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Part III. Data Link Layer

Part III. Data Link Layer. Chapter 12. Multiple Access. COMP 3270 Computer Networks Computing Science Thompson Rivers University. Course Outline. Overview Physical Layer Digital transmission Analog transmission Bandwidth utilization … Data Link Layer Error detection

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Part III. Data Link Layer

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  1. Part III. Data Link Layer Chapter 12. Multiple Access COMP 3270 Computer Networks Computing Science Thompson Rivers University

  2. Course Outline • Overview • Physical Layer • Digital transmission • Analog transmission • Bandwidth utilization • … • Data Link Layer • Error detection • Data link control • Multiple access • Ethernet; Wireless LANs; Connecting LANs • Network Layer • Logical addressing • IPv4, protocols • Delivery, forwarding and routing

  3. Learning Objectives • Describe how CSMA/CD works, in terms of CS, MA and CD. • List two types of channelization.

  4. Data link layer divided into two functionality-oriented sublayers Hardware independent Hardware dependent Example:

  5. Taxonomy of multiple-access protocols discussed in this chapter -> Ethernet

  6. 1. RANDOM ACCESS In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. Topics discussed in this section: • Carrier Sense Multiple Access (CSMA) • Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

  7. ALOHA MA (Multiple Access): • Transmit whenever data is ready to send • Acknowledgement ☺ How to improve?

  8. CSMA (Carrier Sense Multiple Access) • In wired, not wireless, networks • Algorithm: • Sense if there is carrier signal in the link; • If there is not, then send data; • Otherwise, wait a little bit; • ☺ What if there is signal collision?

  9. Space/time model of the collision in CSMA B Why not detect this signal collision early? C

  10. CSMA/CD (CSMA / Collision Detection) • In wired, not wireless, networks • Algorithm: • CSMA first; • List and detect if there is any collision during the transmission of data; • If so, abort the transmission and retransmit the data;

  11. 3. CHANNELIZATION Wireless networks, Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we discuss two channelization protocols. Topics discussed in this section: • Frequency-Division Multiple Access (FDMA) • Time-Division Multiple Access (TDMA)

  12. FDMA

  13. In FDMA, the available bandwidth of the common channel is divided into bands that are separated by guard bands.

  14. TDMA

  15. In TDMA, the bandwidth is just one channel that is timeshared between different stations.

  16. ☺ Can we combine FDMA+TDMA?

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