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 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.

10.5 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)

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.

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.

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.

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 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

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)

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.

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.