Optical Fibre Communication Systems

Optical Fibre Communication Systems
Lecture 7 – Optical Switches Professor Z Ghassemlooy Northumbria Communications Laboratory School of Computing, Engineering and Information Sciences The University of Northumbria U.K.

Contents Network Systems Network Trends Switch Fabric Type of Switches
Optical Cross Connects Optical Cross Connects Architecture Large Scale Switches Optical Router Applications

Development Milestones
2004 International Engineering Consortium

Network Network Connectivity Network Span Data Rates Service Types
Point to Point: one to one Broadcast: one to many Multicast: many to many Network Span Local / Metro Area Network Wide Area Network Long Haul Network Data Rates Voice 64kbps Video 155Mbps, etc. Service Types Constant or Variable bit rate Messaging Quality of Service

Fully Connected, Un-switched Network
Ports Problem limited and could not scale to thousands or millions of users Solution - switched network

Switched Network Pervasive, high-bandwidth, reliable, transparent

Optical Network - Issues
Capacity 2.5 Gb/s Gb/s Gb/s Larger Control (switching) Electronics 10 Gb/s (GaAs, InP) can deliver low order optical cross connects (16 x 16) > 10 Gb/s ??(mainly power dissipation) Optical Reconfiguration: Static or dynamic

Optical Network Elements
Dense Wavelength Division Multiplexing Optical Add/Drop Multiplexers (OADM) Optical Gateways: A critical network element. A common transport structure to cater for variety of bit rates and signal formats, ranging from asynchronous legacy networks to 10–Gbps SONET systems, a mix of standard SONET and ATM services.

Switching - Electrical
Right now, the optical switches have electrical core, where Light pulses are converted back into electrical signals so that their route across the middle of the switch can be handled by conventional ASICs (application specific integrated circuits). This has a number of advantages: Enabling the switches to handle smaller bandwidths than whole wavelengths, which fits in with current market requirements. Easier network management, because standards are in place and products are available. Optical equivalents are not, at present. But, there are concerns that electrical cores won’t be able to cope with the explosion in the number of wavelengths in telecom networks (deployment of DWDM). Until recently, state-of-the-art ASIC technology wouldn’t support anything more than a 512-by-512-port electrical core, and carriers demanding for at least double this capacity.

Optical Network Elements - Switches
Optical Bidirectional Line Switched Rings Optical Cross-Connect (OXC) Efficient use of existing optical fibre facilities at the optical level becomes critical as service providers started moving wavelengths around the glob. Routing and grooming are key areas, and that is where OXCs are used. International Engineering Consortium, 2004

Optical Switches To provide high switching speed
To avoid the electronics speed bottleneck I/O interface and switching fabric in optics Switching control and switching fabric in optics Switches act as routers and redirect the optical signals in a specific direction. It uses a simple 2x2 switch as a building block Main feature: Switching time (msecs - to- sub nsecs)

All Optical Switches That’s the theory. But, things are turning out a little different in practice. Vendors are finding ways of building larger scale electrical cores, with switch of many thousands of ports. This may encourage carriers to put off decisions on moving to all-optical switches. Does this mean that is the end of the idea of all-optical networks? Well, not really. All that it might do is delay things.

Electrical vs. Optical - Cross Connects
Number of ports 1024 32 64 16 8 512 256 128 Optical 10 MHz DS3 100 MHz OC3 OC12 1 GHz OC48 OC192 100 GHz Electrical 10 GHz Data rate Electrical Limits High power consumption: e.g. 1024x1024: 4 kW Jitter: very large Large switches Need OE/EO conversion Bipolar or GaAs M C Wu

Switching: Types Circuit Switching: E.g. Telephone
Continuous streams no bursts no buffers Connections are created and removed Buffering does not exist in circuit-switches Packet Switching: Uses store & forward The configuration may change per packet Switching/forwarding is based on the destination address mapping Switching table is used to provide the mapping Switching table changes according to network dynamics (e.g. congestion, failure)

Switching Fabric Electro-optical 2 x 2 switching elements are the key devices in the fabrication of N x N optical data path. The switching elements rely on the electro-optic effect (i.e., the application of an electric field to an electro-optical material changes the refractive index of the material). The result is a 2x2 optical switching element whose state is determined by an electrical control signal. Can be fabricated using LiNbO3 as well as other materials. Optical input output Electrical control Optical input output Electrical control

Switching Fabric – contd.
Switching control Input interface Output Switching fabric

Switching Fabric – contd.
... Optical transport system (1.55 mm WDM) 1.3 mm intra-office Optical Crossconnect (OXC) Transponders Terminating equipment | SONET, ATM, IP...

Connectivity Since a switch work as a permutation that routes input to the outputs, therefore it needs to provide at least N! different configuration A minimum number of Log2(N!) is needed to configure N! different permutation Blocking Non-Blocking

Connectivity - Blocking
Occurs when one reduces the number of crosspoints in order to achieve low crosstalk and less complexity. In some switching architecture internal blocking may be reduced to zero by: Improving the switching control: Wide sense non-blocking Rearranging the switching configuration: Rearrangeably non-blocking

Connectivity– Non-blocking
A new connection can always be made without disturbing the existing connections: Strictly Non-blocking A connection path can always be found no matter what the current switching configuration is or what switching control algorithm is used Wide-Sense Non-blocking A connection path can always be found regardless of the current switching configuration provided a good switching control algorithm is employed No re-routing of the existing connections Rearrangeably Non-blocking The same as wide-sense, but requires re-routing of the existing connections to avoid blocking Use fewer switches Requires more complex control algorithm

Time Division Switching
Interchanges sample (slot) position within a frame: i.e. time slot interchange (TSI) when demultiplexing, position in frame determines output link read and write to shared memory in different order 1 2 3 4 TSI M U X N D E

TSI - Properties Simple
Time taken to read and write to memory is the bottle-neck 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

Space Division Switching
Crossbar Clos Benes Spank - Benes Spanke

Crossbar Architectures
Each sample takes a different path through the switch, depending on its destination Crossbar: Simplest possible space-division switch Wide- sense blocking: When a connection is made it can exclude the possibility of certain other connections being made Crosspoints can be turned on or off Input ports Output ports 1 2 3 4 Sessions: (1,4) (2,2) (3,1) (4,3)

Crossbar Architectures - Blocking
1 2 3 4 Input channels Output channels - Bars Output channels - Cross N X N matrix S/W M inputs x N outputs Switch configuration: “set of input-output pairs simultaneously connected” that define the state of the switch For X crosspoints, each point is either ON or Off, so at most 2X different configurations are supported by the switch. Case 1: - (3,2) ok - (4,3) blocked Optical switching element

Crossbar Architecture - Wide-Sense Non-blocking
Rule: To connect ith input to the jth output, the algorithm sets the switch in the ith row and jth column at the “BAR” state and sets all other switches on its left and below at the “CROSS” state. 1 2 3 4 Input channels Output channels Case 2: - (2,4) ok (3,2) ok (4,3) ok

Crossbar Architectures – 2 Layer
Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81 Penalty is loss of connectivity 3x3 2 5

Crossbar Architectures - 3 Layer
1 1 2 2 3 3 4 4 5 Output ports Input port 5 6 6 7 7 8 8 9 9 Blocking still possible

Crossbar Architectures - 3 Layer
Blocking * 1 2 3 4 5 6 7 8 9 The first four connections have made it impossible for 3rd input to be connected to 7th output The 3rd input can only reach the bottom middle switch The 7th output line can only be reached from the top output switch.

Crossbar Architecture - Features
Architecture: Wide Sense Non-blocking Switch element: N2 (based on 2 x 2) Switch drive: N2 Switch loss: (2N-1).Lse +2Lfs SNR: XT – 10log10(N-1) Where XT; Crosstalk (dB), Lse; Loss/switch element Lfs; Fibre-switch loss

Crossbar Architecture - Properties
Advantages: simple to implement simple control strict sense non-blocking Low crosstalk: Waveguides do not cross each other Disadvantages number of crosspoints = N2 large VLSI space vulnerable to single faults the overall insertion loss is different for each input-output pair: Each path goes through a different number of switches

Time-Space Switching Arch.
MUX 1 2 3 4 2 1 3 4 TSI 4 3 time 1 Each input trunk in a crossbar is preceded with a TSI Delay samples so that they arrive at the right time for the space division switch’s schedule Note: No. of Crosspoints N = 4 (not 16)

Time-Space Switching Arch.
Can flip samples both on input and output trunk Gives more flexibility => lowers call blocking probability TSI Complex in terms of: - Number of cross points - Size of buffers -Speed of the switch bus (internal speed)

Clos Architecture kxk nxp pxn Stage 1 Stage 3 Stage 2 Stage 1 (nxp)
32 64 33 N= 1024 993 n It is a 3-stage network - 1st & 2nd stages are fully connected - 2nd & 3rd stages are fully - 1st & 3rd stages are not directly connected Defined by: (n, k, p, k, n) e.g. (32, 3, 3, 3, 32) (3, 3, 5, 2, 2,) Widely used Stage 1 (nxp) Stage 2(kxk) Stage 3 (pxn)

Clos Architecture In this 3-stage configuration N x N switch has:
2pN + pk2 crosspoints (note N = nk) (compared to N2 for a 1-stage crossbar) If n = k, then the total number of crosspoints = 3pN, which is < N2 if 3p < N. Problem: Internal blocking Larger number of crossovers when p is large.

Clos Architecture – Blocking
If p < 2n-1, blocking can occur as follows: Suppose input 1 want to connect to output 1 (these could be any fixed input and outputs. There are n-1 other inputs at k-switch (stage 1). Suppose they each go to a different switch at stage 2. Similarly, suppose the n-1 outputs in the first switch other than output 1 at the third stage are all busy again using n-1 different switches at stage 2. If p < n -1 + n = 2n -1 then there will be no line that input 1 can use to connect to output 1. If p = 2n -1, then Total Switch Element: 2kn(2n-1) + (2n -1)k2

Clos Architecture – Blocking
If p = 2n -1, then Total Switch Element: 2kn(2n-1) + (2n -1)k2 Since k = N/n, therefore the number of switch elements is minimised when n ~(N/2) 0.5. Thus the number switch elements = 4 (2)0.5 N3/2 – 4N, which is less than N2 for the crossbar switch

Clos Architecture – Non-blocking
If p  2n -1, the Clos network is strict sense non-blocking (i.e. there will free line that can be used to connect input 1 to output 1) If p  n, then the Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches)

Clos Architecture – Example
If N = 1000 and and n = 10, then the number of switches at the: 1st & 3rd stages = N/n = 1000/10 = 100 1st stage = 10 x p 3rd stage = p x 10 2nd stage = p x k x k. If p = 2n -1 = 19, then the resulting switch will be non-blocking. If p < 19, then blocking occurs. For p = 19, the number of crosspoints are given as follow:-

Clos Architecture – Example contd.
In the case of a full 1000 x 1000 crossbar switch, no blocking occurs, requiring 106 crosspoints. For n = 10 and p = 19, the number of crosspoints at 1st and 3rd stages = no. of stages x (n x p) x k = 2 x (10 x 19) x 100 = 38,000 crosspoints 2nd stage (p = 19 crossbars each of size 100 x 100, because N/n = 100. = p x k x k = 19 x 100 x 100 = crosspoints. The total no. of crosspoints = = Vs. the 106 points used by the complete crossbar.

Clos Architecture – Example contd.
Since k = N/n, the number of switch elements k is minimised when n ~(N/2)0.5 = (1000/2) 0.5 =~ 23 instead of 19. then k = N/n = 1000/23 =~ 44 switches in the 1st & 3rd stages, and p = 2(23) -1 = 45. the number of crosspoints at 1st and 3rd stages = no. of stages x (n x p) x k = 2 x (23 x 45) x 44 = the number of crosspoints at 2nd stage = p x k x k = 45 x 44 x 44 = Since n = 23 does not divide 1000 evenly, we actually have 12 extra inputs and outputs that we could switch with this configuration ( 23x44=1012 and = 12). Thus the total number of crosspoints = = best case for a non-blocking switch as compared with the: 1,000,000 for the complete crossbar and about 190,000 for n = 10. This is a factor of over 11 less equipment needed to switch 1000 customers!

Benes Architecture 2  2 N/2  N/2 Benes N NxN switch (N is power of 2) RNB built recursively from Clos network: 1st step Clos(2, N/2, 2, N/2, 2) Rearrangably non-blocking

Benes Architecture - contd.
Number of stages = 2.log2N - 1 Number of 2x2 switches /each stage = N/2 Total number of crosspoints ~N.(log2N -1)/2 For large N, total number of crosspoint = N.log2N Benes network is RNB (not SNB) and so may need re-routing: Modular switch design Multicast switches can be built in a modular fashion by including a copy module in front of the point-to-point switch

Benes Architecture - contd.
1 2 3 4 5 6 7 8 X e.g. Connection sequence 4 to 2 Fails 2 to 1 1 to 5 3 to 3 Note there is no way 4 to 2 connection could be made

Benes Architecture –Non-blocking contd.
Now use different connections e.g. 4 to 2 OK 2 to 1 1 to 5 3 to 3

Three Building Blocks for OXC
International Engineering Consortium, 2004

Optical Switches - Tow-Position Switch
Input port Ii Output ports I1 I2 Control Signal The input signal can be switched to either of the output ports without having any access to the information carried by the input optical signal In the ideal case, the switching must be fast and low-loss. 100% of the light should be passed to one port and 0% to the other port.

Two Position Switch - contd.
The two-position switch requires three fibres with collimating lenses and a prism. B A C Lens Fibre Prisem Light arriving at port A needs to be switched to port C. B A C

Optical Switches - Applications
Provisioning: Used inside optical cross connects to reconfigure them and set-up new path. [ msecs] Protection Switching: To switch traffic from a primary fibre onto another fibre in the case of a failure. [1 to 10 usecs] Packet Switching: 53 byte 10 Gb/s. [1 nsecs] External Modulation: To switch on-off a laser source at a very high speed. [10 psecs << bit duration] Network performance monitoring Reconfiguration and restoration: Fibre networks

Optical Switching - Technologies
Slow Switches (msecs) Free space Mechanical Solid state Fast Switches (nsecs) LiNbO Non-linear InP

Optical Switches - Criteria
Maximum Throughput: Total number of bits/sec switched through. To increase throughput: Increase the number of I/O ports Bit rate of each line Maximum Switching Speed Important: Packet switched Time division multiplexed Minimum Number of Crosspoints As the size of the switch increases, so does the number of crosspoints, thus high cost Multistage switching architecture are used to reduce the number of crosspoints.

Criteria - contd. Minimum Blocking Probability: Important in circuit switching External blocking: when the incoming call request an output port that is blocked. Subject to external traffic conditions Internal blocking: when no input port is available. Subject to the switch design Minimum Delay and Loss Probability Important in packet switching, where buffering is used, which will introduce additional delay. Scalability Replacing an old switch with a new larger switch is costly. Incrementally increasing the size of the existing switching as traffice grows is desirable Broadcasting and Multicasting To provide conferencing and multimedia applications

Criteria - contd. Optical switches with low insertion loss and low crosstalk are needed in broadband optical networks Restoration Reprovisioning Bandwidth on demand Conventional optical switches cannot satisfy all the requirements: Solid-state guided-wave switches (electro-optic, thermo-optic, SOA): limited expandability due to high crosstalk, loss, and power consumption Optomechanical switches: excellent insertion loss and crosstalk, but are bulky, expensive, and suffer from poor reliability and scalability

Optical Switches - Types
Waveguide Electro-optic effect Semiconductor optical amplifier LiNbO - InP Thermo-optic effect - SiO2 / Si - Polymer Free Space - Liquid crystal - Mechanical / fibre - Micro-optics (MEM’s) - Fast - Complex - Maturing - Lossy - Slow - Maturity - Reliable - Slow - Low loss & crosstalk - Inherently scalable

Optical Switches - Thermo-Optic Effect
Some materials have strong thermo-optics effect that could be used to guide light in a waveguide. The thermo-optic coefficient is: Silica glass dn/dt = 1 x 10-5 K-1 Polymer dn/dt = -1 x 10-5 K-1 Difference thermo-optic effect results in different switch design. + v Electrodes

Thermo-Optic Switch - Silica
Input Ii I1 I2 Outputs Mach – Zehnder Configuration Heater Analogue Directional coupler

Thermo-Optic Switch - Polymer
Ii I1 I2 PH1 PH2 Y – Junction Configuration Digital If PH1 = PH2 = 0, then I1 = I2 = Ii /2 If PH1 = Pon & PH2 = 0, then I1 = 0, and I2 = Ii If PH1 = 0 & PH2 = Pon, then I1 = Ii, and I2 = 0

Thermo-Optic Switch - Characteristics
15 S/W power (W) ~4 ~ S/W time (ms) 13 Crosstalk 18 Insertion Loss (dB) 256 No. of S/W 16 x 16 Si 8 x 8 Si Poly. 2 x 2 Si Poly. Switch Size Parameters

Mechanical Switches 1st Generation – Mid. 1980’s
Loss Low (0.2 – 0.3 dB) Speed slow (msecs) Size Large Reliability Has moving part Applications: - Instrumentation - Telecom (a few) Size: 8 X 8 Loss: 3 dB Crosstalk: 55 dB Switching time: 10 msecs

Micro Electro Mechanical Switches
Combines optomechanical structures, microactuators, and micro-optical elements on the same substrate Input fibres Output fibres Made using micro-machining Free-space: polarisation independent Independent of: Bit-rate Wavelength Protocol Speed: 1 10 ms 4 x 4 Cross point switch Lens Flat mirror Raised mirror

This tiny electronically tiltable mirror is a building block in devices such as all-optical cross-connects and new types of computer data projectors. I/O Fibers Imaging Lenses Reflector MEMS 2-axis Tilt Mirrors Lightwave

Monolithic integration --> Compact, lightweight, scalable Batch fabrication > Low cost Share the advantages of optomechanical switches without their adverse effects General Characteristics: Low insertion loss (~ 1 dB) Small crosstalk (< - 60 dB) Passive optical switch (independent of wavelength, bit rate, modulation format) No standby power Rugged Scalable to large-scale optical crossconnect switches Moderate speed ( switch time from 100 nsec to 10 msec)

Large Optical Switches - Optical Cross Connects
Switch sizes > 2 X 2 can be implemented by means of cascading small switches. Used in all network control Bit rate at which it functions depends on the applications. 2.5 Gb/s are currently available Different sizes are available, but not up to thousands (at the moment) 1 2 N N X N Cross Connect Control

Optical Cross Connects

Optical Switches Electrical switching and optical cabling: inputs come
from different clock domains resulting in a switch that is generally timing-transparent. Optical switching and optical cabling, clocking and synchronization are not significant issues because the streams are independent. Inputs come from different clock domains, so the switch is completely timing-transparent.

Optical Switches - System Considerations
For a given switch size N, the number of 2x2 switches should be as small as possible. When the number is large it will result in: high cost large optical power loss and crosstalk. A switch with reduced number of crosspoints in each configured path, can have a large internal blocking probability In some switching architectures, the internal blocking probability can be reduced to zero by: using a good switching control or rearranging the current switch configuration

Optical Routers In the core large optical-switching elements have already started to appear to handle optical circuits, Large, centralized IP routers are also appearing, because they're an extremely efficient solution to IP routing. There are a variety of technologies and issues that influence the architecture for these types of network elements. To transport Tbps, new optical technologies have emerged to enable the economic transport of incredible bandwidth over single-mode optical fibrer, including DWDM and OTDM. That means individual optical links can sustain the enormous traffic needed to support the continuing growth of IP data.

Optical Routers High-power, low-noise optical amplifiers-or erbium-doped fiber amplifiers (EDFAs)-and pulse-shaping technologies mean the high-bit-rate optical signals do not require electronic regeneration except on the very longest fiber spans. New fibres with larger cross-sectional areas mean a large number of high-bit-rate signals can be wavelength-multiplexed onto a single fiber. Thus, it is becoming affordable to actually construct links that can support Tbps of capacity between routing and switching centres.

Network Problems - Scalability
The bottleneck at the core of the expanding network is at the junction points of the fibre bundles: I.e the switching and routing centres. With Tbps links, a huge amount of data converges into a single central office (CO) (see Figure 1). New routers emerge only to be swamped with traffic within months.

Network Problems - Scalability
Solution: Use of cluster of several routers (or crossconnects). However, clustering is not a good long-term solution, because: a cluster of crossconnects requires interconnecting links between the crossconnects. As the number of switches in the cluster grows beyond about 4 or 5, the interconnecting links consume most of the ports. Clustered routers have the same problem. the IP traffic must transit more and more devices, and the latency (the delay of IP packets) and jitter (delay variance) of the cluster grow quickly. the hot-spot problem, where one of the small routers in a cluster can be overwhelmed by temporary traffic dynamics in the network that do not exceed the combined node capacity. This swamping effect also increases the delay of that saturated small router.

Large, Centralized Router
Current trend in XCs is to use large micro-electromechanical systems (MEMS)-based OXCs for core node protection and grooming of DWDM traffic. Similarly, large centralized routers are an efficient alternative to solving bottleneck problems: by avoiding the hot-spot problems of distributed routers, eliminating clustering problems, and permitting global scheduling. A centralized (single-hop), synchronous, large non-blocking switch fabric has the best latency and throughput performance of all router topologies. It also scales better than a clustered system-and it results in less complicated system software for the network element.

IP Routers + Optical Network Elements
ONE Optical Network End Customer A V Lehmen, Telecordia Tech.

Optical Layer Capability: Reconfigurability
IP Router IP Router IP Router IP Router OXC - A OXC - C OXC - B IP Router OXC - D Crossconnects are reconfigurable: Can provide restoration capability Provide connectivity between any two routers A V Lehmen, Telecordia Tech.

Architecture 1: Large Routers + High capacity Fibres
Z Access lines All traffic flows through routers Optics just transports the data from one point to another IP layer can handle restoration Network is ‘simple’ But….. - more hops translates into more packet delays - each router has to deal with thru traffic as well as terminating traffic A V Lehmen, Telecordia Tech.

Architecture 2: Small Routers + OXC
Router interconnectivity through OXC’s Only terminating traffic goes through routers Thru traffic carried on optical ‘bypass’ Restoration can be done at the optical layer Network can handle other types of traffic as well But: network has more NE’s, and is more complicated A V Lehmen, Telecordia Tech.

Dynamic Set-Up of Optical Connection
IP Router IP Router IP Router IP Router 1. Router requests a new optical connection 3. Path set-up message propagates through network 4. Connection is established and routers are notified OXC - A OXC - C OXC - B A V Lehmen, Telecordia Tech. 2. OXC A makes admission and routing decisions

OXC – Router-Selector Architecture
1 N Type I - 1 x N & N x 1 optical switches Type II - 1 x N passive optical splitter - N x 1 Optical switches

Strictly non-blocking
OXC – Router - Feature log2N(3+Lse)+2Lfs (2Nlog2N)Lse+4Lfs Switch Loss XT-10log10(log2N) 2XT-10log10(log2N) SNR Nlog2N 2Nlog2N Switch Drive N(N-1) 2N(N-1) Switch Element Strictly non-blocking Architecture TypeII Type I Where XT; Crosstalk (dB), Lse; Loss/switch element Lfs; Fibre-switch loss

OXC + Wavelength Converters

Optical Switches: - A comparison
Characteristic Traditional Optical Switches Next Generation Optical Switches Switching Speed >1ms <1µsec Multicast Not available Dynamic power partition between ports Integrated VOA functionality High dynamic range VOA Reliability ~10 Million cycles (Mech.dev.) ~10 Billion cycles (Opto-elect.) Insertion loss Low Cross talk High Scalability Medium-High

Optical Gateway Cross-Connect
Performs digital grooming, traditional multiplexing, and routing of lower-speed circuits in mesh or ring network configurations. Specifically, it brings in lower rate SONET/SDH layer OC-3/STM-1, OC-12/STM-4 and OC-48/STM-16 rates and electrical DS-3, STS-1 and STM-1e rates and grooms them into higher rate optical signals. Alcatel. 2001

IP-router with Tb/s throughput can be built with
fast tunable lasers & NxN optical mux Buffer From Input Port Output T-Tx 40 G mod 40 G mod 40 G mod 40G Rx retiming T-Tx 40G Rx Sche- duler T-Tx 40G Rx T-Tx 40G Rx Clock Yamada et al., 1998

Router & Optical Switch
CHIARO- OptIPuter Optical Switch Workshop

The Optical Future- Tomorrow's Architecture
Services are consolidated onto a single access line at the user site and fed into a Sonet multi-service provisioning platform at the carrier’s POP (point of presence). Several POPs feed traffic into a terabit switch capable of handling all traffic—including IP, ATM and TDM. The terabit switches sit at the edge of a three-tier network of optical switches—local, regional and long distance-each of which has a mesh topology. DWDM is used throughout the network and access lines. Where fiber is scarce, FDM (frequency division multiplexing) is used to pack as much traffic as possible into wavelengths. Light signals no longer need regeneration on long distance routes.

Separate access networks carry telephony and data into the carrier’s point of presence. Voice traffic runs over a TDM (time division multiplexer) network running over a Sonet (synchronous optical network) backbone. IP traffic is shunted onto an ATM backbone running over other Sonet channels. The Sonet backbone comprises three tiers of rings at the local, regional and national level, interconnected by add-drop multiplexers and cross-connects. DWDM (dense wave division multiplexing) is in use in the regional and national rings, but not the local rings. Light signals need regenerating on long distance routes.

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