Switching

Definition of Terms

• Switching is a method in which communication devices are connected to one another efficiently.
• A switch is intermediary hardware or software that links devices together temporarily.
• There are three fundamental switching methods: circuit switching, packet switching, and message switching. 
• Circuit switching uses either of two technologies: the space-division switch or the time-division switch. 
• In a space-division switch, the path from one device to another is spatially separate from other paths.
• A crossbar is the most common space-division switch.  It connects n inputs to m outputs via n × m crosspoints.
• Multistage switches can reduce the number of crosspoints needed, but blocking may result.
• Blocking occurs when not every input has its own unique path to every output.
• In a time-division switch, the inputs are divided in time, using TDM.   A control unit sends the input to the correct output device. 
• The time-slot interchange and the TDM bus are two types of time-division switches.
• Space- and time-division switches may be combined.
• A telephone network is an example of a circuit-switched network. 
•  Switching at the physical layer in the traditional telephone network uses the circuit-switching approach.
• In a datagram network, each packet is treated independently of all others. Packets in this approach are referred to as datagrams. There are no setup or teardown phases.
• A switch in a datagram network uses a routing table that is based on the destination address.
• The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet.
• Switching in the Internet is done by using the datagram approach to packet switching at the network layer.
• A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. 
• In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand.
• Switching at the data link layer in a switched WAN is normally implemented by using virtual-circuit techniques.
• The address field defines the end-to-end (source to destination) addressing.
• A switch in a packet-switched network has a different structure from a switch used in a circuit-switched network.We can say that a packet switch has four types of components: input ports, output ports, a routing processor, and switching fabric.
• The United States is divided into more than 200 local exchange carriers (ILECs) and competitive local exchange carriers (CLECs). Inter-LATA services are handled by interexchange carriers (IXCs). 
• Telephone companies provide digital services such as switched/56 services and digital data services. 
• The AT&T monopoly was broken in 1984 through a government suit. 

Three Fundamental Switching Methods

• Circuit switching - a direct physical connection between two devices is created by space-division switches, time-division switches, or both. Circuit switching takes place at the physical layer. In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase.
• Packet switching - there is no resource allocation for a packet. This means that there is no reserved bandwidth on the links, and there is no scheduled processing time for each packet. Resources are allocated on demand. Packet switching uses either the virtual circuit approach or the datagram approach.
• Message switching - has been phased out in general communications but still has networking applications.

Today's Networks Three Broad Categories

• Circuit-switched networks - made of a set of switches connected by physical links, in which each link is divided into n channels.
• Packet-switched networks - there is no resource reservation; resources are allocated on demand. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol. Packet-switched networks can also be divided into two subcategories: virtual-circuit networks and datagram networks
• Message-switched -

Taxonomy of switched networks

Three Phases in a Circuit-switched network

• Setup Phase - The end systems are normally connected through dedicated lines to the switches, so connection setup means creating dedicated channels between the switches.
• Data Transfer Phase - After the establishment of the dedicated circuit (channels), the two parties can transfer data.
• Teardown Phase - When one of the parties needs to disconnect, a signal is sent to each switch to release the resources.

Telephone system has three major components

• Local loops - a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz).  The first three digits of a local telephone number define the office, and the next four digits define the local loop number.
• Trunks - transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing.
• Switching Offices - A switch connects several local loops or trunks and allows a connection between different subscribers.

Formulas

• In a three-stage switch, the total number of crosspoints is 2kN + k(N/n)² which is much smaller than the number of crosspoints in a single-stage switch (N2). 
• According to the Clos criterion:
  n = (N/2)^1/2
  k > 2n – 1
  Crosspoints ≥ 4N [(2N)^1/2 – 1]
  

Note: You can proceed to take the multiple choice exam regarding this topic. Switching - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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