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The Physical Layer. Summary (See Physic and LineCode, plus Chapters 3, 4 5, 6 ,7 )

The Physical Layer. Summary (See Physic.ppt and LineCode.ppt, plus Chapters 3, 4 5, 6 ,7 ) 1.Properties of Copper, Radio, and Fibre media 2.Bandwidth. 3. Attenuation. The decibel scale. The Signal to Noise ratio (S/N). Repeaters 4. Shannon’s Theorem

thomas-williams
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The Physical Layer. Summary (See Physic and LineCode, plus Chapters 3, 4 5, 6 ,7 )

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  1. The Physical Layer. Summary (See Physic.ppt and LineCode.ppt, plus Chapters 3, 4 5, 6 ,7) 1.Properties of Copper, Radio, and Fibre media 2.Bandwidth. 3. Attenuation. The decibel scale. The Signal to Noise ratio (S/N). Repeaters 4. Shannon’s Theorem 5. Line Codes. NRZ, Manchester, AMI, HDB3, B8ZS 5.The Amplitude, Frequency and Phase of an Analog Signal 6.Modulation. ASK, FSK, PSK, QAM . 7.The Baud Rate v Bit Rate of a Line 9.Multiplexing. TDM, FDM, WDM

  2. Ethernet

  3. Figure 1.12Categories of networks

  4. Figure 1.7Categories of topology

  5. Figure 1.15WAN

  6. Figure 1.8Fully connected mesh topology (for five devices)

  7. Figure 1.10Bus topology

  8. Figure 1.9Star topology

  9. Figure 1.11Ring topology

  10. Ethernet. (HUB Based) • LANs are mostly based on Ethernet technology. IBM uses Token Rings. • Ethernetis a contention-based, shared medium, and broadcast based. (Hubs) • Because it is broadcast based, there can be only one message in transmission at a time. • Because it is broadcast based, the transmitted message is received by all network devices. • Because of the shared medium aspect, if two or more devices attempt to transmit at the • same time, there will be a collision. A collision represents a waste of bandwidth. • 6. To minimise collisions, Ethernet implements a Medium Access Control (MAC) protocol. • 7. CSMA/CD…Carrier Sense Multiple Access/Collision Detection • 8. In terms of the OSI stack, Ethernet represents the physical layer plus a minimal DL layer • 9. Ethernet messages are called frames. Ethernet is ‘best-effort’ datagram delivery. • It operates in CL mode. There are no guarantees of delivery. There is no recovery from • errors. There is no flow control. • There is a minimum and a maximum frame size. Frames contain a header and payload. • The max payload size is 1500 bytes. The Min size is 64 bytes. • The frame header contains a start of frame marker, SA, DA, and a ‘type’ field. (DIX) • The purpose of the ‘type’ field is to indicate what upper layer message type is being • carried in the payload. e.g IP packet or ARP • Ethernet addresses are physical, cannot be changed. SA and DA are 6 bytes in size. • The earliest Ethernet networks provided 10Mbps transmission speeds. Today speeds of • 100 Mbps and 1000 Mbps are in use. • Ethernet works well under light load conditions. Under heavy load conditions, the network • degrades because of collisions.

  11. In the DIX frame format, the ‘type’ field specifies the upper layer message that is being carried in the frame’s payload. For example, when an ARP message is being carried, the type field is set to 080616, and when an RARP message is being carried the type field is set to 803516. When IP messages are being carried the type field is set to 080016, When Xerox XNS traffic is being carried, the type field is set to 060016, and when Novell IPX traffic is being carried the type field is set to 813716.

  12. A bridged network

  13. In the configuration above the switch is like the multi-port bridge in the previous slide. The switch is a specialised hardwire unit with several ports. In an Ethernet switch, typically each port can have either a single station connected, in which case it has all the bandwidth, or else a hub can be connected where the switch port’s bandwidth is shared between all the stations connected to the hub. The switch is a high speed backplane to interconnect ports.

  14. Summary. • The Data Link Layer (in Ethernet) • MAC and LLC sublayers, see another slide (8) • Ethernet is a broadcast network, only one message in transmission at a time, • contention and collisions occur • The Ethernet frame, SFD, DA,SA, Type, Payload, Pad and CRC. See another slide (13) • Ethernet provides ‘best effort’ (datagram) transport, frames with a failed CRC are • discarded. • Error Control: Discard bad frames.No recovery in Ethernet. Recovery is for higher • layer protocols (e.g LLC or TCP) • Flow Control: Ethernet provides no flow control between communicating stations. • Ethernet Addresses are fixed at time of manufacture of network card. Addresses are • 6 bytes long. You can check the Ethernet address of your PC by typing C:\ipconfig/all • at the Dos prompt. (10) Normally there is little reason to know the Ethernet address. See • another slide for an example of how an Ethernet address is written. (11) • The ‘Type’ field identifies the type of messsage which is carried in the payload of the • Ethernet frame. The payload may encapsulate an IP packet from the network layer, or • an ARP packet for example. • The CRC (2 bytes) is for the detection of damaged frames. The CRC is a unique function • of the contents of the frame, header & payload. If one or more bits change a different • CRC results. The sender calculates a CRC and sends it with the frame to destination. At • the destination, the receiver calculates an independent CRC based on the frame received. • (which may or may not be in error).The receiver compares its own CRC with the • sender’s CRC. If different, conclude that the frame was damaged, discard.

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