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.


• 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)


• 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.


• Location of Signaling– User to network– Within the network (computer to computer)

• Common Channel Signaling– Inchannel Signaling: Inband and 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.


• Signaling System Number 7 (cont.)– Signaling Network Elements

• Signaling point (SP)--any point in the signaling network capable of handling SS7 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.10.10).– 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.

10.6 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.


• 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.


• 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.


• 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.


• 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)

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.


• 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.


• 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.

Ch. 10 Circuit Switching and Packet Switching

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