Section 4 Switching Methods

Switching Methods: Circuit and Packet Switching

Professor Izhak Rubin Electrical Engineering Department

UCLA [email protected]

© 2014-2015 by Professor Izhak Rubin

Prof. Izhak Rubin 2

Switching Methods

Circuit Switching

Example: Telephone Networks

Packet Switching: store & forward switching

Connectionless, datagram

Example: INTERNET

Connection oriented, virtual circuit switching

Examples: X.25, BISDN (Broadband Integrated Services Digital Networks) ATM (Asynchronous Transfer Mode) Networks, Frame Relay Networks

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Line Sharing: Fixed vs. Statistical

Time frame Time frame Time frame

1 2 3 4 1 2 3 4 1 2 3 4

Circuit switched transmission across TDM circuit 1

Packet switched transmission of packets

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Circuit switching

N1

N2

N5

N3

N4

(N1,N5)

(N1,N4)

(N1,N5)

(N1,N4)

Link L(1,2)

Messages (N1,N5) (N1,N4)

Circuit Switching concept: Connection Oriented; user signals the network; network sets up a connection; a circuit is established to support transport of messages that are part of the connection; circuit consists of dedicated link capacity resources along the selected path.

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Circuit switching: Circuits


1 2 3 4 1 2 3 4 1 2 3 4 Link (N1,N3)


1 2 3 4 1 2 3 4 1 2 3 4 Link (N3,N5)

Circuit (N1,N5) established across links (N1,N3) and (N3, N5) through dedicated TDM slots; e.g., slot 1 in each frame is dedicated to this circuit

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Circuit switching: Circuits


1 2 3 4 1 2 3 4 1 2 3 4 Link (N1,N3)


1 2 3 4 1 2 3 4 1 2 3 4 Link (N3,N4)

Circuit (N1,N4) established across links (N1,N3) and (N3,N4) through dedicated TDM slots

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Circuit switching: Principles

N1

N2

N5

N3

N4

(N1,N5)

(N1,N4)

(N1,N5)

(N1,N4)

Link L(1,2)

Circuit switch

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Principle of circuit switching

A connection is established

A dedicated-bandwidth circuit is allocated to each connection across a selected path/route

The circuit consists of a tandem collection of link-circuits along the route links

While multiple link-circuits can be multiplexed across a link, a single connection is permitted to use the circuit BW resources

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Example: Circuit allocated for a connection

Circuit/path (N1, N5) consists of link-circuits/links (N1, N3) and (N3, N5). This circuit is dedicated to an (N1, N5) connection.

Circuit entails: Selected path: N1-N3-N5 Dedicated time slots (or other communications capacity

resources) allocated across each link along the path.

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Circuit switching: signaling

Phases of circuit switching Phase 1 (signaling): circuit set-up phase

The signaling network is used to set-up a circuit for the requesting connection

In-band vs. out-of-band (common channel signaling – CCS) signaling channels and networks

Route is selected

Circuit switches are notified; switching matrices are set


Switching Table at a Circuit Switch

Connection ID Line-In Time Slot-In Line-Out Time Slot-Out

Set by signaling system


Set by signaling data received from preceding switch


Set by switch in selecting available slots


Time-Space-Time Switching

TD_Demux

TD_Mux

Sp

ace

Sw

itch

ing


Signaling

Example: time-space-time circuit switch whereby incoming/outgoing links (lines) and time-slots are recorded in the switching matrix of each switch across the route.

The signaling network is used to set-up a circuit (FDX or HDX) for the requesting connection

Multicast circuits can also be set-up for conference sessions (using conference bridges)


Circuit Switching: Phases

Phase 2: Information transport Information belonging to a connection is

transported across the circuit established for this connection

Phase 3 (signaling): circuit disestablishment phase When the user signals the network that the

session has terminated the connection is disestablished and the circuit is torn-down; the circuit’s resources are returned to the pool of available communications transport resources.


Circuit switching: performance

Delay

Circuit set-up delay – key delay measure

Information transmission latency across circuit switch (such as for time-slot interchange) – minimal – of the order of a time frame

Circuit tear-down delay

Blocking and throughput

Grade of service (GOS) = probability that a connection request is blocked (Pb) – key call throughput measure

Information throughput per connection = circuit capacity


Features of Circuit Switching Networking Operations: Advantages

Guaranteed quality of service (QoS) for transport across the circuit configured by the signaling system

Message throughput and delay performance can be guaranteed to messages that are issued by admitted connections


Features of Circuit Switching Networking Operations: Advantages

Good capacity utilization of circuit resources when source generated message traffic or streams at a steady basis or at a relatively high duty cycle, so that the circuit is kept busy high fraction of the time

Due to configuration of switching tables by the signaling system, the switching operation can be implemented to be hardware focused and proceed at ultra high rates.


Features of Circuit Switching Networking Operations: Disadvantages

Requires the implementation, operation and maintenance of a signaling system

Low capacity utilization of circuit resources when the source is bursty, generating message traffic at a low duty cycle, so that the circuit capacity is unused high fraction of the time Example: interactive personal user

terminal generated data applications


Connectionless (Datagram) packet switching: principles

N1

N2

N5

N3

N4

(N1,N5)

(N1,N4)

(N1,N5)

(N1,N4)

Messages segmented into packets

(N1,N4)

(N1,N4)

(N1,N5)

(N1,N5)

(N1,N5)

(N1,N4)

Packets assembled into messages

Packet header information field


Connectionless (Datagram) packet switching: Statistical Multiplexing

Under a packet switching operation, messages that belong to a single flow are not bound to be transmitted across the communications lines only during pre-set slots

Rather, a flow’s messages occupy capacity resources of communications lines only when generated and transported across them.

Thus, messages (belong to multitudes of flows) share the communications lines across which they are transported in a statistical multiplexing manner.


Datagram Networking Principles

Messages are segmented into network-layer packets

Each packet carries in its header information sufficient for its routing across the network

Under network-layer connectionless operation, no connection is set-up prior to the start of transport of packets across the network. Each packet contains an header which identifies the packet’s source and destination nodes.

A packet is switched and routed in the network on a store-and-forward basis, as a “datagram”


Datagram Switch Operation

Header is read; using the routing discipline and routing table, the output line is determined

The packet is switched to the output line

The packet is queued at the output line buffer; it is transmitted across the output line on a FCFS or priority basis


Connectionless (datagram) packet switching: switch/router system

Routing table

Switching fabric

Output line

Output line

Header processor

Input Buffers

Output Buffers

Router: Provides switching and routing services; engages in calculating routes and configuring its routing table.

Switching/routing rate measured in packets/sec

Spatial diversity induces variable message rate feeding output queues, leading to queuing delays at the output buffers


Illustrative Switching Table at a Datagram (IP) Router

Destination Address

Mask Line-Out Distance Measure

Time validity

IP Address

e.g., subnetwork and hierarchical routing structure; to reduce table size, group addresses of packets that travel along the same outgoing link

along the ‘shortest’ (best) path

e.g., number of hops, delay, throughput or $cost oriented measures

Forwarding entry is discarded as its validity time expires


Connection oriented (VCS) packet switching: Principles

N1

N2

N5

N3

N4

(N1,N5)

(N1,N4)

(N1,N5)

(N1,N4)

Link L(1,2)

packet switch

Signaling packets

packet


Connection oriented (VCS) packet switching: Principles

Connection oriented: a signaling system is used to setup connections

Packet switching method: communications lines are shared on a statistical multiplexing basis

Route is selected during signaling setup phase

Virtual circuit identifies the selected route for a given connection

Virtual circuit identifier (VCI) is used to identify packets that belong to the same connection

VCI is swapped at each switch


Phases of connection oriented packet switching – virtual circuit switching

Phase 1 (signaling): virtual circuit set-up phase

The signaling network is used to set-up a logical path/connection (virtual circuit – VC)

A virtual circuit identifier (VCI) is selected to identify the logical connection

The VC table of each switch is updated


Switching Table at a Virtual Circuit Switch

Connection ID Line-In VCI-In Line-Out VCI-Out



Set by signaling data received from preceding switch


Set by switch


Virtual circuit switching networking phases (Cont.)

Phase 2: information transport

Information belonging to a connection is transported across the established VC

Phase 3 (signaling): call is terminated; VC disestablishment phase


Connection oriented (VCS) packet switching: network transport

N1

N2

N5

N3

N4

(N1,N5)

(N1,N4)

(N1,N5)

(N1,N4)

Link L(1,2)

Messages (N1,N5) (N1,N4)

packet

VC(N1,N3) VC(N1,N4) VC(N1,N4)

VC(N1,N3)


VCS Switch Operation

Header is read; using the VC routing table, the output line is determined

The packet is switched to the output line

The packet is queued at the output line buffer; it is transmitted across the output line on a FCFS or priority basis


VCS: Switch System

VC Routing table

Switching fabric

Output line

Output line

Header processor

Input Buffers

Output Buffers

Signaling system (or network management) is used for calculating routes and configuring the VC routing table.

Switching/routing rate measured in packets/sec

Spatial diversity induces variable message rate feeding output queues, leading to queuing delays at the output buffers

VCI

VCI

VCI

VCI


Datagram Packet switching: performance

Delay

Queueing and processing delays at packet switch processors

Queueing delays and transmission latencies across lines

Routing Operation

Router engaged in setting up and continuous dynamic (and periodic) updating of the routing tables



Effective link utilization through statistical multiplexing

Flow is typically not provided QoS guarantees

Easy to operate when packets are provided Best Effort type services



No signaling based end-to end connection setup and connection oriented resource prescriptions imply simple and flexible interfaces between IP networks, leading to modular and flexible growth of networking coverage and of mixing of heterogeneous networking technologies Easy to support nets that employ distinct

communications media and operate over different environmental conditions



Rapid reaction to link / node failures / degradations

Periodic and dynamlic updating of forwarding entries in the routing tables in fast reaction to such failures

A packet arriving at a router can be rapidly directed to an alternate route, rather then be discarded or queued until a new route is established.


VCS Packet switching: performance

Delay VC set-up delay Queueing and processing delays at packet switch

processors Queueing delays and transmission latencies across

lines

Blocking and throughput Grade of service (GOS) = probability that a call

connection request is blocked (Pb) Information throughput per connection is variable

depending on the source; accommodates VBR (variable bit rate connections)



Requires a signaling subsystem for dynamic connection setup

Effective use can be made of the connection subsystem, in conjunction with a Call Admission Control (CAC) process to admit a call and (statistically) configure resources across the VC so that call packets are (statistically) guaranteed desired QoS



Upon failure, the connection and associated VC must be re-configured, inducing delays

Switching nodes require a simplified hardware based operation, leading to higher switching speeds

Internetworking involves splicing of VCs.


Packet Switching

Key phenomena:

Statistical multiplexing across the links

Occurrences of congestion hot spots

As a result: employ adaptive flow and congestion control mechanisms; call admission controls; adaptive routing


QoS Oriented Operation over IP Networks

Signaling / control system implemented to select routes and statistically assign capacity resources at the switches in conjunction with a label switching based operation Multi Protocol Label Switching (MPLS)

Integrated Services approach Through reservation process

e.g., Asynchronous Transfer Mode (ATM) networks

Differentiated Services approach Through the use of packet service preference

identifiers (DSCIs)

Section 4 Switching Methods

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