Switched Communication Network

Course: Switching and Routing Lecture 2 Switched Communication Network Ch. 9 Data and Computer Communications Tenth Edition by William Stallings Introduction • How information can be transmitted over a communications link? • Communication networks can be classified based on the way in which the nodes exchange information: Communication Network Switched Communication Network Broadcast Communication ➢ Broadcast communication networks − information transmitted by any node is received by every other node in the network • examples: usually in LANs (Ethernet, Wlan) − ➢ Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem) Switched communication networks − information is transmitted to a sub-set of designated nodes • examples: WANs (Telephony Network, Internet) − Problem: how to forward information to intended node(s) • this is done by special nodes (routers, switches) running routing protocols Switched Communication Network For transmission of data beyond a local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes; this switched network design is typically used to implement LANs as well. The switching nodes are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination. Communication Network Switched Communication Network Switched network Broadcast Communication Switched Communication Network In a switched communication network, data entering the network from a station are routed to the destination by being switched from node to node. Mainframe When a message reaches a connecting device, a decision needs to be made to select one of the output ports through which the packet needs to be send out. In other words, the connecting device acts as a switch that connects one port to another port. Personal computer C 2 1 Server 3 B D 5 Personal computer 4 A E Switching Node 6 7 Personal computer Figure 9.1 Simple Switching Network F Personal computer Switched Communications Networks Switching nodes - provide a switching facility that move data between nodes Stations – devices attached to the network Nodes – switching devices that provide communication Communications network – collection of nodes Switched communication network – data entering the network from a station are routed to the destination by being switched from node to node Switched communication networks Switched Communication Network Circuit-Switched Broadcast Communication Packet-Switched Two different technologies are used in wide area switched networks: • circuit switching and • packet switching. ✓ These two technologies differ in the way the nodes switch information from one link to another on the way from source to destination. ✓ Since the invention of the telephone, circuit switching has been the dominant technology for voice communications, and it has remained so well into the digital era. ✓ Around 1970, research began on a new form of architecture for long distance digital data communications: packet switching. ✓ Although the technology of packet switching has evolved substantially since that time, it is remarkable that (1) the basic technology of packet switching is fundamentally the same today as it was in the early 1970s networks, and (2) packet switching remains one of the few effective technologies for long-distance data communications. Communication Network Circuit Switching Switched Communication Network Circuit-Switched Broadcast Communication Packet-Switched ➢ Communication via circuit switching implies that there is a dedicated communication path between two stations. ➢ That path is a connected sequence of links between network nodes. • On each physical link, a logical channel is dedicated to the connection by using FDM or TDM.. Long-distance office Long-distance office End Office End Office Digital PBX A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n Figure 9.2 Example Connection Over a Public Circuit-Switching Network channels. circuit-switching • Circuit switching in a network operates almost the same way as the telephone system works. • A complete end-to-end path must exist before communication can take place. • The computer initiating the data transfer must ask for a connection to the destination. • Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer. Circuit Switching…..2 Communication via circuit switching involves three phases: Circuit establishment. Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established. Data transfer. Data can now be transmitted from A through the network to E. The transmission may be analog or digital, depending on the nature of the network. Generally, the connection is full duplex. Circuit disconnect. After some period of data transfer, the connection is terminated, usually by the action of one of the two stations. Signals must be propagated to nodes 4, 5, and 6 to deallocate the dedicated resources. Three phases Mainframe channel capacity must be reserved between each pair of nodes in the path, and each node must have available internal switching capacity to handle the requested connection. Personal computer Establish C 2 1 Server 3 B D 5 Personal computer 4 A E Switching Node 6 7 Personal computer Disconnect Personal computer Figure 9.1 Simple Switching Network Transfer F Circuit Switching…3 ➢Circuit switching can be rather inefficient • Channel capacity dedicated for duration of connection even if no data are being transferred, capacity is wasted. ❑ For a voice connection, utilization may be rather high, but it still does not approach 100%. ❑ For a client/server or terminal-to-computer connection, the capacity may be idle during most of the time of the connection. ❑ In terms of performance, there is a delay prior to signal transfer for call establishment. ❑ However, once the circuit is established, the network is effectively transparent to the users. Information is transmitted at a fixed data rate with no delay other than the propagation delay through the transmission links. The delay at each node is negligible. Circuit Switching…4 Long-distance office Long-distance office End Office End Office Digital PBX Figure 9.2 Example Connection Over a Public Circuit-Switching Network Circuit switching was developed to handle voice traffic but is now also used for data traffic. The best-known example of a circuit-switching network is the public telephone network. Circuit Switching…4 ❑Circuit switching was developed to handle voice traffic but is now also used for data traffic. • This is actually a collection of national networks interconnected to form the international service. Although originally designed and implemented to service analog telephone subscribers, it handles substantial data traffic via modem and is gradually being converted to a digital network. ❑Driven by applications that handle voice traffic • Key requirement is no transmission delay and no variation in delay ❑Efficient for analog transmission of voice signals ❑Inefficient for digital transmission • In today’s digital world, its inefficiencies are more apparent. ❑Transparent • Once a circuit is established it appears as a direct connection; no special logic is needed Delay in a circuit-switched network In circuit switching, the whole message is sent from the source to the destination without being divided into packets. Advantages and disadvantages of Circuit switching Advantages: • The communication channel (once established) is dedicated. Disadvantages: • Possible long wait to establish a connection, (10 seconds, more on long- distance or international calls.) during which no data can be transmitted. • More expensive than any other switching techniques, because a dedicated path is required for each connection. • Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it. •When the resources are allocated during the entire duration of the connection, these resources are unavailable to other connections. Packet-Switching Networks Switched Communication Network Circuit-Switched Broadcast Communication Packet-Switched The packet-switching network is a distributed collection of packet-switching nodes. Each station attaches to a node, and the collection of nodes is referred to as a communications network. Packet-Switching Networks ➢ Circuit switching was designed for voice: The long-haul circuit-switching telecommunications network was originally designed to handle voice traffic. However, as the circuit-switching network began to be used increasingly for data connections, two shortcomings became apparent: • In a typical client/server data connection (e.g., personal computer user logged on to a database server), much of the time the line is idle. Thus, with data connections, a circuit-switching approach is inefficient. • In a circuit-switching network, the connection provides for transmission at a constant data rate. Thus, each of the two devices that are connected must transmit and receive at the same data rate as the other. This limits the utility of the network in interconnecting a variety of host computers and workstations. ➢ Packet switching was designed for data • Transmitted in small packets: In packet switching the Data are transmitted in short packets. A typical upper bound on packet length is 1000 octets (bytes). If a source has a longer message to send, the message is broken up into a series of packets. • Packets contain user data and control information: Each packet contains a portion (or all for a short message) of the user’s data plus some control information. • Control information includes routing (addressing):The control information, at a minimum, includes the information that the network requires to be able to route the packet through the network and deliver it to the intended destination. • Packets are received, stored briefly (buffered) and passed on to the next node: At each node en-route, the packet is received, stored briefly, and passed on to the next node. Packet-Switching Networks…2 If a station has a message to send through a packet-switching network that is of length greater than the maximum packet size, it breaks the message up into packets and sends these packets, one at a time, to the network. Application data control information (packet header) Packet-Switching Network packet Figure 9.10 The Use of Packets Packet Switching…4 ▪ ▪ Data are sent as formatted bit-sequences, socalled packets. Packets have the following structure: Header Data Trailer • Header and Trailer carry control information (e.g., destination address, check sum) ▪ ▪ ▪ ◼ ◼ ◼ ◼ Each packet is passed through the network from node to node along some path (Routing) At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-andForward Networks) Typically no capacity is allocated for packets Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward Packet Switching…5 A node in a packet switching network incoming links Node outgoing links Memory Packet format specification specifies where the address is Data 1 client Server: address = 1 In a packet-switched network, there is no resource reservation; resources are allocated on demand. The advantages of packet switching The of packet switching provides the following: (flexibility, resource sharing, robustness, responsiveness, low a cost) ➢Channel efficiency can be greater compared to circuit- switched systems, because more devices are sharing the channel (many packets over time) ➢Data rate conversion: Two stations of different data rates can exchange packets because each connects to its node at its proper data rate.(connect to local node at own speed). ⚫Nodes buffer data if required to equalize rates ➢Packets accepted even when network is busy (no blocking): When traffic becomes heavy on a circuit-switching network, some calls are blocked; that is, the network refuses to accept additional connection requests until the load on the network decreases. On a packet-switching network, packets are still accepted, but delivery delay increases. ➢Priorities can be used: If a node has a number of packets queued for transmission, it can transmit the higher-priority packets first. These packets will therefore experience less delay than lower-priority packets. ➢Broadcasting can be achieved with the use of broadcast address appended in the packet. Switching Techniques ➢ Station breaks long message into packets ➢ Packets sent one at a time to the network ➢ Packets can be handled in two ways: Datagram Virtual circuit • Each packet is treated independently with no reference to previous packets • A preplanned route is established before any packets are sent Switched Communication Network Circuit-Switched Packet-Switched Datagram Virtual Circuit The packet-switched network layer of the Internet was originally designed as a connectionless service (datagram switching), but recently there is a tendency to change this to a connection-oriented service (virtual circuit switching). Datagram Switching ✓ The network layer is designed as a packet-switched network. ✓ This means that the packet at the source is divided into manageable packets, normally called datagrams. ✓ Individual datagrams are then transferred from the source to the destination. ✓ The received datagrams are assembled at the destination before recreating the original message. ✓ If the message is going to pass through needs to be divided into packets a packet-switched network, it of fixed or variable size. The size of the packet is determined by the network and the governing protocol. Datagram-Switching Communication Network Switched Communication Network Circuit-Switched Broadcast Communication Packet-Switched Datagram switching Virtual Circuit ❖ Each packet is independently switched − each packet header contains destination address ❖ No resources are pre-allocated (reserved) in advance - Example: IP networks Routing table in a datagram network A datagram network with four switches (routers) A switch in a datagram network uses a routing table that is based on the destination address. Routing table in a datagram network Legend Routing table Destination Output address interface A 1 B 2 H 3 Destination address SA DA Data 2 1 3 SA: Source address DA: Destination address Send the packet out of interface 2 SA DA Data 4 In a connectionless datagram-switched network, the forwarding decision is based on the destination address of the packet. DATAGRAM NETWORKS…2 Delay in a datagram network Switching in the Internet is done by using the datagram approach to packet switching at the network layer. Disadvantages of datagrams ◼ Major drawback: ◼ What happens if two packets arrive at a switch and both need to go to the same output interface. Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone or is delayed. This would never happen on a circuit switched network. queuing delay and packet loss increased. VIRTUAL-CIRCUIT NETWORKS Communication Network Switched Communication Network Circuit-Switched Broadcast Communication Packet-Switched Datagram Virtual Circuit A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. Datagram • Each packet is treated independently with no reference to previous packets Virtual circuit • A preplanned route is established before any packets are sent VIRTUAL-CIRCUIT NETWORKS…2 We have seen that a circuit-switching network provides a transparent communications path for attached devices that makes it appear that the two communicating stations have a direct link. But with virtual circuits, this service enables any subscriber to the network to set up logical connections, called virtual circuits, to other subscribers. ➢ Hybrid of circuit switching and packet switching − data is transmitted as packets − all packets from one packet stream are sent along a pre-established path (=virtual circuit) ➢ Guarantees in-sequence delivery of packets ➢ However: Packets from different virtual circuits may be interleaved Mainframe C Personal computer Server D Personal computer B Personal computer Packet-Switching Network E Personal computer A Solid line = physical link Dashed line = virtual circuit ➢ Example: ATM, MPLS networks Figure 9.14 The Use of Virtual Circuits F ❑ Communication via virtual circuits switching takes place in three phases 1. VC establishment 1. data transfer 1. VC disconnect ❑ Note: packet headers don’t need to contain the full destination address of the packet VIRTUAL-CIRCUIT NETWORKS…2 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 Virtual-circuit identifier In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand. Virtual circuit table In the connection setup phase, it is necessary to establish a “connection state” in each of the switches between the source and destination hosts. The connection state for a single connection consists of an entry in a “VC table” in each switch through which the connection passes. One entry in the VC table on a single switch contains: ■ a virtual circuit identifier (VCI) that uniquely identifies the connection at this switch and which will be carried inside the header of the packets that belong to this connection, ■ an incoming interface on which packets for this VC arrive at the switch, ■ an outgoing interface in which packets for this VC leave the switch, ■ a potentially different VCI that will be used for outgoing packets. The semantics of one such entry is as follows: if a packet arrives on the designated incoming interface and that packet contains the designated VCI value in its header, then that packet should be sent out the specified outgoing interface, with the specified outgoing VCI value having been first placed in its header. Thus, the VCI is not a globally significant identifier for the connection; rather, it has significance only on a given link (i.e., it has link-local scope). Whenever a new connection is created, we need to assign a new VCI for that connection on each link that the connection will traverse. Let us assume that a network administrator wants to manually create a new virtual connection from host A to host B. First, the administrator needs to identify a path through the network from A to B. In the example network of Figure, there is only one such path, but in general, this may not be the case. The administrator then picks a VCI value that is currently unused on each link for the connection. For the purposes of our example, let us suppose that the VCI value 5 is chosen for the link from host A to switch 1 and that 11 is chosen for the link from switch 1 to switch 2. In that case, switch 1 needs to have an entry in its VC table configured as shown in Table 3.2 Fig. An example of a virtual circuit network. Approaches to establishing connection state There are two broad approaches to establishing connection state. One is to have a network administrator configure the state, in which case the virtual circuit is “permanent (PVC) . ” It can also be deleted by the administrator, so a permanent virtual circuit might best be thought of as a long-lived or administratively configured VC. Alternatively, a host can send messages into the network to cause the state to be established. This is referred to as signaling (SVC) , and the resulting virtual circuits are said to be switched. The salient characteristic of a SVC is that a host may set up and delete such a VC dynamically without the involvement of a network administrator. Permanent versus Switched Virtual Circuits • A source and a destination may choose to have a permanent virtual circuit (PVC). In this case, the connection setup is simple. The corresponding table entry is recorded for all switches by the administrator. • PVC connections have two drawbacks. o First, they are costly because two parties pay for the connection all the time even when it is not in use. o Second, a connection is created from one source to one single destination. If a source needs connections with several destinations, it needs a PVC for each connection. • An alternate approach is the switched virtual circuit (SVC). The SVC creates a temporary, short connection that exists only when data are being transferred between source and destination. An SVC requires establishing and terminating phases. In a connection-oriented packet switched network (virtual-circuit), the forwarding decision is based on the VCI of the packet. Switch and tables in a virtual-circuit network Source-to destination data transfer in a virtualcircuit Network If two stations wish to exchange data over an extended period of time, there are certain advantages to virtual circuits. First, the network may provide services related to the virtual circuit, including sequencing and error control. Sequencing refers to the fact that, because all packets follow the same route, they arrive in the original order. Error control is a service that assures not only that packets arrive in proper sequence, but also that all packets arrive correctly. it is not necessary to make a routing decision for each packet at each node. Setup request in a virtual-circuit network Setup acknowledgment in a virtual-circuit network Sending request packet in a virtual-circuit network A to B Incoming Legend Outgoing A to B Request packet Port Label Port Label 1 14 3 Network 1 R1 R2 2 4 A to B 1 Virtual circuit 3 A R5 2 Network A to B 1 2 R3 Incoming 3 Outgoing Port Label Port Label 3 1 66 A to B 3 2 A to B 1 4 R4 Incoming 4 3 A to B B Outgoing Port Label Port Label 4 1 22 A to B Setup acknowledgement in a virtual-circuit network A to B Incoming Legend Outgoing Acknowledge packet Port Label Port Label 1 14 3 Network 4 14 R1 1 Virtual circuit R2 2 4 3 A Network R5 3 66 1 2 2 3 22 22 R3 Incoming 66 Outgoing Port Label Port Label 3 1 66 A to B 2 1 3 77 1 4 R4 Incoming 77 Outgoing Port Label Port Label 4 1 22 A to B B Flow of one packet in an established virtual circuit A to B Incoming Legend Outgoing A B Data Datagram Port Label Port Label 66 1 14 3 Network R1 14 A B Data 1 Virtual circuit R2 2 4 3 A R5 Network 66 A B Data 12 3 R3 Incoming 2 22 A B Data Outgoing Port Label Port Label 3 1 66 22 A to B 3 77 A B Data 1 R4 Incoming 4 Outgoing Port Label Port Label 77 4 1 22 A to B Delay in a virtual-circuit network Virtual Circuits vs. Datagram ➢Virtual circuits ⚫Network can provide sequencing and error control ⚫Packets are forwarded more quickly ⚫Less reliable ➢Datagram ⚫No call setup phase ⚫More flexible ⚫More reliable There have been a number of successful examples of virtual circuit technologies over the years, notably X.25, Frame Relay, ATM and MPLS. One of the most common applications of virtual circuits for many years was the construction of virtual private networks (VPNs). Switching at Physical Layer At the physical layer, we can have only circuit switching. There are no packets exchanged at the physical layer. The switches at the physical layer allow signals to travel in one path or another. Switching at Data-Link Layer At the data-link layer, we can have packet switching. However, the term packet in this case means frames or cells. Packet switching at the data-link layer is normally done using a virtual-circuit approach. Switching at Network Layer At the network layer, we can have packet switching. In this case, either a virtual-circuit approach or a datagram approach can be used. Currently the Internet uses a datagram approach. Advantages of the datagram approach over virtual circuits • One advantage of the datagram approach is that the call setup phase is avoided -(Thus, if a station wishes to send only one or a few packets, datagram delivery will be quicker). • Another advantage of the datagram service is that, because it is more primitive, it is more flexible. -(For example, if congestion develops in one part of the network, incoming datagrams can be routed away from the congestion. With the use of virtual circuits, packets follow a predefined route, and thus it is more difficult for the network to adapt to congestion). • A third advantage is that datagram delivery is inherently more reliable. -(With the use of virtual circuits, if a node fails, all virtual circuits that pass through that node are lost. With datagram delivery, if a node fails, subsequent packets may find an alternate route that bypasses that node). Packet-Switching vs. Circuit-Switching • A related issue is that the destination addresses used in datagram networks are longer than circuit numbers used in virtual circuit networks because they have a global meaning. • Yet another issue is the amount of table space required in router memory. A datagram network needs to have an entry for every possible destination, whereas a virtualcircuit network just needs an entry for each virtual circuit. Thank You 10/8/2025 45