Islamic University Of Gaza Data Communication Faculty of Engineering Discussion Computer Department Chapter 10 Eng. Ahmed M. Ayash Date: 19/12/2012 Chapter 10 CIRCUIT SWITCHING AND PACKET SWITCHING Switched Network Consists of a series of interlinked nodes called switched. (Switching nodes). Figure 10.1 a simple network. 1
The devices attached to the network may be referred to as stations. The stations may be computers, terminals, telephones, or other communicating devices. Node-station links are generally dedicated point-to-point links. Node-node links are usually multiplexed, using either frequency division multiplexing (FDM) or time division multiplexing (TDM). Switching Network technologies Figure 10.2 Switching Network technologies. Circuit Switching: uses a dedicated path between two stations for communication. The link can be permanent (leased line) or temporary (telephone). Each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. Was designed for voice. Switching take place at physical layer Figure 10.3 Circuit Switching 2
Communication via circuit switching involves three phases: Circuit establishment (setup) - create dedicated channel by sending a request. Data transfer. Circuit disconnect (teardown) - Signal is sent to each switch to release resources. Example1: A path in a digital circuit-switched network has a data rate of 1 Mbps. The exchange of 1000 bits is required for the setup and teardown phases. The distance between two parties is 5000 km. answer the following questions if the propagation speed is 2 X 10 8 m/s: a. What is the total delay if 1000 bits of data are exchanged during the data transfer phase? b. What is the total delay if 100,000 bits of data are exchanged during the data transfer phase? c. What is the total delay if 1,000,000 bits of data are exchanged during the data transfer phase? We assume that the setup phase is a two-way communication and the teardown phase is a one-way communication. These two phases are common for all three cases. The delay for these two phases can be calculated as three propagation delays and three transmission delays or 3 [(5000 km)/ (2 10 8 m/s)] + 3 [(1000 bits/1 Mbps)] = 75 ms + 3 ms = 78 ms We assume that the data transfer is in one direction; the total delay is then delay for setup and teardown + propagation delay + transmission delay a. 78 + 25 + 1 = 104 ms b. 78 + 25 + 100 = 203 ms c. 78 + 25 + 1000 = 1103 ms Packet switching: was designed for data and transmitted in small packets. Packets contains user data and control info 3
Packet Switching Techniques o Datagram Network o Virtual-Circuit Network Datagram Network: each packet is treated independently, with no reference to packets that have gone before. Datagram switching is done at network layer. Figure 10.4 Datagram switching A switch in a datagram network uses a routing table that is based on the destination address. The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet. 4
Example2: The following Figure shows a switch (router) in a datagram network. Find the output port for packets with the following destination addresses: Packet 1: 7176 Packet 2: 1233 Packet 3: 8766 Packet 4: 9144 Packet 1: 2 Packet 2: 3 Packet 3: 3 Packet 4: 2 Example3: Five equal-size datagrams belonging to the same message leave for the destination one after another. However, they travel through different paths as shown in the following table: We assume that the delay for each switch (including waiting and processing) is 3, 10, 20, 7, and 20 ms respectively. Assuming that the propagation speed is 2 x 10 8 m/s. Find the order the datagrams arrive at the destination and the delay for each. Ignore any other delays in transmission. The arrival timed is calculated as: First: (3200 Km) / (2 x 10 8 m/s) + (3 + 20 + 20) = 59.0 ms Second: (11700 Km) / (2 x 10 8 m/s) + (3 + 10 + 20) = 91.5 ms Third: (12200 Km) / (2 x 10 8 m/s) + (3 + 10+ 20 + 20) = 114.0 ms Fourth: (10200 Km) / (2 x 10 8 m/s) + (3 + 7 + 20) = 81.0 ms Fifth: (10700 Km) / (2 x 10 8 m/s) + (3 + 7 + 20 + 20) = 103.5 ms The order of arrival is: 3 5 2 4 1 5
Virtual-Circuit Network: Packets form a single message travel along the same path. Implemented in data link layer. Each packet contains a virtual circuit identifier instead of destination address. Three phases to transfer data. Example4: 6 Figure 10.5 Virtual-Circuit Network The following Figure shows a switch in a virtual circuit network. Find the output port and the output VCI for packets with the following input port and input VCI addresses: Packet 1: 3, 78 Packet 2: 2, 92 Packet 3: 4, 56 Packet 4: 2, 71 Packet 1: 2, 70 Packet 2: 1, 45 Packet 3: 3, 11 Packet 4: 4, 41
X.25 and Frame relay X.25 is the standard protocol for the interface between an end system and a packet-switching network. defines three layers o Physical - Interface between attached station and link to node. o Link - Link Access Protocol Balanced (LAPB) o Packet - External virtual circuits Figure 10.6 X.25 Use of Virtual Circuits Frame relay is a form of packet switching that provides a streamlined interface compared to X.25, with improved performance. Designed to be more efficient than X.25 Designed to eliminate much of the overhead that X.25 imposes on end user systems and on the packet-switching network. 7