Lecture 2

What is Switching? Switching is the process of creating a route (path) between a source and a desired destination(s) and relaying data (or call) via this route Switching is divide into Path selection Route establishment Data forwarding

Types of Switching Systems Distributed, Centralized, Hybrid Switching Space, Time, Frequency/Wavelength Switching Circuit and Packet Switching Manual and Automatic

Automatic Switching Electromechanical Strowger Step-by-step switch Crossbar exchange Electronic Digital Space Time

Electromechanical Switches Electromechanical switches use relays and electromechanical devices to perform the switching They are all space switches (frequency also possible) Electromechanical time switches are not practical as we’ll see later

Signaling Signaling in old systems is restricted to the dialing process performed by the user There are two dialing techniques Pulse dialing Dual Tone Multi-frequency (DTMF or Tone) dialing

Pulse Dialing The oldest Central office supplies 50V DC to the user terminal When handle is on, the circuit is open When the user lifts the handle, the circuit is closed and current flows in the circuit Numbers are represented by pulses created by opening and closing the circuit a certain number of times 1 = open circuit one time, 2 = open circuit two times, …, 0 = open circuit 10 times

Pulse Dialing Pulse duration is 33 1 / 3 ms with 200 ms pause between numbers Dialing is assumed finished if circuit remains for more than 200 ms Slow

DTMF Dialing Two frequencies per number 40ms pause between numbers Much faster

Strowger Step-by-step Switching Consists of a group of uni-selectors and two-motion selectors

Two-motion selectors

Step-by Step Switching Calling 5345

User lifts the phone, current flows in the circuit Central office recognizes user has lifted the phone User’s selector hunter starts moving in search for an idle selector Once a selector is connected, it sends the dial tone to the user The user dials the number 5 The selector moves vertically one step for each pulse, thus reaching row 5 Within this row, the selector searches for an idle line Of no line is found, a busy signal is returned

Calling 5345 The user dial the number 3 This is passed to the second group selector via selector 1. The second selector moves to row 3 and searches for an empty line to the final selector The user dials the number 4 This is passed through the group selectors to the final selector The final selector moves vertically to row 4 and stops The user dials the number 5 The final selector moves horizontally to column 5 thus connecting to user 5345 If at any point, no free line is found, a busy signal is returned

Disadvantages A lot of moving parts-> needs regular maintenance Alternative routes if a certain row is busy are not possible Low system lifetime The capacity of the system is reduced if short codes are allocated, e.g. 999 code means 999x are useless The Strowger system accepts 7-9 pulses/s. Fast dialing may lead to incorrect operation To use DTMF dialing, a DTMF to pulse dialing converter is required

Crossbar Exchange Makes use of relays to perform the switching The relays used in the crossbar are controlled by 2 coils Applying currents in certain directions closes the relay Inverting the current in one of the coils opens the relay

Crossbar Exchange Structure The Exchange consists of horizontal bars (connected to the inputs) and vertical bars (connected to the outputs) A relay exists at the intersection of each two bars By activating or deactivating the relays, the input can be connected to the desired output(s)

I1 I2 I3 I4 O1O2O3O4 Crossbar Exchange Structure Crossbar is represented by a state vector (vector is [4, 1, 3, 2] for figure) Unconnected inputs are set to  Broadcast and multicast are easy to implement An output can be connected to one input only at a time

Crossbar Exchange Structure All the relays are connected to a central control system The central control units are called markers (2-3 per exchange) System can use step-by-step (old exchanges) or DTMF dialing (newer ones) Devices known as registers are used to receive the call request and setup the path Electronic crossbars used transistor switches instead of relays Crossbar switches do not require heavy motion, thus they have longer lifetimes and require less maintenance

Types of Crossbar Switches Symmetric Nin = Nout Asymmetric Nin  Nout Speed up Slow down

Asymmetric Crossbars Speedup Nin < Nout Speedup ratio = Nout/Nin Speedup ratio > 1 I1 I2 O1O2O3O4

Asymmetric Crossbars Slow down Nin > Nout Speedup ratio = Nout/Nin Speedup ratio < 1 Connection/output vector = [ , 1, , 2] I1 I2 I3 I4 O1O2

Analog Time Division Switching Shared Bus

Analog Time Division Switching The sampling frequency of each channel must satisfy Nyquist law (  2* bandwidth) A real system uses a set of electronic switches instead of a rotating arrow! The switches are enabled (connect to bus) or disabled (disconnect from bus) using a control unit There are two types of control units Cyclic Control Memory-based Control

Cyclic Control Two control units are required, one for the input lines and the other for the output lines The decoder uses the count value to enable a certain input/output By synchronizing the input and output control units, we can connect different inputs to different outputs k =  log 2 (N) , N is the number of input/output lines Clock Modulo N Counter k to 2 k Decoder N lines to control switches … k lines/bits

Memory Based Control Cyclic control forces a certain sequence for the input output connections, e.g. 1-1, 2-2 means 3-3, 4-4, etc By adding a memory element between the counter and the decoder, any input-output connections are possible It is also possible to connect the same input/output to several outputs/inputs at different times The counter cycles reading the values stored in the memory The decoder decodes the read value Only one memory based control is necessary, the other can be memory based or cyclic control

Memory Based Control Clock Modulo N Counter k to 2 k Decoder N lines to control switches … k lines/bits

Capacity of Time Division Switching System The capacity (C) is given by: C = (Time between samples in any line)/(Time to Tx sample (t s )) For memory based control t s = t i + t m + t d + t t Where: t i  time to increment the counter t m  time to read the memory (zero for cyclic control) t d  time to decode the address and select the input/output t t  time to transfer the sample through the bus Time between samples = 1/sampling frequency

Limitations Synchronization is essential for the operation of the system. For large networks this is a major concern The sampling process in analog system does not use all the energy of the signal leading to a large loss in signal power The signal bandwidth increases thus increasing the noise power Hence the SNR becomes very low in analog time switching and this is why it’s not practical

Digital Switching Digital Switches use digital circuits and logic to switch digital signals For analog signals, A/D and D/A converters must be used

Digital Transmission Pros and Cons Advantages of digital transmission Low SNR and SIR requirements Signal regeneration Ease of storage and processing Ease of multiplexing Lower cost and power requirements Disadvantages Greater bandwidth Need for synchronization

Modes of Digital Transmission Asynchronous Transmission only when there is data Start and stop bit(s) indicate beginning/end of frame Idle periods separate frames (varying length) Limitations High overhead Timing errors, since each device is using internal clock Framing errors, missed or false start/stop or timing errors Large overhead + idle gaps -> low bit rate StartData bitsStopStartData bitsStopStartData bitsStop Gap

Modes of Digital Transmission Synchronous Continuous transmission, if no data, a special sequence of bits is sent High bit rates Disadvantages Wasted power if no data is available Synchronization from a single clock is required 8-bit flag Data bitsControl field 8-bit flag PreamblePost-amble

Digital Space Switch (Digital Crossbar) I1 I2 I3 ININ O1 O2 OMOM … MUX … … … … 1 2 … M 1 2 … M Control 1 Control 2

Digital Crossbar Control units (registers) must select one or none of the inputs The control units have a size   log 2 (N+1)  One control unit is capable of operating the switch but usually two are used If the switch connections are to be changed, the new configuration is loaded in the non-active control unit The control then is switched to this unit

Digital Crossbar The previous configuration is known as a single stage crossbar or single stage switch The complexity of the crossbar is calculated by the number of cross connections (N  M) or N 2 for symmetric crossbars For large N, this becomes large making the MUX and memory very large and causing power and capacitance problems The processing load also increases as new configurations are required more often (more calls) For large switches, multistage configurations are used