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

.