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  • 1. 141-623 Communication networks and networking for managers Suntorn Witosurapot Contact Address: Phone : 074 287369 or Email: wsuntorn@coe.psu.ac.th November 2003
  • 2. Chapter 3 A Small Ethernet PC Network
  • 3. Review: Architecture Reference Presentation OSI TCP/IP Application Application Session Transport Network Transport Internet Data Link Physical Data Link Physical General Function Application Communication Internetworking Single Network Transmission
  • 4. Review: a View of Horizontal Layered Message Communication in TCP/IP PC Ethernet Switches Routers (IP) Message Web Server Message PC Web Server
  • 5. Review: a View of Logical Message Communication in TCP/IP HTTP Msg Application Layer Proc Proc = Process Application Layer Proc Transport Layer Proc TCP Msg Transport Layer Proc Network Layer Proc IP Packet Network Layer Proc Network Layer Proc Client PC Server Ethernet Switch Router Data Link Layer Proc Eth Frame Data Link Layer Proc Data Link Layer Proc Data Link Layer Proc Physical Layer Proc Physical Layer Proc Physical Layer Proc Physical Layer Proc
  • 6. Review: Communication at All Layers on the Source Host HTTP Msg Application Process TCP Hdr HTTP Msg Transport Process TCP Hdr IP Hdr HTTP Msg Internet Process DL Hdr TCP Hdr IP Hdr DL Hdr HTTP Msg Data Link Process
  • 7. Preview: An Ethernet PC network
    • Our goals:
    • Understand Ethernet architecture and its components
      • Able to differentiate between hubs and switches and reasons for the growing dominance of switches
      • Understand the division of data link into the medium access control and logical link control layers
  • 8. Elements of a Small Ethernet PC Network Access Router A1-44-D5-1F-AA-4C D4-47-55-C4-B6-9F To the Internet Ethernet Switch or Hub B2-CD-13-5B-E4-65 C3-2D-55-3B-A9-4F UTP UTP UTP UTP UTP Broadband Modem Client Client Server Server
  • 9. Network Interface Card (NIC)
    • Each station must have a network interface card (NIC)
    • This is true for both clients and servers
    • The NIC implements LAN protocols
    Physical Data Link
  • 10. Ethernet Standards
    • Ethernet standards dominate in small LANs
      • Governs switches, wiring and NIC (network interface cards)
    • IEEE (Institute of Electrical and Electronics Engineers) Standardization
      • 802 LAN/MAN Standards Committee (802 Committee) creates LAN standards
        • 802.3 Working Group creates Ethernet standards
        • So Ethernet standards are also known as 802.3 standards.
    Physical Data Link
  • 11. Ethernet Standards (cont.)
    • Physical Layer Ethernet Standards Using UTP
      • 802.3 10 Base-T
        • 10 Mbps
      • 802.3 100 Base-TX, 100Base-FX
        • 100 Mbps
        • 10/100 operation (auto-sensing)
      • 802.3 1000Base-T
        • Gigabit Ethernet (?? Mbps)
    Physical Data Link T = twisted pair; F = fiber optics;
  • 12. Purchasing and Installing UTP
    • Wiring Quality Categories
      • Governed by the TIA/EIA-568 standard
      • Categories 3, 4, 5, 5e (enhanced) and 6 (released in 2002 )
        • Higher numbers indicate better quality
      • Category 5e is recommended for new buildings
      • Category 5 or 5e is required for 100Base-TX
      • Category 5 or 5e is required for 1000Base-TX (Gigabit Ethernet), but 5 is marginal
    Physical Data Link
  • 13. Hubs and Switches:
    • Available either in a form of
      • Desktop box, or
      • 19’’ Stackable/Rack-Mountable module
  • 14. Hub Operation A1-44-D5-1F-AA-4C D4-47-55-C4-B6-9F Ethernet Hub B2-CD-13-5B-E4-65 C3-2D-55-3B-A9-4F UTP UTP UTP UTP Client Client Server Server Station A1-44-D5-1F-AA-4C transmits a bit. Hub broadcasts the bit out all other ports.
  • 15. Hub Operation A1-44-D5-1F-AA-4C D4-47-55-C4-B6-9F Ethernet Hub B2-CD-13-5B-E4-65 C3-2D-55-3B-A9-4F UTP UTP UTP UTP Client Client Server Server While station A1-44-D5-1F-AA-4C is transmitting. If station B2-CD-13-5B-E4-65 wishes to transmit, it must wait. Waiting Transmitting
  • 16. Hub Operation: Waiting to Transmit with a Hub
    • Waiting creates latency (delay)
      • Becomes intolerable with many stations
    • Not a problem for very small networks
    • Speed of hub is shared by all stations—each gets only a fraction
  • 17. Switch Operation A1-44-D5-1F-AA-4C on switch port 10 D4-47-55-C4-B6-9F on switch port 16 Ethernet Switch B2-CD-13-5B-E4-65 on switch port 13 C3-2D-55-3B-A9-4F on switch port 15 UTP UTP UTP UTP Client Client Server Server A switch sends a frame out a single port — the one to the receiver Frame Frame Frame
  • 18. Switch Operation A1-44-D5-1F-AA-4C on switch port 10 D4-47-55-C4-B6-9F on switch port 16 Ethernet Switch B2-CD-13-5B-E4-65 on switch port 13 C3-2D-55-3B-A9-4F on switch port 15 UTP UTP UTP UTP Client Client Server Server Multiple simultaneous conversations are possible! Frame Frame Frame
  • 19. Ethernet Switching Table A1-44-D5-1F-AA-4C on switch port 10 D4-47-55-C4-B6-9F on switch port 16 Ethernet Switch B2-CD-13-5B-E4-65 on switch port 13 C3-2D-55-3B-A9-4F on switch port 15 UTP UTP UTP UTP Client Client Server Server Frame Frame Frame
    • Switching Table
    • Port Station
    • A1-44-D5-1F-AA-4C
    • B2-CD-13-5B-E4-65
    • C3-2D-55-3B-A9-4F
    • 16 D4-47-55-C4-B6-9F
    15 C3-2D-55-3B-A9-4F
  • 20. Ethernet Switches
    • They are link-layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination
    • Switches isolates collision domains since it buffers frames
    • When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit
  • 21. Ethernet Switches: Advantages
    • Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage
    • Can connect different type Ethernet since it is a store and forward device
    • Transparent : no need for any change to hosts LAN adapters
  • 22. 802 Standards Layering Internet Layer Media Access Control Layer Data Link Layer Logical Link Control Layer TCP/IP Internet Layer Standards Other Internet Layer Standards 802.2 Other MAC Standards (802.5, etc.) Ethernet 802.3 MAC Layer Standards Physical Layer 10 Base-T 100 Base-TX 1000 Base-TX Other Physical Layer Standards (802.5, etc.)
  • 23. Octet
    • A collection of 8 bits
    • Called a byte in computer science
    • The term “Octet” is widely used in networking
    Octet = group of 8 bits
  • 24. Ethernet Frame Start of Frame Delimiter (1 Octet) 10101011 Ends synchronization Signals start of content of frame 1010101010101010101010101010101010… Synchronizes the receiver’s clock with the sender’s clock Like a boat racing calling out “Hui, Ley, Hui” Preamble (7 Octets) 10101010 … Begins synchronization Field Description
  • 25. Ethernet Frame (cont.) Destination Address (6 Octets) Of Receiving NIC 48 Bits Expressed for humans in Hexadecimal Example: A1-34-CD-7B-DF-47 Unique NIC value set at factory Field Description Source Address (6 Octets) Of Sending NIC 48 Bits Expressed for humans in Hexadecimal Example: 34-D5-E6-6A-BD-B0 Unique NIC value set at factory
  • 26. Ethernet Frame: MAC Addresses
    • Ethernet addresses are called MAC addresses
    • They are station (NIC) address at the MAC layer
    • Usually expressed for humans in hexadecimal
      • Base 16: Each symbol represents 4 bits (for example, 0101=5 hex)
      • Convert each four bits to a hex symbol between 0 and F
      • Put dashes between each pair
      • Example: A1-34-CD-7B-DF-89
    Physical MAC
  • 27. Ethernet Frame: Hexadecimal Notation 0 0001 0010 0011 0100 0101 1 2 3 4 5 4 Bits (Base 2*) Decimal (Base 10) Hexadecimal (Base 16) 0 hex 1 hex 2 hex 3 hex 4 hex 5 hex 0000 *2 4 =16 combinations For example, A1-34-CD-7B-DF hex begins with 1010 0001
  • 28. Ethernet Frame: Hexadecimal Notation (cont.) 4 Bits (Base 2) Decimal (Base 10) Hexadecimal (Base 16 8 1001 1010 1011 9 10 11 8 hex 9 hex A hex B hex 1000 0110 0111 6 7 6 hex 7 hex 12 1101 1110 1111 13 14 15 C hex D hex E hex F hex 1100 Example: 1100 1101 1110 1111 = CD-EF
  • 29. Ethernet Frame (cont.) Ethernet 802.3 MAC Header LLC 802.2 Header IP Packet Ethernet Trailer Ethernet Data Field: LLC Frame Preamble (7 octets) Start of Frame Delimiter (1 octet) Dest. Address (6 octets) Src. Address (6 octets) Length (2 octets) Frame Check Sequence (4 octets) LLC Data Field: IP Packet Physical MAC Direction
  • 30. Ethernet Frame (cont.) Field Description Length (2 Octets) Length of Data Field in octets 1,500 (decimal) maximum LLC Frame Containing IP Packet PAD Needed if data field < 46 octet min., (so that Data Field + PAD will be 46 Octets) Physical MAC Data Field (variable)
  • 31. Ethernet Frame (cont.) Frame Check Sequence (4 Octets) Error detection field; If error is detected, frame is discarded
    • The FCS field is for error checking.
    • Both sender and receiver compute it from the other bits in the frame.
    • If the two computed values are different, there has been an error.
    • In case of error, the receiver discards the frame; there is no request for retransmission.
    Physical MAC Field Description
  • 32. MAC Protocols
    • We consider broadcast channels
    • Problem: how to determine who gets to use the channel when there is competition for it
    • Two basic schemes: Static channel allocation and Dynamic channel allocation
    Physical MAC
    • Frequency Division Multiplexing (FDM)
    • Time Division Multiplexing (TDM)
    • Aloha
    • Carrier Sense Multiple Access (CSMA)
  • 33. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
    • With Hubs, NICs must engage in CSMA/CD
      • CSMA/CD is not used with switches
    • Two processes
    • 1. Carrier Sense Multiple Access (CSMA)
      • If a NIC wishes to transmit, it must listen for traffic
        • If there is no traffic , the NIC may transmit
        • If there is traffic , the NIC must wait to transmit until no traffic is being transmitted; then it may send
  • 34. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) (cont.)
    • 2. Collision Detection (CD)
    • If there is a collision ( by 2 or more stations transmitting at the same time ),
      • All NICs stop transmitting and wait for a random amount of time
      • The first NIC that finishes its wait may transmit
        • but only if there is no traffic!
        • If there is traffic, the NIC must wait until there is no traffic
  • 35. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) (cont.)
    • 3. Collision Detection (CD)
      • If there are multiple collisions,
        • The random wait is increased each time
        • After 16 collisions, the sending NIC discards the frame
  • 36. CSMA/CD: a few details time A B C data interframe gap (IFG): 96 bits collision enforcement jam signal : 32 bits collision backOff time: multiples of the slot time: 512 bits
  • 37. CSMA/CD: Why collision occurs? A B C D t 0 t 1 t 2 t 3 A B C D A B C D A B C D
  • 38. Recap: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
    • Hubs do not implement it when talking to stations
    • NICs implement it when talking to hub
    CSMA/CD with Hubs
  • 39. Recap : Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
    • Three basic elements for describing CSMA/CD
      • CSMA —transmit only if line is clear; waiting otherwise
      • CD Part 1 —handling a single collision
      • CD Part 2 —handling of multiple collisions
  • 40. NIC-Switch Communication
    • No possibility of collisions with switches
    • With switches, NICs operate in full-duplex mode
      • Turn off CSMA/CD
      • Can transmit and receive any time — called Full Duplex operation
  • 41. Network Interface Cards (NICs) PC Card NICs snap into external PC card slots on notebooks. RJ-45 Jack Internal NICs fit into slots on the motherboard. Must open PC to install.
  • 42. Recap : Ethernet
    • 802.3 Physical Layer Standards
      • 10Base-T, 100Base-TX, gigabit Ethernet, etc.
      • Media and signaling
    • 802.3 MAC Layer Standards
      • Frame Organization
      • Hub and Switch Operation
      • CSMA/CD versus Full-Duplex
    • NICs Handle Both Layers, Plus LLC
  • 43. 802 Standards Layering Internet Layer Media Access Control Layer Data Link Layer Logical Link Control Layer TCP/IP Internet Layer Standards Other Internet Layer Standards 802.2 Other MAC Standards (802.5, etc.) Ethernet 802.3 MAC Layer Standards Physical Layer 10 Base-T 100 Base-TX 1000 Base-TX Other Physical Layer Standards (802.5, etc.)
  • 44. Logical Link Control (LLC) Layer
    • 802.2 LLC Standard is used in all 802 LANs
      • All MAC layer standards interact only with 802.2 at the layer above them
      • All internet layer standards interact only with 802.2 at the layer below them
    LLC 802.2 Ethernet MAC Wireless MAC IP IP MAC LLC Physical
  • 45. Logical Link Control (LLC) Layer (cont.)
    • Optional Error Correction
      • Error detection, and retransmission
      • Almost never used
      • NICs rarely allow user to control LLC functionality
    MAC LLC Physical
  • 46. Ethernet Frame Organization Including LLC
    • As noted earlier, here is how Ethernet frames are organized, including the LLC header
    Ethernet Header 802.2 LLC Header IP or Other Packet Ethernet Trailer Ethernet Data Field: LLC Frame LLC Data Field Ethernet Frame MAC LLC Physical
  • 47. Internetworking
    • To extend / connect networks using the same or different tech.
      • LAN  LAN
      • LAN  WAN
      • WAN  WAN
      • LAN  WAN  LAN
  • 48. Why not just one big LAN?
    • Limited amount of supportable traffic: on single LAN, all stations must share bandwidth
    • Large “collision domain” (can collide with many stations)
    • Limited length and number of stations, e.g.
      • 802.3 (Ethernet) specifies maximum cable length
  • 49. Ethernet Switching Hubs
    • Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top.
    • Each connected LAN referred to as LAN segment
    • Hub Advantages:
      • simple, inexpensive device
      • Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions
      • extends maximum distance between node pairs (100m per Hub)
  • 50. Ethernet Switching Hubs: Backbone
    • Interconnection without a backbone is not recommended due to
      • - single point of failure at Computer Science hub
      • - all traffic between EE and SE must path over CS segment
      Backbone hub
  • 51. Example: 100Mbps Ethernet Backbone strategy
  • 52. Internet Access
    • Desktop access router for very small group
      • Usually has built-in switch to connect several PCs
      • The access router/switch usually has a built-in DHCP server so that all PCs can share a single IP address given to the user by the ISP
    18 cm (7 in) wide
  • 53. Using an Access Router Broadband Modem PC PC WAN Port to ISP UTP UTP UTP Access router has built-in switch, DHCP server for PCs and NAT Firewall (will be studied later) Auto-configured by Access Router
  • 54. Linux/Windows Allows PCs to Share a Connection Broadband Modem PC With 2 NICs Other PC Or Switch ISP