Introduction to Computer Networking: Multiplexing, Switching, and Layering

Posted on Oct 18, 2024 in Computers

Multiplexing Techniques in Circuit Switching

What are the two most common multiplexing techniques used in circuit switching?

One is frequency division multiplexing (FDM), which partitions the bandwidth in a link using frequency bands, such as in FM radio. Another is time division multiplexing (TDM), which partitions time in a link with a revolving frame, giving each connection the same slot in the revolving frame.

Circuit Switching vs. Packet Switching

a. Suppose that all of the network sources send data at a constant bit rate. Would packet-switching or circuit-switching be more desirable in this case? Why?

b. Suppose that all of the network sources are bursty—that they only occasionally have data to send. Would packet-switching or circuit switching be more desirable in this case? Why?

a. Circuit-switching is more desirable here because there are no statistical multiplexing gains to be had, and by using circuits, each connection will get a constant amount of bandwidth that matches its CBR (Constant bitrate).

b. Packet-switching is better here because there are statistical multiplexing gains—when a source does not have data to send, it will not be allocated bandwidth; it would rather be unused when the source had nothing to send. With packet-switching, this bandwidth is available for use by other sources.

Analyzing Link Sharing with Circuit and Packet Switching

Suppose users share a 1 Mbps link. Also suppose each user requires 500 kbps when transmitting, but each user transmits only 10 percent of the time.

a. When circuit switching is used, how many users can be supported?

b. For the remainder of this question, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit simultaneously? Why will there be a queuing delay if three users transmit simultaneously?

c. Find the probability that a given user is transmitting.

d. Suppose there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

a. Two users, each getting a dedicated 500 kbps.

b. If two users transmit simultaneously, the input rate is a constant 1 Mbps. Since the link is also 1 Mbps, there will be no queuing delay. If three users transmit simultaneously, then the input rate is a constant 1.5 Mbps. Since the maximum output rate is 1 Mbps, bits will build up in a queue at a rate of 0.5 Mbps. After one second, the queue length will be approximately 500,000 bits.

c. Since each user is transmitting 10 percent of the time, the probability that a user is transmitting at any instant is 0.1.

d. Assuming that all three users are transmitting independently, the probability that all of them are transmitting simultaneously is (0.1)^3 or, 0.001. Since the queue only grows when all three users are transmitting, the fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is .001.

Calculating Packet Propagation and Transmission Delay

How long does it take a packet of length 1,000 bytes to propagate over a link of distance 5,000 km, propagation speed 2.5 × 108 m/s, and transmission rate 1 Mbps (include the transmission delay)?

Part 1: Computing the transmission delay:

1000 bytes = 8000 bits.

Transmitting rate is 1Mbps or, 106 bits/sec, or, 103 bits/msec. Thus, 8000 bits will require 8000/1000=8msec to be transmitted.

Part 2: Computing the propagation delay:

Now, the propagation speed given as: (2.5 × 108 m/s) or, (2.5 × 105 km/s) or, (2.5 × 1022 km/msec), or, 250 km/msec. Thus to travel 5000km it will take 5000/250 = 20mses.

The propagation delay including the transmission delay is (20+8) or, 28msec.

TCP/IP Reference Model

For the TCP/IP reference model,

a. What are the 4 protocol layers, in the Internet above the physical layer?

b. A router processes up to which layer?

c. A link-layer switch processes up to which layer?

a. Above the physical layer there are (bottom to top) link, network, transport, and application layers.

b. A router processes up through the network layer.

c. A link-layer switch processes up through the link layer.

OSI and TCP/IP Layers and Their Tasks

Mention the names of the OSI and TCP/IP layers and briefly mention the tasks of each of the OSI layers.

OSI:

  1. Application: Functions needed for users
  2. Presentation: Converts different presentations
  3. Session: Manages task dialog
  4. Transport: Provides end-to-end delivery
  5. Network: Sends packets over multiple links
  6. Data link: Sends frames of information
  7. Physical: Sends bits as signals

TCP/IP layer:

  • Application
  • Transport
  • Network
  • Link