HYBRID OPTICAL WIRELESS ACCESS NETWORKS
A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
Wei-Tao Shaw March 2009
UMI Number: 3351490 Copyright 2009 by Shaw, Wei-Tao
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© Copyright by Wei-Tao Shaw 2009 All Rights Reserved
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
/40*&p&. (Leonid G. Kazovsky)
Principal Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
(Donald C. Cox)
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
(David A. B. Miller)
Approved for the University Committee on Graduate Studies.
^k./. A£ in
Abstract Next generation access network will require flexible deployment, large backbone capacity, upgrade ability, scalable to user number and demand, and economically feasible. One example is to provide ubiquitous, blanketed broadband access service in metropolitan area. Such requirements are instrinsically impossible to meet if the network is designed with any single access technology. On the other hand, a hybrid optical and wireless access network would combine high optical capacity and flexible wireless deployment that is economic and scalable. This dissertation focuses on hybrid optical and wireless access networks that consists of wireless mesh network based on multi-hop wireless communications and novel optical backhaul networks. Two novel optical backhaul networks are proposed, analyzed, and experimentally evaluated. The first optical backhaul network is a reconfigurable architecture based on the Time Division Multiplexing Passive Optical Networks. The reconfigurable architecture can optimize the network efficiency and performance by dynamic bandwidth allocation.
An experimental testbed is built
to demonstrate the feasibility for realisitc application. The second optical backhaul network is based on a grid structure to provide broadband, scalable, blanket-cover, and cost-effective connectivity. Advanced optical devices are employed to achieve centralized control, bandwidth scalability, resource sharing, statistical multiplexing gain, a n d ease of deployement and m a n a g e m e n t . Experimental t e s t b e d is built for
performance evaluation and demonstration of QoS capability. An integrated routing paradigm is developed to enahnce the wireless mesh network performance by leveraging the optical backhaul. Finally, a novel feedback-based burst-mode clock and data recovery architecture is proposed for both optical backhaul networks. It can rapidly
IV
recover clock for data sampling and provide jitter tolerance that feed-forward CDR circuits cannot achieve.
Acknowledgement I am grateful for working with many wonderful people at Stanford. These people made these years one of the most memorable times in my life. First of all, I want to thank my advisor Professor Leonid Kazovsky for many invaluable advice and constant support during my study in the Photonics and Networking Research Laboratory (PNRL). I appreciate the research environment he created that allowed me to explore various interesting research topics. Under his advice throughout the years, I not only learned how to solve problems, but also built the vision to future research directions. I would also like to thank my associated advisor, Professor Donald Cox, from whom I took one of my first classes in Stanford. He is a very knowledgeable and nice teacher in class. Later we worked together on a joint research project, Grid Reconfigurable Optical and Wireless Network, and he gave me many valuable advices and came up with good ideas. It is indeed my pleasure to work with him. Another person I would like to thank is Professor David Miller. He is a very kind and patient Professor when students need his help. I closely followed his research, and it helped my research directly or indirectly. The project sponsors are imperative to my resrach and myself. I am so grateful for the Industrial Technology Research Institute (ITRI) in Taiwan and the National Science Foundation (NSF). I also thank my past and current group mates in PNRL. I received uncountable help from each of them in different aspects of my life. Without them, my time in the office and laboratory would not be as enjoyable and memorable. Finally I want to thank my family, especially my uncle Jean, who is the most important person in my life. Since I was a kid, he has influenced me deeply in many aspects. His never changing love, care, patience, and support made me what I am
VI
and who I am. Although he has left me, I will always remember him with love. I am also so grateful for my dear wife, Minny. She and her love made me complete.
Contents Abstract
iv
Acknowledgement
vi
1
2
Introduction
1
1.1
Communication Network Hierarchy
2
1.2
Bottleneck: Access Networks
5
1.3
Brief Introduction to Access Technologies
6
1.3.1
Digital Subscriber Line
6
1.3.2
Hybrid Fiber Coax
7
1.3.3
Passive Optical Networks
8
1.3.4
Wireless Access Technologies
8
1.4
Convergence of Optical and Wireless Access
10
1.5
Dissertation Outline
11
Enabling Optical and Wireless Access Technologies
12
2.1
Passive Optical Networks
12
2.1.1
TDM PON
13
2:1.2
WDM PON
16
2.1.3
PON Deployment Method and Cost
18
2.1.4
Summary of Optical Access Technologies
19
2.2
Enabling Wireless Access Technologies
19
2.2.1
Multi-Hop Wireless Communication Network
19
2.2.2
Wireless Mesh Networks
21 vm
2.2.3
Client Wireless Mesh Networks
22
2.2.4
Infrastructure Wireless Mesh Networks
23
2.2.5
PHY and MAC Layers of WMN
25
2.2.6
Routing Algorithms of Wireless Multi-Hop Networks
26
2.2.7
Capacity Scalability of WMN
28
2.2.8
Backhaul Solutions for WMN
34
2.2.9
Candidate Solutions for Broadband Backhaul in WMN . . . .
36
2.2.10 Summary of Wireless Access Technologies 3
Hybrid Architecture Enabling S m o o t h Wireless Access Upgrade
41
3.1
Introduction
41
3.2
Network Architecture
42
3.2.1
44
3.3 3.4
4
40
Upgrading Path
Reconfigurability of the Proposed Optical Backhaul
49
3.3.1
51
Performance simulation of the reconfigurable optical backhaul
Experimental testbed of reconfigurable optical backhaul
56
3.4.1
Handshaking protocol for network reconfiguration
56
3.4.2
Enabling Devices
60
3.4.3
Experimental Results
68
Next-Generation Hybrid Access Network
71
4.1
Introduction
71
4.2
Requirements of Next-Generation Optical Backhaul Network
72
4.3
A Novel Optical Grid Backhaul
73
4.4
Optical Grid Unit
75
4.4.1
Node Structures of the Central Hub
78
4.4.2
Node Structure of the Optical Terminal
80
4.5
Multiplexing of Tunable Lasers
85
4.5.1
Statistical Multiplexing Gain of Tunable Lasers
86
4.5.2
Simulations of Packet Delay and Jitter Improvement due to Statistical Multiplexing Gain
4.6
Optimization between Bandwidth Scalability and Cost-efficiency . . . IX
88 91
4.7
4.8
5
98
4.7.1
Enabling devices
99
4.7.2
Downstream Transmission Experiment
103
4.7.3
Upstream transmission experiment
105
4.7.4
QoS Experiments of the Optical Backhaul
110
Integrated Routing Algorithm for Optical Grid Unit
115
4.8.1
Integrated Routing Algorithm
116
4.8.2
Algorithm Simulation
120
B u r s t - M o d e Clock and D a t a Recovery Technique
126
5.1
Introduction
126
5.2
Clock and Data recovery in conventional optical communication systems 128
5.3
Review of Up-to-date Burst-mode CDR techniques
131
5.3.1
Digital ring oscillator
131
5.3.2
Instantaneously locked clock and data recovery circuit
136
5.3.3
Nonlinear clock extraction circuit
139
5.3.4
Summary among the three burst-mode CDR techniques . . . .
145
5.4
6
System testbed of the optical grid unit
Dual-loop burst-mode CDR technique
146
5.4.1
Simplified Delay lock loop (DLL)
147
5.4.2
Charge Pump DLL (CPDLL)
151
5.4.3
Charge pump phase lock loop (CPPLL)
163
5.4.4
The third-order CPPLL
170
5.4.5
Simulation of dual-loop CDR structure
171
Conclusion
179
Bibliography
182
x
List of Tables 1.1
Multimedia applications and their bandwidth requirements
2
1.2
Comparison of bandwidth and reach for popular access technologies .
9
2.1
Cost of fiber deployment for PON
19
2.2
Summary of I E E E 8 0 2 . i l standards
25
5.1
Summary of three CDR techniques
145
XI
List of Figures 1.1
Modern Communication Network Hierarchy
3
1.2
Synergy of Optical and Wireless to Construct Access Network . . . .
10
2.1
Passive optical networks
13
2.2
TDM PON
14
2.3
WDM PON
18
2.4
IEEE 802.16 WiMAX: Single-Hop wireless Communication Network .
20
2.5
IEEE 802.16J: Mobile Multiple Relay WiMAX Network
20
2.6
Generic Wireless Mesh Network (WMN) . . .
22
2.7
Client Wireless Mesh Network
23
2.8
Infrastructure Wireless Mesh Network
24
2.9
Two Categories of Routing Algorithms in Wireless Ad-Hoc Networks
26
2.10 Scalibility in one-dimensional exemplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: Aggregated data rate, CMR-MR'
Link capacity between adjacent MRs, CMR-user'- Link ca-
pacity between MR and users, DMR- Total user demand within area served by a MR
28
2.10 Scalibility in one-dimensional examplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: CMR-MR'-
Aggregated data rate,
Link capacity between adjacent MRs, CMR-user' Link ca-
pacity between MR and users, DMR' Total user demand within area served by a MR (con't)
29
xn
2.10 Scalibility in one-dimensional exemplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: CMR-MR'-
Aggregated data rate,
Link capacity between adjacent MRs, CMR-user'- Link ca-
pacity between MR and users, DMR'- Total user demand within area served by a MR (con't)
30
2.10 Scalibility in one-dimensional examplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: Aggregated data rate, CMR-MR-
Link capacity between adjacent MRs, CMR-user' Link ca-
pacity between MR and users, DMR- Total user demand within area served by a MR (con't)
31
2.10 Scalibility in one-dimensional exemplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: Aggregated data rate, CMR-MR-
Link capacity between adjacent MRs, CMR-user- Link ca-
pacity between MR and users, DMR:
Total user demand within area
served by a MR
31
2.11 1-Dimensional WMN
32
2.12 Throughput per router and overall capacity of one-dimentional WMN
33
2.12 Throughput per router and overall capacity of one-dimentional WMN
34
2.12 Throughput per router and overall capacity of one-dimentional WMN
35
2.12 Throughput per router and overall capacity of one-dimentional WMN
36
2.13 Hierarchical Wireless Access Network (proposed by Google-Earthlink to San Francisco City)
37
2.14 Fundamental limit of transmission distance of copper wire technologies
38
2.15 Comparison of broadband backhaul technology canidate for WMN.
39
3.1
.
Hybrid optical wireless access network as proposed by Google and Earthlink for San Francisco Metro Wireless Networks Project
3.2
Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network
3.2
43 45
Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't) .
xiii
46
3.2
Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't) .
3.2
Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't) .
3.2
46 47
Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network
48
3.3
Reconfigurable Optical Backhaul
49
3.4
System architecture of the central office to facilitate the reconfigurable optical backhaul
51
3.5
Traffic throughput at P I in figure 3-4
53
3.6
Buffer depth and packet rejection ratio of PON1 in both architectures
54
3.7
Throughput of each PON and combined throughput of both architectures; measured at P2 in
3.8 3.9
figure3-4
55
Long-term average packet delay of both PONs (varying load for PON1 and fixed load for PON2)
57
Experimental testbed of reconfigurable optical backhaul
58
3.10 Timing diagram of the reconfiguration protocol
59
3.11 State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU
61
3.11 State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
61
3.11 State diagrams of the RGI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
62
3.11 State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
62
3.11 State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU
63
3.12 Optical tunable receiver used on the reconfigurable optical backhaul experimental testbed
64
3.13 Transient response of the optical tunable receiver in figure3-12 . . . .
65
3.14 FPGA and 1.25Gbps SerDes board
66
xiv
figure3-ll
3.15 Functionalities implemented on the two FPGA boards
67
3.16 Control packet format
69
3.17 Experimental result of the reconfiguration protocol
70
4.1
GROW-Net architecture
74
4.1
GROW-Net architecture (con't)
74
4.1
GROW-Net architecture (con't)
76
4.2
Optical grid unit structure
77
4.3
Node structure of central hub
78
4.4
Tunable optical transmitter at the central office
79
4.5
DWDM optical filter used in optical terminal
81
4.6
Wavelength duplexer implemented by DWDM filter
82
4.7
RSOA structure and upstream transmission
83
4.8
Incremental bandwidth scalability
85
4.9
M / M / l queue model of tunable optical transmitter
86
4.10 Packet delay and jitter improvement due to statistical multiplexing gain 89 4.10 Packet delay and jitter improvement due to statistical multiplexing gain (con't)
90
4.11 Bandwidth scalability of optical grid unit
92
4.11 Bandwidth scalability of optical grid unit (con't)
94
4.11 Bandwidth scalability of optical grid unit (con't)
95
4.12 Optical loss along the longest path
96
4.13 Further scale-up by splitting the optical grid unit
97
4.14 System testbed of the optical grid unit
98
4.15 Fast tunable laser
100
4.15 Fast tunable laser (con't)
101
4.15 Fast tunable laser (con't)
102
4.16 RSOA module employed on the system testbed
103
4.17 Downstream transmission experiment
104
4.17 Downstream Transmission Experiment (con't)
104
4.17 Downstream Transmission Experiment (con't)
105
xv
4.17 Downstream Transmission Experiment (con't)
106
4.18 Upstream transmission experiment
107
4.18 Upstream transmission experiment (con't)
108
4.18 Upstream transmission experiment (con't)
109
4.19 QoS implementation and experiment results on the system testbed . .
Ill
4.19 QoS implementation and experiment results on the system testbed (con't) 113 4.19 QoS implementation and experiment results on the system testbed (con't) 113 4.19 QoS implementation and experiment results on the system testbed (con't) 114 4.19 QoS implementation and experiment results on the system testbed (con't) 114 4.20 Appropriate optical link allocation to improve WMN performance . .
116
4.21 System operation of proposed integrated routing algorithm
117
4.22 Bounded flooding of link state information
118
4.23 Simulation scenario
121
4.24 Normalized average throughput distributions
122
4.24 Normalized average throughput distributions (con't)
123
4.25 Average throughput and packet delay comparison
124
4.25 Average throughput and packet delay comparison (con't)
125
5.1
Burst-mode transmission on optical grid unit and burst-mode receiver
127
5.2
Burst-mode clock and data recovery
128
5.3
PLL CDR
128
5.4
LPF responses and eye diagram at different loop bandwidth
130
5.5
Digital ring oscillator
132
5.6
Two feedback loops in DR_OSC CDR
132
5.7
A single pulse applied to the DR-OSC CDR
134
5.8
Pulse train applied to the DR-OSC CDR
134
5.9
Simulation of DR-OSC CDR
135
5.9
Simulation of DR-OSC CDR (con't)
135
5.10 Instantaneous clock and data recovery circuit
136
5.11 Gated Voltage Controlled Oscillator
137
5.12 The outputs of GVCO's and resultant clock
138
xvi
5.13 Nonlinear clock extraction circuit
139
5.14 Frequency spectrum of random non return to zero (NRZ) signal . . .
140
5.15 The math model of the nonlinear clock recovery circuit
140
5.16 Input and output waveforms and corresponding average frequency spectrum
142
5.17 Waveforms of NRZ burst input and recovered clock
143
5.18 Impact of BPF bandwidth on the recovered clock quality
144
5.19 Dual-loop burst-mode CDR circuit architecture
146
5.20 Delay lock loop (DLL)
147
5.21 Simulation results of control voltage of the VCDL with different LPF bandwidth
150
5.22 VCDL output clock waveforms with different loop gains
151
5.23 Charge pump delay lock loop (CPDLL)
152
5.24 Hogge phase detector
153
5.25 Waveform of the nodes in Hogge PD when the incoming data and the clock are aligned
154
5.26 Waveform of the nodes in Hogge PD when the clock leads the incoming data for 0.02ns (bit period is 0.1ns, or lOGbps) 5.27 The control voltages of the DLLs with different loop gains
155 157
5.28 Phase relationships between input data and DLL output clock at different points of the burst
158
5.29 Control voltage of unequal frequency situations
159
5.30 Phase tracking error of VCDL output clock in the presence of clock frequency deviation
161
5.31 VCDL output clock rising edge variation during the random payload
162
5.32 Charge pump phase lock loop (CPPLL)
164
5.33 Root locus plot of 2nd-order CPPLL
165
5.34 VCO control voltage (yctri) of 2nd-order under-damping CPPLL . . .
166
5.35 VCO control voltage (Vctr.j) of 2nd-order over-damping CPPLL . . . .
168
xvn
5.36 VCO control voltage (yctri) responses to (1) input data consisting of alternating logic zeros and ones and (2) the real packet consisting of preamble and random payload
169
5.37 Third-order CPPLL
170
5.38 Locking dynamics comparison between 2nd-order and 3rd-order CPPLLsl71 5.39 Dual-loop CDR architecture
172
5.40 The VCDL control voltages under different conditions
174
5.41 Input data and clock phase relationship at different moments of time
176
5.42 The response of VCO control voltage
177
5.43 Jitter tolerance masks of dual-loop CDR structure and typical CPPLL 178
xvm
Chapter 1 Introduction Since the first deployment of the Advanced Research Projects Agency Network [1] in 1969, communication networks have dramatically changed how people live, work, and interact. Historically, communication networks transport three types of services: voice, video, and data, together referred to as triple play. Conventional voice traffic is a two-way, point-to-point, continuous 3.4 kHz, analog signal with a very stringent delay requirement. The standard TV signal is a broadcasting, point-to-multipoint, continuous 6 MHz analog signal. Data traffic is typically bursty with varying bandwidth and delay requirements. Since the traffic characteristics of voice, data and video services and their requirements for quality of service (QoS) were fundamentally different, three major types of networks were built to render these services separately in a cost-effective manner: the public switched telephone networks (PSTN) for voice conversation, the hybrid fiber coax (HFC) networks for video distribution, and the Internet for data transfer. Due to advances in digital communication technology, voice and video signals have been digitized to accommodate different transport platforms. Emerging multimedia applications such as video-on-demand, e-learning, interactive gaming, etc. are often packaged and transported with voice, data and video services. Fueled by advances in computer technology, these applications are growing in size and demanding more and more bandwidth for transport. Tablel-1 lists common end user applications and their bandwidth requirements. As demand continues to grow, the required bandwidth may 1
CHAPTER
1.
2
INTRODUCTION
Table 1.1: Multimedia applications and their bandwidth requirements. Application Bandwidth Latency Other Requirements Voice over IP (VoIP)
64 kb/s
200 ms
Protection
Video conference File sharing
2Mb/s
200 ms
Protection
3Mb/s
1s
SDTV
4.5 Mb/s/ch
10 s
Interactive game
5Mb/s
200 ms
Telemedicine
8Mb/s
Protection
Real time video
10 Mb/s
50 ms 200 ms
Video on demand HDTV
10 Mb/s/ch 10 Mb/s/ch
10 s 10 s
Low packet loss Multicasting
Network-hosted software
25 Mb/s
200 ms
security
Multicasting
Content distribution
even exceed 50 Mbps in the foreseeable future. Breakneck increases in bandwidth demand spurred rapid evolution of communication technologies and fierce competition between service providers. Driven by user demands and stiff competition, service providers began to integrate these services and applications together and strived to build new communication infrastructure to meet the new challenges. As a result, we have witnessed rapid development of communication infrastructure around the world and the explosive growth of the Internet within the past decade. The sum of today's communication network is an exceptionally complicated system, covering the entire globe. Such an intricate system is built and managed on a hierarchical structure, which consists of four network layers. These network layers cooperate to achieve the ultimate goal: anyone, anywhere, anytime and any media communications.
1.1
Communication Network Hierarchy
The hierarchical structure of today's communication network consists of local area, access, metropolitan area and wide area networks as illustrated in Figure 1-1. Local area networks (LANs) connect computers and other electronic devices
CHAPTER 1.
3
INTRODUCTION
Backbone Networks Continent-to-Continent Coast-to-Coast Typical Distance > 1000km 10Gbps~Tbps
London
-
TECHNOLOGIES29
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i D,
l* ^%.A» r t ; U f«. At Dm>ired ,.--'' \ .
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Figure 2.10: Scalibility in one-dimensional Router, GR: Wireless Gateway Router, Dagg: capacity between adjacent MRs, CMR-userDMR- Total user demand within area served CMR-MR)
examplar WMN. MR: Wireless Mesh Aggregated data rate, CMR-MR' Link Link capacity between MR and users, by a MR (con't)
are independent. The gateway router (GR), which has backhaul connection
to the Internet, is the outlet of the entire system, where we assume that there is only upstream traffic. As traffic load from end users increases under each mesh router, the link capacity between the mesh router and end users may be exhausted as indicated in figure 210(b). To reduce the load on each mesh router, we can deploy more mesh routers in a given area with lower transmission power as in figure 2-10(c), which is analogous to cell splitting in cellular systems. Note that after cell splitting, the pipe between the gateway router and its adjacent mesh router needs to conduct the aggregated flows (Dagg) from more mesh routers; in this exemple the number increases from three (figure 2-10(a)) to five (figure 2-10(c)). Also note that the pipe connecting the routers may become thinner after cell splitting because of the increased co-channel interference from other routers. Since the link connecting the gateway router and its adjacent mesh router needs to aggregate traffic based on a larger amount of mesh routers, the aggregated load {Dagg) may soon exhaust the pipe capacity connecting the gateway router and its adjacent mesh router, as shown in figure 2-10(d). The bandwidth insufficiency of this link will choke
CHAPTER 2. ENABLING
OPTICAL AND WIRELESS ACCESS
Wired Backhaul
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Figure 2.10: Scalibility in one-dimensional Router, GR: Wireless Gateway Router, Dagg: capacity between adjacent MRs, CMR-User' DMR- Total user demand within area served
exemplar WMN. MR: Wireless Mesh Aggregated data rate, CMR-MR'- Link Link capacity between MR and users, by a MR (con't)
each mesh router's throughput and the overall network capacity of the WMN. In other words, after cell splitting, although each router's average load is reduced, the throughput of each router and overall capacity of the WMN may not necessarily increase. To improve the performance, a straightforward solution is to upgrade the link capacity by exploiting more frequency channels and radio interfaces on the routers [12], i.e. widening the pipes of the model. However, since the number of available channels in any wireless system is limited and the right of using the licensed bands is very expensive, this approach is limited by either the finite capacity or cost. Instead of resorting to upgrade the capacity of wireless links, the throughput per router and overall capacity can be enhanced by scaling the number of gateway routers accordingly with that of the mesh routers. This can be accomplished by replacing the middle mesh router with another gateway router as illustrated in figure 2-10(e). After placing this gateway router, traffic flow from a few mesh routers can be routed to the new gateway router and thus the bottleneck at the original gateway router link is mitigated. After qualitative insight into the scalability issues of WMN and the proposed solution, let's analyze the maximum throughput per router and the overall capacity
CHAPTER 2. ENABLING
OPTICAL AND WIRELESS ACCESS
TECHNOLOGIES?,!
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Figure 2.10: Scalibility in one-dimensional examplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: Aggregated data rate, CMR-MR- Link capacity between adjacent MRs, CMR-User'- Link capacity between MR and users, DMR- Total user demand within area served by a MR (con't)
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Figure 2.10: Scalibility in one-dimensional exemplar WMN. MR: Wireless Mesh Router, GR: Wireless Gateway Router, Dagg: Aggregated data rate, CMR-MR- Link capacity between adjacent MRs, CMR-User'- Link capacity between MR and users, DMR'- Total user demand within area served by a MR
CHAPTER 2. ENABLING
OPTICAL AND WIRELESS ACCESS
TECHNOLOGIES^
of a one-dimensional WMN. In this analysis, we first assume that there is only one gateway router at the end of the one-dimensional WMN as in figure 2-11, and the link speed of the higher layer of links among the mesh and gateway routers is 54Mbps and does not interfere with the links of the lower layer. We further assume that the distance between any two adjacent mesh routers of the WMN is a constant A, as shown in figure 2-11, and all the routers are identical in transmission power, receiver sensitivity, and type of antenna (omni-directional). •"• Interference Range
- - Transmission Range
>.
. Rr.ntr-
IWPP MAC
PHY A*" -ft P" - f i
llllllsr — g _ r
........
W End user
Figure 3.1: Hybrid optical wireless access network as proposed by Google and Earthlink for San Francisco Metro Wireless Networks Project. Under this hybrid optical-wireless architecture, the upstream traffic is first received by a nearby wireless mesh router via the links of the lower layer and then relayed to
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one of the nearby wireless gateway routers through multi-hop communications. In figure 3-1, mesh router 4 aggregates traffic of nearby end users, and relays it over router 3, 2, and 1 to reach gateway router A. Once traffic reaches the gateway router, it is forwarded toward the central office over the optical backhaul. For downstream traffic, packets are first routed to one of the gateway routers, such as B in figure 3-1 and forwarded on specific route, e.g. through router 5, 3, and 4, to the end user. Since the optical backhaul and WMN are implemented with different technologies, such as Ethernet PON (EPON) and Wi-Fi, interoperability is needed at the interface between the ONU and wireless gateway router. To address this issue, the optical backhaul and WMN can either be fused at the networking layer using an IP router, or employ application-specific integrated circuits (ASIC) designed to translate the packet formats. The integration of the point-to-multipoint optical backhaul and WMN paves multiple routes between the central hub and the end user. Therefore an integrated routing paradigm that can dynamically choose the optimum route is essential for a hybrid optical-wireless network. Although routing in WMNs by itself is a challenging issue, as described in previous sections, we envision that the optical backhaul can help to collect some network conditions in WMNs, such as link status, traffic load, interference, etc, and help to determine the optimum route. [39] proposed an integrated routing algorithm to achieve load balancing as congestion occurs in the wireless mesh network. This integrated routing algorithm will be discussed in chapter 5.
3.2.1
Upgrading Path
The proposed hybrid architecture is designed to smoothly upgrade a hierarchical wireless access network in figure 2-13(a) and (b). The upgrading path proposed in [38] and [39] begins with the replacement of the wireless links at the backhaul layer in figure 2-13(b). The top layer aggregates traffic from the lower layers. Consequently this layer will become the first bandwidth bottleneck. Figure 3-2(a) shows the upstream wireless segments of the ring network (figure 213(b)), which are closer to the central office. These wireless links aggregate traffic from
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CHAPTER 3. HYBRID ARCHITECTURE
UK- - . - . . » Central Office
ENABLING
SMOOTH WIRELESS ACCESS
Backhaul Layer M -
^ ^ f "
" " ~ /J^AggTegatTorT Tower
(a) A segment of wireless link in figure3-l
Figure 3.2: Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network the downstream segments along the ring, so they need to be upgraded first. Figure 3-2 (b) shows the upgrade of the first wireless link: a fiber is deployed to replace the wireless link. A TDM PON stream at a pair of wavelengths (Al pair) is allotted to facilitate communications between the central office and the first aggregation tower. The Al pair consists of two different wavelengths for separating the downstream and upstream traffic. At the first aggregation tower, a low-loss optical wavelength add/drop is installed to add or drop the wavelength pair Al. The wireless links under the first aggregation tower (i.e. the capacity injection layer in figure2-13) and the wireless link between the first and the second aggregation tower (i.e. the backhaul layer in figure2-13) remain intact. Note that the first aggregation tower enjoys the full bandwidth provisioned by the TDM-PON stream and if needed, more wavelength pairs that carry TDM-PON streams can be allotted in addition to Al pair. Figure 3-2(c) illustrates that the upgrade proceeds to the second wireless link in the backhaul layer. In figure 3-2(c), a A2 pair is allotted to carry new TDMPON stream, which bypasses the first aggregation tower and drops at the second aggregation tower. The replacement can be applied to the subsequent wireless links until the entire backhaul layer is upgraded. As the wireless links in the ring network can be upgraded with optical links, the wireless links between t h e aggregation a n d access towers can also b e upgraded with
optical links. Note that in [33], a point-to-point wireless link is dedicated to facilitate communication between the aggregation tower and a nearby access tower. So point-tomultipoint topology of the aggregation and access towers is based on many expensive point-to-point links. To upgrade these links, we can install an ONU by the access
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
A's Add/Drop
.>*|gfr. Central Office
Aggregation Tower
Aggregation Tower
ONU
; •
Avg. Bandwidth Allocation
: :
(b) Upgrade of upper wireless link
Figure 3.2: Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't)
A's Add/Drop A's Add/Drop
A1.A2 Central Office
.-'M""M •
\\
.*&•, Aggregation /*^i Tower
ONU I
Avg. Bandwidth Allocation
:
Avg. Bandwidth Allocation
(c) Further upgrade of upper wireless link
Figure 3.2: Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't)
CHAPTER 3. HYBRID ARCHITECTURE
Central Office
i Splitter
ENABLING
SMOOTH WIRELESS ACCESS
I Aggregation splitter & X s * . ^ Tower
Access Toswer 0NUi0NUf0NU?0MU5 ", a b e d " k«
Avg. Bandwidth Allocation
Avg. Bandwidth Allocation
(d) Upgrade of lower wireless link
Figure 3.2: Smooth upgrading path of the wireless backhaul proposed by Google and Earthlink for San Francisco City wireless access network (con't) tower to replace the wireless link. Figure 3-2(d) shows that as the wireless links are gradually upgraded, the point-to-multipoint MAC protocol of TDM-PON will automatically manage the point-to-multipoint network and its bandwidth allocation. Once the bottleneck in the backhaul layer is resolved by upgrading to optical links, the capacity injection layer will become the next bandwidth bottleneck. The upgrade can be realized with the same principle by further deploying ONU at wireless gateway routers to gradually replace wireless links, as shown in figure 3-2(e). If a certain district needs more bandwidth, (for example the district X in figure 3-2(e)), another TDM PON stream on a different wavelength pair can be allotted. Passive optical filters will be required at the ONU to separate different wavelengths. Note that the upgrade will eventually result in the hybrid optical and wireless architecture illustrated in figure 3-1.
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A-band Add/Drop (1.2
OLT1 Packet Rejection Ratio 25 20 15 Packet Rejection Ratio (%) 1Q
-J
1 2 4
L_
3 4 5 Time (sec)
6
7
8
PON1 input loading: 0.2 -vl.2
Figure 3.6: Buffer depth and packet rejection ratio of PON1 in both architectures
CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
Fixed Backhaul Architecture
Throughput (Gbps) 0.8 0.6
MMiLiMimimiMmikmAMh 1 2 t
3 4 5 Time (sec)
6
8
P0N1 input loading: 0.2 -^1.2
Reconfigurable Backhaul Architecture I
1.5 Throughput (Gbps) 1
1 2 t
3 4 5 Time (sec)
6
8
PON1 input loading: 0.2 ->1.2 Figure 3.7: Throughput of each PON and combined throughput of both architectures; measured at P2 in figure3-4
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scales with different loads for P 0 N 1 and fixed load for P0N2. According to the linear scale, the fixed architecture's performance reaches its limit as load reaches 90%, while the reconfigurable system does not until the overall load reaches 180%. On the log scale, after passing 90%, the reconfigurable backhaul is insensitive to load increasing due to reconfiguration.
3.4
Experimental testbed of reconfigurable optical backhaul
After reviewing the performance improvement from dynamic load balancing realized by a reconfigurable backhaul, it is desirable to investigate its feasibility, performance, and compatibility to TDM PON technologies. For these purposes, an experimental testbed was built as illustrated in figure 3-9 [39]. It consists of two nodes: one emulates the central office having two OLTs and the other node emulates the ONU; both nodes are connected with a 100m single mode optical fiber. The two OLTs are equipped with fixed optical transceivers and the ONU is installed with a optical tunable transceiver. The TDM PON standard we selected is the IEEE 802.3ah EPON standard, and the functionalities of the OLT and the ONU as specified in EPON standards are programmed in commercial Field Programmable Gate Array (FPGA) chips. To facilitate a reliable network reconfiguration, the so called "reconfiguration control interfaces (RCIs)" are implemented in the FPGAs at the ONU and the central office (as shown in figure 3-9). These RCIs sit at higher level above the MAC layer defined by EPON standard, and coordinate with a handshaking protocol to achieve safe network reconfiguration.
3.4.1
Handshaking protocol for network reconfiguration
The handshaking protocol can be illustrated by the timing diagram as in figure 3-10. The red dashed lines indicate the transmission of RCI reconfiguration commands, and the solid lines signify the MPCPDU packets specified in the EPON standard. As traffic load needs to be re-balanced, the system bandwidth management module
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P0N2 Loading 20% 14000 12000 -Fixed Backhaul 10000
Average Packet sooo Delay (us) 6000
-Reconfigurable Backhaul
(Linear-Scale)
4000 2000 0.
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40
60
80
100
120
140
160
180
140
160
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Traffic Loading (%) PON2 Loading 20%
10a
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Average Packet Delay (us)
-Reconfigurable Backhaul
10*
(Log-Scale) 10
10
20
40
60
80
100
120
Traffic Loading (%)
Figure 3.8: Long-term average packet delay of both PONs (varying load for PON1 and fixed load for PON2)
CHAPTER 3. HYBRID ARCHITECTURE
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Tunable TX: Tunable transmitter Tunable RX: Tunable receiver RCI: Reconfiguration Control Interface
Central Office
0LT1 RCI
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Figure 3.9: Experimental testbed of reconfigurable optical backhaul will issue a network reconfiguration trigger to the RCI at the central office. Upon reception of the trigger signal, the RCI will coordinate with the RCIs at the ONU(s) to deregister from a heavily loaded PON (e.g. P0N1) and register to a lightly loaded PON (e.g. P0N2). The detailed steps are described as follows: (1) The RCI at the central office sends new wavelength information, e.g. A 2d,u of the lightly loaded OLT (i.e. 0LT2) to the heavily loaded OLT (i.e. OLT1), and the OLT will arrange the next available data packet to deliver the new wavelength information to the ONU to be deregistered. Note that this information can be piggybacked on data packets the specified by TDM PON standards. (2) Upon receiving the RCI information, the ONU passes it to the RCI behind ONU. (3) The RCI behind the ONU stores the new wavelength information, and generates an acknowledgement (ACK) signal and passes it onto the ONU for transmission.
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System BW RCI Module .Reconfiguration
0LT1 0LT2
ONU
Reconfiguration J - SOps
Dashed arrows: RCI reconfiguration commands Black arrows: EPON MPCPDU commands
Figure 3.10: Timing diagram of the reconfiguration protocol
RCI
CHAPTER 3. HYBRID ARCHITECTURE
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The ONU will then arrange the next available data packet to deliver the ACK. (4) Upon reception of the ACK signal from upstream packets, the heavily loaded OLT passes it to the RCI at the central office. The RCI will proceed to instruct the heavily loaded OLT to start the standard deregistration process specified in TDM PON standards [2], [3]. (5) After the deregistration ACK is delivered by the ONU, the RCI at the ONU will tune the wavelengths of tunable transceiver to the new wavelengths of the designated OLT (e.g. from A ld)U t o A 2d,u)(6) After receiving the deregistration ACK from the ONU, the RCI at the central office will wait for a certain amount of time (the waiting time is preset according to the tuning time of the optical tunable transceiver at the ONU), and instruct the designated OLT (e.g. OLT2) to start the standard discovery process defined in TDM PON standards [2], [3] to discover and register the new ONU. To realize and examine the handshaking protocol, figure 3-11 (a)-(e) summarizes the state diagrams of RCI's at the central hub, the heavily and lightly loaded OLTs, the reconfigurable ONU, and the RCI at the reconfigurable ONU which are implemented in the FPGAs shown on figure 3-9. Note that the discovery, de-registration, registration processes are programmed according to the IEEE 802.3ah EPON standards.
3.4.2
Enabling Devices
This section introduces the two enabling devices in details on the experimental testbed. The tunable transmitter (laser) will be introduced in chapter 4.
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Idle Receive reconfig. command from System Bandwidth Management
Instruct heavily loaded OLT to send reconfig. info. & deregister the to-be-reconfigured ONU Inform System Bandwidth Management of Reconfig. failure
Wait lor ACK from heavily loaded OLT Receive ACK from heavily loaded GLT
Timer expires
Wait for reconfig. period of the ONU transceiver Timer expires OR incorrect ONU registration
Instruct designated OLT to perform autodiscovery process
I Wait for ONU registration from designated OLT
Receive ACK from designated OLT
(a) State diagram of the reconfiguration control interface at the central office
Figure 3.11: State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU
Receive recc nfig. instruction fr ymRazonfig. Ctrl, interfac Send PHY info, of designated OLT to the to-be-deregistered ONU Inform reconfig. Ctrl. Interface of reconfig. failure
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1
Wait for ACK from the to-be-deregistered ONU Receive ACKfrom the ONU(s) Deregister the ONU
Inform Reconfig. CM. Interface of ONU deregisteretion (b) State diagram of the heavily loaded OLT
Figure 3.11: State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
Idle (Normal Operation) Autodiscovery fails after N trials Report registration failure to Reconfig. Ctrl. Interface
Receive instruction from Reconfig. Ctrl. Interface to perform the autodiscovery process
Autodiscovery
Autodiscovery , performed periodically
New ONU ^registered Report registration success & ONU's MAC address to Reconfig. Ctrl. Interface
(c) State diagram of the designated lightly loaded OLT
Figure 3.11: State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
Idle (Normal operation) Receive reconfig. info. Inform Reconfig. Ctrl. Interface at ONU of upcoming Reconfig.
T Deregistration
Reconfig. the transceiver Autodiscovery Process
(d) State diagram of the reconfigured ONU
Figure 3.11: State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU (con't)
CHAPTER 3. HYBRID ARCHITECTURE
Tune transceiver to theA pair of original OLT
,
ENABLING
SMOOTH WIRELESS ACCESS
Receive reconfig, instruction & Apair of the designated OLT Deregistration from current PON system
1 Tune the transceiver to the A pair of designated OLT
• registration fails
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(e) State diagram of the reconfiguration control interface at the reconfigured ONU
Figure 3.11: State diagrams of the RCI's in the central office, heavily and lightly loaded OLTs, ONU, and RCI behind ONU Optical Tunable Reciver The tunable receiver at the ONU is implemented with an MEMS tunable filter, as shown in figure 3-12, which has a wide tuning range, from 1591nm to 1525nm, corresponding to the control voltage of OV to 35V. Throughout the tuning range, the insertion loss is less than ldB. To examine the transient response of the tunable filter, a lightbeam at 1586nm is continuously transmitted to the tunable filter, and the tunable filter tunes to receive between 1591nm and 1586nm by changing its control voltage. As shown in figure 3-13, after the control voltage is changed, it takes 33.6^s for the filter to stabilize to receive the 1586nm light. Since the tuning time is much longer than that of the tunable transmitter, the reconfiguration overhead incurred in the PHY layer is dominated by the tunable receiver. To accommodate the tuning time, the period between de-registration (step 2: de-register ONU from heavily loaded PON) and re-registration (step 4: re-register the ONU to lightly loaded PON) issued by the RCI at the central office is thus programmed as 50/is in the FPGA.
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Figure 3.12: Optical tunable receiver used on the reconfigurable optical backhaul experimental testbed F P G A and SerDes Figure 3-14 shows the PC board which has the field programmable gates array (FPGA) and the SerDes (serializer-deserializer) we used on the testbed. The FPGA is made by Altera Inc. and can parallelly process 16 bits running at a 77.6MHz basic clock rate. For the transmitter (TX) function, the serializer of the SerDes combines the 16 parallel bits within 1 basic clock cycle to make a 1.25Gbps data stream. For the receiver (RX) function, the deserializer segregates the 1.25Gbps serial data stream into 16 data streams at 77.6MHz rate. The OLT, ONU, and RCI functionalities depicted in the state diagrams, are programmed in the FPGAs. Figure 3-15(a) and (b) depict the configuration of the OLT and RCI at the central office and the ONU and its associated RCI. On each board, there are two FPGA boards, which are synchronized to the same basic clock and can communicate via the central bridge. The state diagrams are programmed in Static Random Access Memory (SRAM). In the next section we will see how these FPGAs coordinate to implement the reconfiguration process.
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
3.4.3
Experimental Results
To facilitate the communication of RCI on the ONU and central office sides, a control packet is used on the testbed and its format is shown in figure 3-16. It consists of the following parts: 1. Preamble (60 Bytes): for clock and level recovery; specifically to facilitate upstream reception. 2. Sync Bytes (4 Bytes): delimiter for bit synchronization 3. Frame ID: for statistics and error checking purposes 4. TX: sender ID 5. RX: receiver ID 6. Frame type: indicates the type of MAC frame 7. Payload: carries RCI commands such as de-registration, ACK, new wavelength information, etc. The experimental results are shown in figure 3-17, which demonstrates the ONU deregistration from the OLT1, the reconfiguration period, and the ONU discovery and registration performed by the OLT2. The results are captured by the HP16500B Logic Analyzer, which allows data rate up to 4Gb/s, and can observe at the same time up to 32 waveforms and 0/1 transitions in a digital system in order to monitor the system as well as verify its performance. The signals marked with numbers in figure 3-17 are outputs measured on the two FPGA boards on the central office and ONU sides, and they indicate the following events: (1) OLT1 sends deregistration message;
CHAPTER
3. HYBRID ARCHITECTURE
To facilitate upstream reception
Sender ID
ENABLING
SMOOTH WIRELESS ACCESS
RCI commands: Reconfiguration & ACK New wavelength information Dummy bits
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2) ONU receives the deregistration message; 3) ONU sends the ACK; 4) OLT1 receives the ACK; 5) after the 50/us reconfiguration period, OLT2 sends discovery gate message; 6) ONU receives the discovery gate message; 7) ONU sends the registration request; 8) OLT2 receives the registration request; 9) OLT2 sends the registration message; 10) ONU receives the registration message; 11) ONU sends the registration ACK;
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CHAPTER 3. HYBRID ARCHITECTURE ENABLING SMOOTH WIRELESS ACCESS UPGRA
(12) 0LT2 receives the registration ACK.
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