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					          Data Link Layer
Lecture 22




                          Imran Ahmed
    University of Management & Technology

                                            1
                     Agenda

• Introduction & services

• Error detection and correction

• Multiple access protocols

• LAN addresses and ARP

• Ethernet

• Hubs, bridges and switches


                                   2
                       Ethernet
• IEEE divides the baseband category into five different
  standards:-
   – 10Base5
   – 10Base2
   – 10Base-T
   – 1Base5
   – 100Base-T
• IEEE defines only one specification for the broadband
  category:-
   – 10Base36


                                                           3
                    Ethernet
• Access Method (CSMA/CD):
  – When a station wants to transmit, it checks the
    cable. If the cable is busy, the station waits until
    it goes idle; otherwise it transmits immediately.
  – If two or more stations simultaneously begin
    transmitting on an idle cable, they will collide.




                                                       4
                Ethernet
• Addressing: Each station on an Ethernet
  network has its own NIC. Which has a
  unique address of 48-bits.
• Signaling:     The baseband system uses
  Manchester digital encoding and 10Base36
  system uses Differential PSK.
• Data Rate:     Ethernet LANs can support
  data rates between 1 to 100 Mbps.



                                             5
                    Ethernet
• Frame Format:       IEEE 802.3 specifies
  one type of frame containing seven fields:-
  –   Preamble
  –   Start Frame Delimiter (SFD)
  –   Destination Address (DA)
  –   Source Address (SA)
  –   802.2 Frame
  –   CRC
  –   Length of PDU

                                                6
MAC Frame




            7
                    Ethernet
• Preamble:
  – It contains 7-bytes of alternating 0s and 1s that
    alert the receiving system to the coming frame
    and enable it to synchronize its input timing.
  – HDLC combined the alert, timing and start
    synchronization into a single field: the flag.
  – IEEE 802.3 divides these three functions
    between the preamble and the second field
    (SFD).


                                                        8
                    Ethernet
• Start Frame Delimiter (SFD): The second field (1-
  byte) of the 802.3 frame signals the beginning of
  the frame. The SFD tells the receiver that
  everything that follows is data, starting with the
  addresses.
• Destination Address (DA): The DA field is
  allotted 6-bytes and contains the physical address
  of the packet’s next destination.




                                                       9
                    Ethernet
• Source Address (SA): The source address field is
  allotted 6-bytes and contains the physical address
  of the last device to forward the packet.
• PDU:        The next 2-bytes indicate the number of
  bytes in the coming PDU.
• 802.2 Frame: This field contains the entire data.
• CRC:        The last field in the 802.3 frame
  contains the error detection mechanism.



                                                   10
11
                 Ethernet (10Base5)
• 10base5 (thicknet) has a bus topology LAN that
  uses baseband signaling and has a maximum
  segment length of 500 meters.
• The physical connectors and cables utilized by
  10base5 include coaxial cable, NIC, transceivers
  and attachment unit interface (AUI) cables
   – RG-8 Cable
   – Each station is linked to its corresponding transceiver
     by an AUI, also called transceiver cable.



                                                               12
10BASE5




          13
Ethernet Segments




                    14
15
             Ethernet (10Base2)
• 10Base2 (thinnet) provides an inexpensive
  alternative to 10base5 Ethernet with the
  same data rate.
  – The disadvantage is shorter range (185 meters).
  – It uses RG-58 coaxial cable and BNC-T type
    connectors.




                                                  16
10BASE2




          17
18
              Ethernet (10Base-T)
• 10Base-T (twisted-pair), a star topology LAN
  using UTP cable instead of coaxial cable.
• It supports a data rate of 10 Mbps and has a
  maximum length (hub to station) of 100 meters.
• Instead of individual transceivers, 10base-T
  Ethernet places all of its networking operations in
  an intelligent hub with a port of each station.
• Stations are linked into the hub by four-pair RJ-45
  cable.


                                                    19
10BASET




          20
21
             Ethernet (1Base5)
• 1Base5 (Star LAN) is an AT&T product
  used infrequently today because of its slow
  speed (only 1Mbps).




                                                22
               Switched Ethernet
• Switched Ethernet is an attempt to improve the
  performance of 10Base-T Ethernet.
• The 10Base-T Ethernet is a shared media network, which
  means that the entire media is involved in each
  transmission.
• This is because of topology, though physically a star, is
  logically a bus.
• For example: Station A is sending a frame to station E. The
  frame is received by the hub and is sent to every station
  (broadcast). All of the cabling in the system is involved in
  this transmission.



                                                            23
               Switched Ethernet
• However, if we replace the hub with a switch (a device that
  can recognize the destination address and can route the
  frame to the port to which the destination station is
  connected), the rest of the media are not involved in the
  transmission process.
• This means that the switch can receive another frame from
  another station at the same time and can route this frame to
  its own final destination.
• For example: When station A is sending a frame to station
  E, station B can also send a frame to station D without any
  collision.




                                                            24
 Hubs
   &
Switches




           25
              Gigabit Ethernet
• Gigabit Ethernet operates at 1000 Mbps or 1 Gbps
  approx.
• In Gigabit Ethernet, the MAC layer and the access
  method remain the same, but the collision domain
  is reduced.
• Gigabit is mainly designed to use optical fiber,
  although the protocol does not eliminate the use of
  twisted pair cables.
• Gigabit Ethernet usually serves as a backbone to
  connect Fast Ethernet networks.


                                                    26
                    Gigabit Ethernet


Feature   1000Base-SX 1000Base-LX 1000base- 1000Base-T
                                  CX

Medium    Optical Fiber   Optical Fiber   STP          UTP

Signal    Short-wave      Long-wave       Electrical   Electrical
          laser           Laser

Max.     550 m            550 m (multi- 25 m           25 m
Distance                  mode)
                          5000 m
                          (single-mode)


                                                                    27

				
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posted:6/25/2011
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