differential Manchester encoding </li></ul>
  25. 25. Serial Protocols <ul><li>Used when a connection is between two points only </li></ul><ul><li>Based on direct links, leased lines, or modem- based access through POTS </li></ul><ul><li>Relatively slow speeds </li></ul><ul><li>Improvements have led to more sophisticated protocols across serial lines called nomadic workstations </li></ul>
  26. 26. Slip <ul><li>Early protocol to send IP over a serial link </li></ul><ul><li>Never formalized into a standard </li></ul><ul><li>Start and end of an IP datagram is detected by reading bytes from a serial connection </li></ul><ul><li>No longer predominant </li></ul><ul><li>Limitations arise from its simplicity </li></ul><ul><li>Fixed mapped IP - no dynamic address allocation </li></ul>
  27. 27. PPP <ul><li>Point to Point Protocol </li></ul><ul><li>Modern implementation of SLIP </li></ul><ul><li>Standardized </li></ul><ul><li>Based on the HDLC standards </li></ul><ul><li>Dynamic IP - many things can be negotiated about the current session </li></ul>
  28. 28. Features of PPP <ul><li>Error Checking </li></ul><ul><li>Synchronous/asynchronous </li></ul><ul><li>Provides weak (PAP) and strong (CHAP) authentication </li></ul>
  29. 29. Three components to PPP <ul><li>Encapsulation mechanism - includes and additional field that identifies the protocol using the link </li></ul><ul><li>Link Control Protocol - Link control protocol us used to establish, configure and test the data-link connection </li></ul><ul><li>Network Control Protocol - used to negotiate the network layer protocol </li></ul>
  30. 30. Things to review <ul><li>IP Addressing </li></ul><ul><li>Netmasks </li></ul><ul><li>Reserved Addresses </li></ul><ul><li>Broadcast Addresses </li></ul><ul><li>Multicasting </li></ul><ul><li>Network Interfaces </li></ul>