Circuit Switching and Packet Switching

What is Circuit Switching and Packet Switching ? 


This blog explains the protocols used for Circuit Switching (voice call) and Packet Switching (Data or Voip) communications.
What is Packet Switching
What is Circuit Switching
Circuit Switching vs Packet Switching

 What is a Circuit or Circuit Definition?

In telecom, circuit is a dedicated reserved path between two endpoints. Two entities may connects directly, or there can be multiple hopes between endpoints . For voice in PSTN , SS7 is the protocol for voice circuit.  SS7 have E1/T1 trunks between two endpoints (SSP or telephone exchange) of SS7.  During call a SSP reserves a voice circuit. If calling and called SSPs connects to each other directly, then one E1/T1 physical line connects directly, if via other intermediate SSPs, multiple physical lines creates a voice path.

Example: There are 2 cities A and C. But there is no physical link between telephony switching center A and C . There is an exchange B between A and C. When a call setup  from A to, B physical circuit is reserved between A and B , and B and C. All communications between a A and C will be via B.

What is Circuit Switching?

In circuit switching a dedicated channel or path is required in both the directions, before any communication starts. The channel may be a direct channel or via many hopes. Data flow in form of of bytes or bits. The Call setup over PSTN is one of the example. In this a voice circuit is reserved from a voice trunk. The Signaling used is ISUP over E1/T1. Isup belongs the the SS7 protocol suit. SS7 switching reserves a fixed bandwidth for the communication in both the directions, in voice call the circuit bandwidth is fixed according to E1 or T1 channel.

ISUP Call Setup:

In this section we are explaining circuit switching example with the example of ISUP protocol. ISUP protocol belongs to the legacy SS7 protocol for voice call signaling.When a mobile user dials a number, originating SSP starts ISUP signaling. A channel  is reserved from originating switching centre to the terminating switching centre. If a new call is initiated from the same switching center , new channel is allocated. A channel a identified by CIC ( Channel identification Code).

Circuit Switching

What is Packet?

A packet have user data and a protocol header. Header have information which is used by the protocol to provide services to its user. Protocol header may have routing and other information. Routing information have source and destination address.  The routing decisions are taken on every intermediate node or router. For example , if a person calls from India to USA over a IP phone. The source address in packet is the IP address of caller in India and destination address  is the IP address of phone in USA.  The packet is routed via many routes (e.g router in UK).

 What is Packet Switching

In packet switching , no dedicated circuit is reserved before any communication. Data from one node to other flows in terms of packets. The data is divided into small packets. Each packet have full source and destination address in packet header and user data. Intermediate node or routers , routes packets based on destination header parameter.  In telecom , the voip call is the example of packet switching. In this two RTP (Real Time Protocol) endpoints are set , no dedicated channels are setup for voice packets.
After Setup of endpoints , same ip ( ip lines, router etc.) network can be use for multiple calls.
Example Of Voip Call setup with SIP or Session Initiation Protocol. The SIP uses UDP over IP, this is signaling protocol in Voip Call setup. If setup RTP endpoints for media flow from source to destination.  RTP also uses UDP over IP.


Packet Switching Example
SIP Call Flow

Difference between circuit switching and packet switching/circuit switching vs packet switching

  • Circuit switching have resource reservation , while in packet switching there is no resource reservation.
  • In Packet switching , data is divided into small units called packets. These packets can take a path which is shared by other packet switching nodes. In Circuit Switching , if there is no communication then there is no data on a dedicated channel.
  • In Circuit switching the resources are reserved for a call or circuit. If the call is idle then there will be no utilization of circuit from any other call. This makes circuit switching less efficient for resource utilization. While in circuit switching same resources or bandwidth can share by multiple calls. This makes packet switching more efficient.
  • Because of resource reservation in circuit switching , the Quality Of Service is guaranteed. While in packet switching Quality of Service is not guaranteed.
  • Packet Switching is more cost effective as compared to circuit switching , due to sharing of resources.
  • In packet based switching each packet follow a different path routing decisions are made on each intermediate nodes or routers .
  • If  there is communication lost in circuit , user gets the indication while in packet based network not.

Virtual Circuit Switching:

This is the way for achieving circuit switching type communication over a packet switching network. In this software level resources ( e.g message window) are fixed for a circuit, while physical resources (e.g IP link , IP routers ) are share among multiple circuit. In Virtual Circuit Switching , a connection is setup over network. Virtual Circuit, gives the impression to user that there is a dedicated path reserved from source to destination.

Functionalities of Virtual Circuit Switching.

Data Segmentation, to meet the requirements on underlying packet switching network.
Data Reassembly, to deliver the compete messages same as it was sent from originator.
Retransmission, this is done for sending a packet again , which has been lost on underlying packet switching network.
Flow Control, this is to avoid packet loss , due to congestion in network or a slow receiver.
In Virtual Circuit switching a connection is setup before any user data. While in packet network no setup of initial connection.
In Virtual Circuit switching user application on receiving side received in same order as they send , while this is not in packet switching.

Example of Virtual Circuit Switching Protocols:

TCP is the transport layer protocol. Which is used to setup a virtual connection between a sources and destination application. TCP uses underlying IP layer, which is an implementation of packet switching network layer. The TCP connection is identified by a connection identifier. Each connection is a unique pair of TCP end points. A TCP end point is the pair of IP address and a port.
Example of Virtual Circuit:
If 2 machines those are connected via packet switching network (e.g IP), over a port 3002. The machines have Ip address like and .
The end point on machine one is {, 3002}. The Endpoint on second machine will be {, 3002}
TCP Connection (a pair of end points) will be [{, 3002}, {, 3002}].
SCTP or Stream Control Transmission Protocol, is a transport layer protocol this is more reliable than TCP. It is used for setting up a virtual connection. The endpoint in SCTP may have more than one IP address as , SCTP provides multi homing capability.

Circuit Network to Packet Network  Conversion:

As we have seen that a call can be connected from packet based network to the circuit based network and vice versa. This is possible via using a gateway between two types of network. Voip to PSTN , is one of the example where an IP phone (even voip software ) can call to a PSTN based device.  The gateway  does the Isup/ss7 signaling to SIP/IP conversion and the conversion of media packets too.

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