UNIT-II DIGITAL SWITCHING

What is it all about? How do we move traffic from one part of the network to another? Connect end-systems to switches, and switches to each other Data arriving to an input port of a switch have to be moved to one or more of the output ports

Types of switching elements Telephone switches switch samples Datagram routers switch datagrams ATM switches switch ATM cells

Classification Packet vs. circuit switches packets have headers and samples don’t Connectionless vs. connection oriented connection oriented switches need a call setup setup is handled in control plane by switch controller connectionless switches deal with self-contained datagrams

Crossbar Simplest possible space-division switch Crosspoints can be turned on or off For multiplexed inputs, need a switching schedule (why?) Internally nonblocking but need N2 crosspoints time taken to set each crosspoint grows quadratically vulnerable to single faults (why?)

Multistage crossbar In a crossbar during each switching time only one crosspoint per row or column is active Can save crosspoints if a crosspoint can attach to more than one input line (why?) This is done in a multistage crossbar Need to rearrange connections every switching time

Multistage crossbar Can suffer internal blocking unless sufficient number of second-level stages Number of crosspoints < N2 Finding a path from input to output requires a depth-first-search Scales better than crossbar, but still not too well 120,000 call switch needs ~250 million crosspoints

Time-space switching Precede each input trunk in a crossbar with a TSI Delay samples so that they arrive at the right time for the space division switch’s schedule

Dealing with blocking Overprovisioning Buffers Backpressure internal links much faster than inputs Buffers at input or output Backpressure if switch fabric doesn’t have buffers, prevent packet from entering until path is available Parallel switch fabrics increases effective switching capacity

Three generations of packet switches Different trade-offs between cost and performance Represent evolution in switching capacity, rather than in technology With same technology, a later generation switch achieves greater capacity, but at greater cost All three generations are represented in current products

First generation switch Most Ethernet switches and cheap packet routers Bottleneck can be CPU, host-adaptor or I/O bus, depending

Example First generation router built with 133 MHz Pentium Copy loop Mean packet size 500 bytes Interrupt takes 10 microseconds, word access take 50 ns Per-packet processing time takes 200 instructions = 1.504 µs Copy loop register <- memory[read_ptr] memory [write_ptr] <- register read_ptr <- read_ptr + 4 write_ptr <- write_ptr + 4 counter <- counter -1 if (counter not 0) branch to top of loop 4 instructions + 2 memory accesses = 130.08 ns Copying packet takes 500/4 *130.08 = 16.26 µs; interrupt 10 µs Total time = 27.764 µs => speed is 144.1 Mbps Amortized interrupt cost balanced by routing protocol cost

Second generation switch Port mapping intelligence in line cards ATM switch guarantees hit in lookup cache Ipsilon IP switching assume underlying ATM network by default, assemble packets if detect a flow, ask upstream to send on a particular VCI, and install entry in port mapper => implicit signaling

Third generation switches Bottleneck in second generation switch is the bus (or ring) Third generation switch provides parallel paths (fabric)

Third generation (contd.) Features self-routing fabric output buffer is a point of contention unless we arbitrate access to fabric potential for unlimited scaling, as long as we can resolve contention for output buffer

The importance of switching in communication Definition: The cost of switching is high Definition: Transfer input sample points to the correct output ports at the correct time Terminology Switching Digital switching (sample points amplitudes are 0's and 1's) PABX Circuit Circuit switching Packet switching

Space division

Voice digitization: Telephone switching W=3KHz, sampling at 2*3=6 or 8KHz 256 levels for quantization (8 bits) Bit rate=64Kb/s Telephone switching Time division multiplexing: time slot (0.1 ms), field, frame; 125ms/0.8=150 channels + time for synchronization and control

Switch architecture Sampling input signals, storing values in memory, placing values in the proper field and frame of the output sequence Need for more channels: hierarchical switching Combining time and space switching

Switching techniques and networking Switching is the technology allowing to get a message between the nodes of a network Crossbar switching: mechanical (in the past) or electronic. Bus and cable switches: computer buses or cables (switching + transportation = network) Token passing approach (similar to the locks used by multiprocessors connected by a bus) Ethernet approach: cable or ring, packets, conflicts, resending Synchronization and Hub switch: star networks (no conflicts)

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. Ethernet Cabling Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

Time division switching Key idea: when demultiplexing, position in frame determines output link Time division switching interchanges sample position within a frame: Time slot interchange (TSI) M U X D E TSI

Time Slot Interchange (TSI) : example sessions: (1,3) (2,1) (3,4) (4,2) 1 2 3 4 1 2 2 4 4 3 2 1 3 1 4 2 3 1 4 3 Read and write to shared memory in different order

TSI Simple to build. Multicast: easy (why?) Limit is the time taken to read and write to memory For 120,000 telephone circuits Each circuit reads and writes memory once every 125 ms. Number of operations per second : 120,000 x 8000 x2 each operation takes around 0.5 ns => impossible with current technology Need to look to other techniques

Time-Space: Example time 1 time 2 2 1 2 1 TSI 3 4 4 3 3 2 1 4 2 1 3 4 3 1 2 4 2 1 4 3 TSI TSI Internal speed = double link speed

From space switch to T-S-T switch k 1 2 .... n 1 2 .... n 1 2 .... k TSI 1 TSI 1 k 1 2 .... k 1 2 .... n 1 2 .... n 1 2 .... TSI 2 TSI 2 k 1 2 .... n 1 2 .... k 1 2 .... n 1 2 .... TSI N/n TSI N/n Replace each first-stage and every last-stage switch by a TSI. Replace multiple intermediate-stage switch with a single time-shared space switch. The intermediate stage switch is a single switch of size: The intermediate stage switch is reprogrammed for each of the k time slots. last updated March 8, 04 M. Veeraraghavan

Time-space-time (TST) switching Allowed to TSI both on input and output Gives more flexibility => lowers call blocking probability TSI

Internal Non-Blocking Types Re-arrangeable Can route any permutation from inputs to outputs. Strict sense non-blocking Given any current connections through the switch. Any unused input can be routed to any unused output. Wide sense non-blocking. There exists a specific routing algorithm, s.t., for any sequence of connections and releases, Any unused input can be routed to any unused output, assuming all the sequence was served by the routing algorithm.

Circuit switching - Space division graph representation transmitter nodes receiver nodes internal nodes Feasible schedule edge disjoint paths. cost function number of crosspoints (complexity of AxB is AB)

Crossbar - example 1 2 3 4 4 1 2 3

Another Example inputs outputs

Another Example outputs inputs sessions: (1,3) (2,6) (3,1) (4,4) (5,2) (6,5) inputs outputs

Clos Network Clos(N, n , k) : N - inputs/outputs; cross-points: 2 (N/n)nk + k(N/n)2 nxk (N/n)x(N/n) kxn 2x2 3x3 2x2 N=6 n=2 k=2 N 2x2 3x3 2x2 k N/n N/n

Clos Network - strict sense non-blocking Holds for k  2n-1 Proof Methodology: Recall: IF [A,B  S and |A|+|B| > |S|] then A∩ B≠Ø S= The k middle switches A = middle switches reachable from the inputs B = middle switches reachable from the outputs Our case: |S|=k |A| ≥ k-(n-1) |B| ≥ k-(n-1)

No.4 ESS TOLL SWITCH Basic Concepts We will define basic telecommunication terms, such as: analog digital bandwidth compression protocols codes and bits

The architecture or protocol suite is the umbrella under which the devices communicate with each other

Congestion All networks are limited in how many peripherals they can support without experiencing too much degradation Today more and more peripherals are being added to networks New ways to eliminate congestion on a network have been developed

Eliminating Congestion Multiplex: to transmit two or more signals over a single channel Compression: reducing the representation of the information, but not the information itself reducing the bandwidth or number of bits needed to encode information or a signal

Multiplexing Several devices can share a telephone line T-1 telephone line will carry 24 communication paths on one high-speed link T-3 provides 672 communication paths on one link

Compression Applications such as: graphics, x-ray images, video are bit intensive Thus require high bandwidth when transmitting Compression reduces the number of bits needed to transfer

Analog and Digital Telephone system developed to transmit speech Spoken words are transmitted as analog sound waves People speak in an analog format, in waves Telephone system was completely analog until 1960

Analog Signals Move down telephone lines as electromagnetic waves The way it travels is expressed in frequency Frequency refers to the number of times per second that a wave oscillates or swings back and forth in a complete cycle from its starting point to its ending point

Analog Signals A complete cycle occurs when a wave starts at a zero point of voltage, goes to the highest positive point of the wave, down to the negative voltage portion, and then back to zero voltage

Analog Signals The higher the speed or frequency, the more complete cycles of a wave are completed in a period of time This speed or frequency is measured in Hertz Hertz: a measurement of frequency in cycles per second, 1 hertz is 1 cycle p/sec

Hertz A wave that oscillates or swings back and forth 10 times per second has a speed of 10 hertz or cycles per second Bandwidth or range of frequencies a service occupies is determined by subtracting the lower range from the higher range 300-3300Hz (voice)= 3300-300= 3000Hz

Analog Services Voice (300 -3300 Hz) Microwave Radio (2-12 GHz) Analog cable TV signals (54-750 MHz) Oscillate between a specific range of of frequencies

Analog versus Digital Analog system can no longer handle the increase in the number of calls that are being generated, was designed for lower volume Digital networks are faster, have more capacity, and are more reliable

Impairments on Analog Services Analog signals loose their power the longer they travel Signal meets resistance in the media (copper, coaxial cable, air), causes fading of the signal or attenuation of the signal Analog signals also pick up noise or electrical energy while travelling from power lines, light sources, and electrical machinery Requires: amplification to inhibit attenuation

Amplification To overcome resistance in a signal, analog signals are amplified while they travel over a medium Drawbacks amplification: also increases level of noise in signal

Digital Signals Advantages digital signals: higher speeds clearer voice quality fewer errors less complex peripheral equipment required

Digital Signals No waves are transmitted Digital signals are transmitted in the form of binary bits Binary = being composed of two parts In telecommunications this means only on or off, one or zero piece(s) of information transmitted

Applications Wideband: Narrowband: TV Cable Connections between telephone offices Narrowband: phone connection to end users

Protocols Enable computers to communicate with each other Spell out the rules of interaction between two or more computers Handle error detection and correction and file transmission

Examples of Protocols Who transmits first? What is the structure of the addresses of devices such as computers? How are errors fixed? How long to wait before disconnecting? How to package data to be sent?

Architecture Ties computers and peripherals together into a coherent whole Forms the network which connects all devices together Layers within architectures have protocols to define functions such as routing, error checking and addressing

OSI Not widely implemented Laid foundation for the concept of open communications among vendors Basic concept of layering of groups of functions into 7 layers Each layer can be changed and developed independently

Bridges Used to connect a small number of LAN’s Provide one common path to connect several LAN’s Easy to configure, all data sent to all devices on a network, appropriate device picks it up, broadcast feature Lack routing and congestion control

Routers Used to connect multiple LAN’s over large distances (differing buildings, cities) More sophisticated than bridges Can handle differing protocols from various LAN’s

Switching Routers Faster than non-switching routers Do not look up in tables where to send data Address placed in the pack sent