Ch. 10 Circuit Switching and Packet Switching

1. Ch. 10 Circuit Switching and Packet Switching

2. 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

3. 10.2 Circuit-Switching Networks • The three phases of a circuit switched connection are • Circuit establishment • Data transfer • Circuit disconnect

4. 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.

5. 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.

6. 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.

7. 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)

8. 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.

9. 10.4 Softswitch Architecture • Specialized software is run on a computer that turns it into a smart phone switch (Fig.10.10). • Performs traditional circuit-switching functions. • Can convert a stream of digitized voice into packets (VoIP). • Media Gateway (MG) performs the physical switching function. • Media Gateway Controller (MGC) performs call processing. • RFC 3015--communications between the two.

10. 10.5 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. 10-8)

11. 10.5 Packet-Switching Principles (p.2) • Two Techniques • Datagram (Fig. 10.9) • Each packet contains addressing information and is routed separately. • Virtual Circuits (Fig. 10.10) • A logical connection is established before any packets are sent; packets follow the same route.

12. 10.5 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 10.11 illustrates this issue.

13. 10.5 Packet-Switching Principles (p.4) • Delay in Switching Networks • Setup Time--connection oriented networks (removed from chapter but not problems) • Transmission Time • Propagation Delay • Nodal Delay--processing time at nodes. • Fig. 10.13 and Table 10.1 compare the performance of circuit switching, datagram packet switching, and virtual-circuit packet switching.

14. 10.6 Packet-Switching Principles (p.5) • Delay inCircuit Switched Networks • Call setup time. • Message transmission time--occurs once at the source. • Propagation delay--sum of all links. • Very little node delay.

15. 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.

16. 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.

17. Problem 10.4 (p.2) • Circuit Switching Delay • Let t0 be the time that the first bit is transmitted at the source node and t1 be the time that the last bit is received at the destination node. • Then let T= t1-t0 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

18. Problem 10.4 (p.3) • Datagram Packet Switch Delay • Let NoPa= 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= (NoPa-1)P/B + N(P/B + D) • Virtual-Circuit Packet Switch Delay • T= S + (NoPa-1)P/B + N(P/B + D)

19. X.25 (no longer in text) • 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. • Virtual circuits are used in an X.25 network.

20. X.25 (p.2) • Three Layers are defined • 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.