channel access method

• a channel access method (or multiple access method)
  • channel partitioning (or channelization) protocols
    • Frequency-division multiple access (FDMA)
    • Time-division multiple access (TDMA)
    • Code-division multiple access (CDMA)
    • Orthogonal frequency-division multiple access (OFDMA)
  • random access (or contention-based) protocols
    • ALOHA
    • CSMA
      • CSMA/CD
      • CSMA/CA
  • controlled access (or taking-turns) protocols
    • token passing
• multiple access protocol

CSMA

• carrier sense multiple access (CSMA)

  • “A contention access method in which each station listens to the line before transmitting data” (Forouzan, 2012)
  • “…means that multiple stations can listen to the link and detect when it is in use or idle” (Peterson, 2020)

• 1-persistent

  • Sense channel
  • If channel is idle
    • transmit frame
      • if transmission is successful, done
      • else, wait random time and continue sensing
  • Else
    • Continue sensing

• non-persistent

  • Sense channel
  • If channel is idle
    • transmit frame
    • if transmission is successful
      • done

    • else

  • Else
    • Wait random backoff time
    • Sense again

• p-persistent

  • Sense channel at slot boundary
  • If channel is idle
    • With probability $p$, transmit frame
    • With probability $1-p$, wait for the beginning of the next time slot and sense again
      • if idle, transmit frame
      • if busy, Wait for Next Time Slot
  • if busy, Wait for Next Time Slot

CSMA/CD

• CSMA/CD (collision detection)

  • “An access method in which stations transmit whenever the transmission medium is available and retransmit when collision occurs” (Forouzan, 2012)
  • “The CSMA method does not specify the procedure following a collision. CSMA/CD augments the algorithm to handle the collision” (Forouzan, 2012)
  • “CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.” (Wikipedia)
  • “With the growing popularity of Ethernet switches in the 1990s, IEEE 802.3 deprecated Ethernet repeaters in 2011, making CSMA/CD and half-duplex operation less common and less important.” (Wikipedia)

• $t_{\text{tx}}\ge 2\cdot t_{\text{prop}}$

• $\text{slot-time}=\frac{n_{\text{min}}}{R}=2t_{\text{prop}}$

  • $t_{\text{prop}}=\frac{d}{v}$ is the propagation delay
  • $n_{\text{min}}=R\cdot 2t_{\text{prop}}$ is the minimum frame size
  • slot time examples:
    • (10 Mbps Ethernet) $51.2\mu \mathrm{s}=\frac{512\text{bits}}{10\times 10^6\text{bps}}$
      • (that implies that $t_{\text{prop}}=\frac{d}{v}=25.6$, thus $d\approx \frac{2}{3}c\times 25.6\mu s\approx 5115\text{m}$, however, some 802.3 standards limit (2.5 km (4 repeaters)) include large safety margins)

• CSMA/CD procedure:

  • $n=0$ (the number of collisions for the frame (up to a maximum value, e.g., $n_{\text{max}}=10$))
  • (1) persistence method
  • while (not done and not collision)
    • transmit
  • if collision detected:
    • send a jamming signal
    • $n=n+1$
    • if $n>n_{\text{max}}$, abort
    • choose random integer $k\in [0,2^n-1]$
    • wait $\text{backoff time}=k\times \text{slot time}$
    • go back to (1)

• jam signal (or jamming signal)

CSMA/CA

• CSMA/CA (collision avoidance)
  • “An access method in wireless LANs that avoids collision by forcing the stations to send reservation messages when they find the channel is idle” (Forouzan, 2012)

Code-division multiple access (CDMA)

• $u\in \{U_1,U_2,\dots ,U_n\}$ is a user (sender)
  • ${\mathbf{c}}_u=[c_{u,1},c_{u,2},\dots ,c_{u,k}]$ is the chiping code for user $u$, where $c_{u,i}\in \{-1,+1\}$ is the $i$-th chip of the code.
  • code length (or spreading factor) $k$ is the number of chips used to represent (spread) a single data bit.
  • $n$ is the number of stations (users)
• The user sends ${\mathbf{s}}_u=b_u{\mathbf{c}}_u$, where $b_u\in \{-1,+1\}$ is the data bit for user $u$. where $b_u=-1$ if the user wants to send data bit 0, and $b_u=+1$ if the user wants to send data bit 1.
• The received signal is $\mathbf{r}=\sum _{u=1}^n{\mathbf{s}}_u=\sum _{u=1}^n{b}_u{\mathbf{c}}_u=\left(\sum _{u=1}^n{b}_u{c}_{u,1},\dots ,\sum _{u=1}^n{b}_u{c}_{u,k}\right)$
• The data bit for user $u$ recovered by receiver is ${\hat{b}}_u=\text{sign}\left(\mathbf{r}\cdot {\mathbf{c}}_u\right)=\frac{1}{k}(\mathbf{r}\cdot {\mathbf{c}}_u)$
  • ${\hat{b}}_u=0$ means user $u$ did not send any data bit.
  • ${\hat{b}}_u=+1$ means user $u$ sent data bit $+1$ (i.e., the user wants to send 1)
  • ${\hat{b}}_u=-1$ means user $u$ sent data bit $-1$ (i.e., the user wants to send 0)
  • $\mathbf{r}\cdot {\mathbf{c}}_u$ is called the correlation value $R_u$

Orthogonal frequency-division multiple access (OFDMA)

• resource element (RE)
• transmission time interval (TTI)
• physical resource block (PRB)
• channel quality indicator (CQI)

References

• Forouzan, B. A. (2012). Data Communications and networking. McGraw-Hill Education.
• Peterson, Larry L. (2020). Computer Networks. Elsevier Science & Technology.