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Ch. 10 Circuit Switching and Packet Switching
10. 1 Switched Communication Networks • Fig. 10. 1 Simple switching network. – End stations are attached to the "cloud". – Inside the cloud are communication network nodes interconnected with transmission lines. – The transmission lines often use multiplexing. – The network is generally not fully connected, but alternate paths exist. • Two technologies for WANs – Circuit Switching – Packet Switching
10. 2 Circuit-Switching Networks • The three phases of a circuit switched connection are – Circuit establishment – Data transfer – Circuit disconnect
10. 2 Circuit-Switching Networks (p. 2) • Four generic architectural components of the public telecommunications network: – Subscribers – Subscriber line (or local loop) – Exchanges – Trunks • Fig. 10. 2 illustrates the public switched telephone network (PSTN). • Fig. 10. 3 illustrates two possible connections over the PSTN.
10. 3 Circuit-Switching Concepts • Fig. 10. 4 Elements of a Circuit-Switch Node – Digital Switch • Provides a transparent signal path between any pair of attached devices. – Control Unit • Establishes connections. • Maintains connections. • Tears down connections. – Network Interface • Functions and hardware needed to connect digital and analog terminals and trunk lines.
10. 3 Circuit-Switching Concepts (p. 2) • Blocking vs. Nonblocking – Relates to the capability of making connections. – A blocking network is one in which blocking is possible. – A nonblocking network permits all stations to be connected (in pairs) as long as the stations are not in use.
10. 3 Circuit-Switching Concepts (p. 2) • Space-Division Switching – Defn: A circuit-switching technique in which each connection through the switch takes a physically separate and dedicated path. – Basic building block--a metallic crosspoint or semiconductor gate. – "Crossbar" Matrix (Fig. 10. 5) – Multi-stage space-division switches reduces the total number of crosspoints required, but increases complexity and introduces the possibility of blocking. (Fig. 10. 6)
10. 3 Circuit-Switching Concepts (p. 3) • Time-Division Switching – Defn: A circuit-switching technique in which time slots in a time-multiplexed stream of data are manipulated to pass data from an input to an output. – All modern circuit switches use digital time division techniques or some combination of space division switching and time division switching.
10. 4 Control Signaling • Signaling Functions – Audible communications with subscriber (dial tone, busy signals, etc. ) – Transmission of number dialed to switches to attempt a connection. – Transmission of information between switches indicating that a call can or cannot be completed. – Transmission of information between switches that a call has ended.
10. 4 Control Signaling (p. 2) • Signaling Functions (cont. ) – A signal to make the phone ring. – Transmission of information for billing. – Transmission of information giving status of equipment or lines. – Transmission of information used in diagnosing and isolating system failures. – Control of special equipment such as satellite channel equipment.
10. 4 Control Signaling (p. 3) • Grouping of Control Signals – Supervisory--binary character (on/off) signals that are related to control functions such as request for service, answer, alerting, idle. – Address--signals that identify a subscriber. – Call information--audible tones that provide information about the status of a call. – Network management--signals that are used for maintenance, trouble shooting, and operation of the network.
10. 4 Control Signaling (p. 4) • Location of Signaling – User to network – Within the network (computer to computer) • Common Channel Signaling – Inchannel Signaling: Inband Out-of-Band-Table 10. 1 – Fig. 10. 7 Inchannel and Common Channel Signaling – Fig. 10. 8 Common Channel Signaling Modes.
10. 4 Control Signaling (p. 5) • Signaling System Number 7 – Designed to support command channel signaling for ISDN. – Control messages are routed through the network to perform call management and network management. – Each message is a short block (or packet) and it is transported over a packet switched network to control the circuit switch network.
10. 4 Control Signaling (p. 6) • Signaling System Number 7 (cont. ) – Signaling Network Elements • Signaling point (SP)--any point in the signaling network capable of handling SS 7 control messages. • Signal transfer point (STP)--signaling point capable of routing control message. • Signaling link--data link that connectws signaling points. • Figure 10. 9 illustrates the Control plane and the Information plane.
10. 5 Softswitch Architecture • Specialized software is run on a computer that turns it into a smart phone switch (Fig. 10). – Performs traditional circuit-switching functions. – Can convert a stream of digitized voice into packets (Vo. IP). • Media Gateway (MG) performs the physical switching function. • Media Gateway Controller (MGC) performs call processing. • RFC 3015 --communications between the two.
10. 6 Packet-Switching Principles • Definition: A method of transmitting messages through a communication network, in which long messages are subdivided into short packets. The packets are then sent through the network to the destination node. (See Fig. 10 -11)
10. 6 Packet-Switching Principles (p. 2) • Two Techniques – Datagram (Fig. 10. 12) • Each packet contains addressing information and is routed separately. – Virtual Circuits (Fig. 10. 13) • A logical connection is established before any packets are sent; packets follow the same route.
10. 1 Packet-Switching Principles (p. 3) • Packet Size – Each packet has overhead. – With a larger packet size • Fewer packets are required (less overhead. ) • But longer queuing delays exist at each packet switch. – Figure 10. 14 illustrates this issue.
10. 6 Packet-Switching Principles (p. 4) • Delay in Switching Networks – Setup Time--connection oriented networks. – Transmission Time – Propagation Delay – Nodal Delay--processing time at nodes. • Fig. 10. 15 and Table 10. 2 compare the performance of circuit switching, datagram packet switching, and virtual-circuit packet switching.
10. 6 Packet-Switching Principles (p. 5) • Delay in Circuit Switched Networks – Call setup time. – Message transmission time--occurs once at the source. – Propagation delay--sum of all links. – Very little node delay.
10. 6 Packet-Switching Principles (p. 6) • Delay in Packet Switching – Connection Setup Time • Required for virtual circuit. • None for datagram. – Packet transmission time and propagation delay occurs on each link. – Processing delay occurs at every node. • Datagram networks may require more than virtual circuit networks.
Problem 10. 4 • Consider the delay across a network. – Let B= data rate on every link. – Let N= the number of links. – Let L= the length of the source message. – Let D= the average delay on a link. – Let S= setup time (when required. ) – Let P= packet size for packet switched networks--fixed length packets. – Let H=the number of bits of overhead in each packet header, for packet switched networks.
Problem 10. 4 (p. 2) • Circuit Switching Delay – Let t 0 be the time that the first bit is transmitted at the source node and t 1 be the time that the last bit is received at the destination node. – Then let T= t 1 -t 0 be the "end-to-end" delay. – Follow the last bit across the network. – No network layer overhead and little nodal delay. – Ignore any data link protocol delay (U=1). – T = S + L/B + N x D
Problem 10. 4 (p. 3) • Datagram Packet Switch Delay – Let No. Pa= Number of Packets= L/(P-H) rounded up (ceiling). – Assume no link level related overhead (U=1. ) – The last packet waits at the source and then is transmitted over every link in a store and forward fashion. – T= (No. Pa-1)P/B + N(P/B + D) • Virtual-Circuit Packet Switch Delay – T= S + (No. Pa-1)P/B + N(P/B + D)
10. 7 X. 25 • First approved in 1976 and revised in 1980, 1984, 1988, 1992, and 1993. • Specifies an interface between a host system and a packet-switched networks. • Almost universally used and is employed for packet-switching in ISDN. • Fig. 10. 16 illustrates the concept of virtual circuits over an X. 25 network.
10. 7 X. 25 (p. 2) • Three Layers are defined--Fig. 10. 17. – X. 21 is the physical layer interface (often EIA-232 is substituted) – LAP-B is the link-level logical interface-it is a subset of HDLC. – Layer 3 has a multi-channel interface-sequence numbers are used to acknowledge packets on each virtual circuit.
10. 8 Frame Relay • Traditional packet switching has the X. 25 protocols – Call control packets are carried on the same channel and the same virtual circuit as data packets. – Multiplexing of virtual circuits takes place at layer 3. – Both layer 2 and layer 3 include flow-control and error-control mechanisms. – Considerable overhead is required.
10. 8 Frame Relay (p. 2) • Frame Relay – Call control signaling is carried on a separate logical connection; intermediate nodes have less processing required. – Multiplexing and switching of logical connections take place at layer 2 instead of layer 3 (eliminating a layer of processing). – No hop-by-hop flow control and error control-(performed at a higher layer if at all). – Less overhead required.
10. 8 Frame Relay (p. 3) • Frame Relay Protocol Architecture – Fig. 10. 18 depicts the protocol architecture. – C-plane protocols are for access control between the subscriber and the network. – U-plane protocols provide end-to-end (user) functionality.
10. 8 Frame Relay (p. 4) • Fig. 10. 19 --LAPF-Core Formats • Similar to LAPD and LAPB except there is no control field. – Only one frame type (for user data). – It is not possible to use in-band signaling. – It is not possible to perform flow control and error control (no sequence numbers). • Address Field--data link connection identifier (DLCI) is similar to virtual circuit numbers in X. 25.