Daniel Livengood, Jijun Lin and Chintan Vaishnav
`Engineering Systems Division
`Massachusetts Institute of Technology
`
`Submitted May 16, 2006
`To Dan Whitney, Joel Moses and Chris Magee
`
`Public Switched Telephone Networks:
`A Network Analysis of Emerging Networks
`
`
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`Page 1
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`Page 1 of 27
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`Unified Patents Exhibit 1007
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`
`Table of Contents
`Nomenclature and Data Sources..................................................................................... 4
`History and Political Economy of PSTN.........................................................................................5
`The period of monopoly (until 1984).............................................................................. 5
`The breakup of the monopoly (after 1984)..................................................................... 8
`Summary of constraints and the current state of PSTN..............................................................10
`Network Analysis...............................................................................................................................12
`Network Modeling Decisions and Assumptions........................................................... 12
`Call scenarios and the networks to analyze .................................................................. 12
`Network Experiments: Dynamically changing Pearson’s Correlation Coefficient ..................17
`Case 1: Randomly add edges within each cluster of Central Offices (COs) ................ 17
`Case 2: Randomly add edges between all Central Offices ........................................... 18
`Case 3: Completely randomly add edges to the Mini Bell network ............................. 21
`Summary of Pearson’s degree correlation experiments................................................ 22
`Network Experiments: Robustness Analysis for Nano Bell........................................................23
`Case 1: Randomly remove nodes.................................................................................. 23
`Case 2: Randomly remove edges.................................................................................. 24
`Contributions to the ESD.342 Project Portfolio ..........................................................................26
`Recommendations for Future Work...............................................................................................26
`References ...........................................................................................................................................27
`
`List of Tables
`Table 1: Call scenarios represented by phone company interactions.........................................12
`Table 2: Network analysis metrics for the five networks modeled.............................................16
`Table 3 Summary of Pearson's degree correlation experiments .................................................22
`Table 4 Number of node failures that Nano 2005 and 2010 can tolerate (500 runs)..............23
`Table 5 Number of clusters existing in the rest of the network with random node failures (50
`runs) .....................................................................................................................................................23
`Table 6 Number of edge failures that Nano 2005 and 2010 can tolerate (500 runs)...............24
`Table 7 Number of clusters existing in the rest of the network with random edge failures (50
`runs) .....................................................................................................................................................24
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`List of Figures
`
`
`Figure 1 PSTN connectivity in 1928.................................................................................................6
`Figure 2 (a) AT&T’s five-level hierarchy in 1970s (b) AT&T’s regional network with five-
`level hierarchy.......................................................................................................................................7
`Figure 3 Level-skipping in pre-1975 networks ................................................................................8
`Figure 4 Regional Bell Operating Companies: then and now .......................................................9
`Figure 5 (a) Dynamic non-hierarchical routing (DNHR) and (b) the new hierarchy ..............10
`Figure 6 Nano Bell Networks: Current (2005) and Future (2010) .............................................13
`Figure 7 Robustness in redundant fiber rings................................................................................14
`Figure 8 Current Mini Bell network ................................................................................................15
`Figure 9 Nano and Mini Bell networks connected .......................................................................16
`Figure 10 Randomly add edges within central offices' clusters...................................................18
`Figure 11 Mini Bell network when r=0 ..........................................................................................18
`Figure 12 Randomly add edges for all central offices...................................................................19
`Figure 13 Mini Bell when r = 0 with 190 added edges.................................................................19
`Figure 14 Mini Bell (a) when r = 0 with 1599 added edges and (b) when r = 0 with 2305
`added edges.........................................................................................................................................20
`Figure 15 Randomly add edges for all central offices and tandems ...........................................21
`Figure 16 Mini Bell (a) when r = 0 with 306 edges and (b) when r = 0.1574 (near peak) with
`1121 edges...........................................................................................................................................22
`Figure 17 Probability distribution of number of node failures that Nano 2005 and 2010 can
`tolerate .................................................................................................................................................24
`Figure 18 Probability distribution of number of edge failures that Nano 2005 and 2010 can
`tolerate .................................................................................................................................................25
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`A recent focus in the area of network analysis has been on the comparison of
`technological, informational, social and biological networks (Newman 2003). Within the
`technological networks category, one comparison of interest is among infrastructure
`networks such as the power generation and distribution networks, the public switched
`telephone networks (PSTN), the Internet, and various transportation networks that we have
`come to rely heavily upon. In this paper we will use network analysis to study the PSTN.
`
`
`Our analysis of the PSTN is focused on wired (copper and fiber) networks. It does
`not entail wireless networks such as microwave, satellite or other radio links. In the United
`States, telecommunications service providers that operate the PSTN fall under three
`categories: interexchange carriers (IXCs) that own networks for long-distance calling,
`incumbent local exchange carriers (ILECs) that own networks for inter and intrastate calling,
`and competitive local exchange carriers (CLECs) that own networks within a state1. For our
`analysis, we looked at the networks of a CLEC and an ILEC serving one US state.
`
`Nomenclature and Data Sources
`
`
`
`For the purpose of our analysis, we will refer to the CLEC whose network we are
`analyzing as a Nano Bell; the ILEC whose network we are analyzing as a Mini Bell; and IXCs,
`which are not a part of our analysis, Maxi Bells. We have decided to use this nomenclature in
`order to protect the identity of the companies that shared the data with us, as per our
`agreement with them. An example of a Nano Bell (CLEC) is Mid-Maine Communications in
`Maine, where Verizon is the Mini Bell (ILEC) and the Maxi Bell (IXC) is a long distance
`company such as AT&T.
`
`
`
`The networks we will analyze – a Nano Bell network and a Mini Bell network – have
`two types of switches: tandem switches and central office switches. A tandem switch switches traffic
`between central offices and forms the core of the network. A central office switch has
`connections to homes’ and offices’ end systems, such as phones, faxes, etc. A central office
`switch often has two parts – the host switch and the remotes. The host is the part of the switch
`that carries out all of the switching functions, whereas the remotes only provide geographical
`coverage but rely entirely on the host for any switching functions its connections may
`require. A tandem switch is equivalent to a Class 4 of AT&T’s original product line. A
`central office switch is typically a relatively smaller Class 5 switch.
`
`
`
` Our data comes from two primary sources. The Mini Bell data was obtained from
`their public website: http://www.qwest.com/iconn/. The Nano Bell data comes from
`network plans a Nano Bell operating in one US state has shared with us under the
`aforementioned confidentiality agreement. The assumptions (discussed later) we made about
`the Nano Bell’s network come from our interviews with an enthusiastic contact person at
`the Nano Bell.
`
`
`
`
`
`1 Our focus on the ownership of the network is deliberate since it is no longer possible to separate the three
`types of providers by the service they provide. Today an IXC, ILEC or a CLEC is allowed to offer local or
`long-distance service.
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`History and Political Economy of PSTN
`
`
`With a technology more than a century old, the US PSTN has a rich history of rapid
`changes in technology, regulation and industry structure. However, keeping in mind the
`focus of this paper, we will discuss the history and political economy of PSTN from the
`network analysis perspective. For our analysis, we have defined telephone switches as nodes
`and the connections between them as edges. Therefore, we will discuss the historical events
`that changed the characteristics of the nodes, the links and the connections between them.
`
`The period of monopoly (until 1984)
`
`
`The telephony, as we know it today, was born with a key breakthrough in 1875-76
`when Alexander Graham Bell developed transducers from voice to electrical signal and vice
`versa. Bell had quickly realized the tremendous potential of his invention and formed the
`Bell Telephone Company in 1877. Although Bell himself stopped working on telephony one
`year after his invention, he left the company to his father-in-law Gardiner Hubbard and
`Watson (Boettinger 1977). Watson invented the ringer, the first switchboard, and many
`other things essential to transforming a laboratory toy into a commercial product. As the
`company grew, they hired Theodore Vail to manage the company. Vail worked hard to
`ensure that the Bell Telephone Company controlled a substantial portion of the telephone
`service in the United States even after the expiration of Bell’s patent in 1894. He also used
`the ever-increasing capital to buy out other telephone companies. By the time he was done,
`the American Telephone and Telegraph Company (AT&T) owned every telephone
`instrument, every telephone switch, and every telephone pole in the country. Vail made sure
`that AT&T would survive the antimonopoly sentiments by promising that every American
`would have access to the telephone network.
`
`
`AT&T’s dominance continued for several decades before it was regulated as a
`natural monopoly with the creation of a federal regulatory body, the Federal
`Communications Commission (FCC). The FCC was established by the Communications Act
`of 1934 and was charged with regulating interstate and international communications by
`radio, television, wire, satellite and cable.
`
`
`The Telecommunications Act imposed universal service obligations on AT&T,
`which led to planned growth (designed network) of PSTN in the subsequent years. Until then,
`the PSTN had been growing (grown network) in areas that were population centers and where
`the installation made business sense. Today, as most of the PSTN is modernized, it would be
`difficult to recreate the picture of what the network looked like during this pre-1934 era of
`grown networks.
`
`
`The initial overriding obstacle to providing universal service was attenuation in
`copper lines, known as the challenge of conquering distance (Fagen, Joel et al. 1975). The
`improvement and general availability of vacuum tubes had a major impact on solving the
`distance challenge. With vacuum tubes, it became possible to interconnect widely separated
`cities with low loss and good quality circuits. In 1925, it was possible to call any big city in
`the continental United States over a circuit of good quality. The switching technology during
`the first half of the twentieth century was dominated by manual switching for both local and
`
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`long-distance calls (Fagen, Joel et al. 1975). Figure 1 shows the 1928 graph of PSTN.
`Initially, improvement in switching technology kept it ahead of the demand. Over time,
`incremental improvements in manual switching hit a limit of reduction in labor costs and
`call-completion time. Around 1925, large volumes of calls began to experience delays while
`waiting for a transmission facility. This made electro-mechanical switching popular during
`1925 through 1950s. The quality of electro-mechanical switches also steadily improved.
`
`
`
`
`Image removed for copyright reasons.
`
`
`
`Two breakthroughs that first necessitated hierarchical switching were: the advent of
`automatic crossbar switching and the ability to do statistical multiplexing on transmission
`lines. Around 1913, the seeds were planted for the next generation of automatic switching
`equipment, what we know as the crossbar switches, when patents for automatic coordinate
`switching were granted to Western Electric2.Crossbar switches initiated the direct distance
`dialing by two customers with no operator in the middle in 1951. The notion of carrying
`traffic other than voice (data and video) also began with the automatic, crossbar switches (of
`course, using appropriate wiring). The point-to-point links could no longer scale to
`accommodate the additional traffic from customer direct dial. A statistical aggregation
`scheme using time division multiplexing (TDM) of voice channels was on the horizon.
`
`
`Availability of solid state electronics post World War II enabled the pulse code
`modulated (PCM) transmission – what we now know as digital transmission – over ordinary
`wire. The PCM used Harry Nyquist’s sampling rate – a discovery he made at Bell Labs in
`1927 (Bellamy 1982) – at which an analog signal must be sampled for its recovery at the
`receiver. The Nyquist rate of sampling enabled the statistical aggregation of voice channels.
`In 1962, the first T1 carrier that multiplexed voice channels of 24-channels over 1.5 Mbps
`was installed and was immediately successful. Subsequently came the higher levels of
`aggregation with T2 (96 channels), T3 (672 channels) and T4 (4032 channels).
`
`
`The combination of automatic crossbar switching and statistical multiplexing on
`transmission lines led to the design of hierarchical networks. By 1970, AT&T had a five-level
`
`2 Unlike the manual system that worked by moving the contact over an appreciable distance to make a
`connection between two lines, the cross-bar switches worked by closing electrical contacts at points in an
`x-y coordinate array.
`
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`hierarchical network shown in Figure 2(a) (Chap11is 1982). Figure 2(b) illustrates AT&T’s
`network with the United States and Canada divided into twelve regions. By this time the
`challenge of distance was replaced with that of efficient operations and management of the
`complex network Two major considerations governed the economics of transmission
`systems then: minimum capacity below which economies—of—scale was not possible and
`minimum distance below which multiplexing was not economical.
`
`Satellite links .g 9
`In
`
`. e
`vvx
`
`S I
`cables
`
`International gateway
`exchange (Centre de Transit 3)
`
`National tandem exchanges
`trunk switching
`
`centres)
`
`centres)
`
`Regional tandem exchanges
`(Secondary trunk switching
`
`Localtandemexchanges
`
`centres)
`
`REGIONAL (ENTERS
`
`Figure by MIT OCW.
`
`Figure by MIT OCW.
`
`While these developments were underway, the invention of the laser in Bell
`laboratories in 1958, and the simultaneous rise of Nyquist and Claude Shannon’s work on
`information theory, had kindled interest in optical transmission. Two other important
`complementary inventions that occurred around that time were: first, the advent of the
`transistor by Bardeen, Shokley and Brattain, and second, the advent of the general purpose
`computer, ENIAC, by Eckert and Mauchly. Ultimately, the manufacture of fiber optics by
`Corning Glass was what led to the next revolution in transmission (Keshav 1997).
`
`The five—level hierarchy has never remained purely a hierarchical tree structure. First,
`level—skipping was exercised as shown in Figure 3 in order to cut down on losses from every
`electro-mechanical contact. This was done by ensuring that there are not more than two
`electro—mechanical contacts in placing a long—distance call. Later, additional alternate routes
`were introduced across levels with the installation of the automatic crossbar switches in
`
`1950s to cater to the higher call volume resulting from customer direct dial.
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`Class 1
`
`Regional Center
`
`Class 2
`
`Sectional Center
`
`Class 4
`
`ToH Center
`
`Class 5
`
`End Oflice
`
`Five-level toll switching plan in use from the 1950s.
`A variety of routings was possible with a maximum
`of nine tnmks in tandem.
`
`Figure by MIT OCW. After Andrews & Hatch, 1971.
`
`The breakup of the monopoly (after 1984)
`
`In the late 1970s, the anti—monopoly sentiments were strong in America. By the early
`1980s there was an alternate long—distance carrier, MCI, but it was believed that AT&T’s
`market power over MCI came from its ownership of the local loops (the last mile
`connection from central office to home). In 1984, the US Government decided to breakup
`AT&T into the long—distance company (AT&T), and seven regional bell operating
`companies (RBOC). At the time of its creation, the RBOCS inherited AT&T’s network,
`including the regional centers
`centers), tandem switches (toll centers) and central
`(end offices) shown in Figure 3, Figure 4 shows the original breakup of AT&T into seven
`RBOCS and the changes they have undergone over time.
`
`An inter—carrier compensation (also known as access charge) regime was established
`defining the per—minute charges AT&T (now an IXC) paid an RBOC (now an ILEC) to help
`the RBOCs can operate and maintain the local networks. AT&T was disallowed to offer
`local service, while the RBOCS were prohibited from offering the long—distance service after
`the breakup.
`
`As the competition in local and long distance service began to emerge, AT&T began
`to use innovative routing schemes to maintain its competitive edge following the breakup of
`1984. In 1987, AT&T installed a dynamic routing system called dynamic non-hierarchical
`routing (DNHR). The objective of this system was to decrease the number of dropped calls,
`especially on high—traffic days like holidays. Originally, the static paths from the five-level
`hierarchy would simply drop a call if it reached a li11k that was blocked. A new “originating
`call control feature” allowed a call to be “cranked back” to the original switch if the direct
`path was blocked. Then, with new technology coming in the form of 4ESSm switches,
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`alternate paths that may not be considered the most direct paths were tried until all possible
`‘two-hop’ routes were exhausted. Only then would a call be dropped (Ash and Oberer,
`1989).
`
`
`Image removed for copyright reasons.
`See: http://en.wikipedia.org/wiki/Image:RBOC_map.png
`
`
`
`As an example, imagine a call wanting to go from Lincoln NE to Seattle WA. In the
`five-level hierarchy, the call from the Plains states would be forced to go to one of the
`Regional Center switches at Denver, St. Louis, or Chicago. If the link from the chosen
`regional center to Seattle was blocked, the call was dropped. There was no way to switch to a
`new Regional Center switch without starting the call over. With DNHR, the call would
`initially try to connect directly from Lincoln to Seattle (i.e. from A to B in Figure 5a). If this
`direct connection was blocked for some reason, the call could “crank back” to Lincoln,
`which could then send it to any other directly connected city (not just the Regional Centers)
`and ultimately on to Seattle via the direct connection from this intermediate city (i.e. from A
`to C to B in Figure 5a). The path could ultimately be Lincoln NE to Dover DE to Seattle
`
` http://en.wikipedia.org/wiki/Regional_Bell_operating_company
`
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`WA, as an example, even though this seems counterintuitive. Quality loss over large
`distances was no longer a constraint on the system. Hence, the DNHR system was able to be
`implemented to handle a different quality constraint: dropped calls.
`
`f\
`
`f\
`
`f\
`
`I\
`
`f\
`
`f\
`
`Central Ofices
`
`End System
`
`Backbone
`
`(3)
`Figure 5 (a) Dynamic nomhierarchical routing (DNHR)
`
`Figure by MIT OCW.
`
`Ultimately, the smaller carriers liked the new dynamic routing systems as well. After
`the breakup, AT&T and MCI still controlled the long—distance market and charged access
`c11arges for RBOCs to use their network. W’ith fixed paths from tl1e five—level hierarchy,
`RBOCs wo11ld be forced to pay the access charges. Thanks to schemes like DNHR, RBOCs
`could set up their dynamical routing to try all of their own paths between cities before
`jumping to one of the IXCS. The PSTN network today only has two or three levels of
`central offices connecting to local and regional tandem as shown in Figure 5b.
`
`Jumping ahead to 1996, the concern for local loop ownership being a bottleneck to
`the growth of local PSTN markets led to the unbundling act of 1996, which required the
`ILECS to share their access tandems with the competitive local exchange carriers (CLECS)
`for FCC-determined usage charges. In the following sections, we will see that due to various
`constraints, networks the ILECS inherited from AT&T compared to the new ones built by
`the CLECS have turned out to be different in structure and hence in capabilities.
`
`Surnrnary of constraints and the current state of PSTN
`
`Before we begin the network analysis discussion, in this section we will summarize
`the constraints that had to be overcome during the evolution of the PSTN to understand the
`current state of the art in wired networks.
`
`The distance constraint is addressed today using optical fibers that can go up to 70
`km (compared to 1 km for copper) witho11t using a repeater. They can carry up to a few
`Gbps (compared to 100 l\/fbps for copper) of traffic. A low—end optical fiber costs nearly the
`same as a high—end copper cable; however, what stalls the installation of fiber is the cost of
`switching electronics. A fiber network interface card costs approximately three to four times
`higher than the electronics to run copper (Barnett, Groth et al. 2004).
`
`Losses in electromechanical contacts used to be a constraint before the 1950s. This
`
`was overcome by level—skipping that ensured not more than two electromechanical
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`connections were made to complete a call. With the introduction of automatic crossbar
`switches in the 1950s that used solid state electronics for switching, the losses in switching
`became secondary to the concern for the ability to handle the additional call volume, as the
`automatic switches enabled customer-to-customer direct dialing with no operator in the
`middle. This introduced routes additional to the existing level-skipping routes.
`
`
`From the regulatory perspective, two constraints affect the evolution of the network.
`First, the access charges imposed for using another carrier’s network leads to routing
`schemes that are non-hierarchical such as DNHR. Second, the equal access obligation
`requires Mini Bells to open their Access tandems to competitors4. On the one hand,
`upgrading the Access tandem switch is difficult for Mini Bell to coordinate as all of the
`connecting carriers must upgrade their side of the network simultaneously. On the other
`hand, Mini Bell has little incentive to upgrade the Access tandem switch since doing so
`would help all of its competitors using the same switch.
`
`
`Many of the other constraints today are operational. For instance, obtaining the
`right-of-way and digging is more expensive than the cost of fiber. This makes it attractive to
`lay dark fiber once a company pays for digging. Also, reliability of electronics in the nodes is
`less of a concern compared to the physical breaks in fiber. This has led to the deployment of
`physically separate redundant fiber rings (discussed later). Finally, legacy is a big constraining
`factor. The network of Mini Bell, having evolved from the legacy AT&T network, has much
`different constraints on their choice of what they can change, as compared to the networks
`of new companies such as the Nano Bells.
`
`
`
`4 Unbundling Act of 1996 required ILECs to designate several Access tandems that other competing
`carriers such as CLECs and IXCs can use to reach local customers. Since in any given state, there was a
`single Mini Bell (ILEC) that owned all local loops to homes, it was believed that it had a potential to stifle
`competition in lieu of such unbundling obligations.
`
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`Network Analysis
`
`
`
`In this section we will carry out a network analysis of a network of a CLEC, a
`network of an ILEC and the interconnection between the two.
`
`Network Modeling Decisions and Assumptions
`
`
`• Nodes in the networks are tandem switches and central offices (aka host
`offices) in both the Mini Bell and Nano Bell networks.
`• Our data for Nano Bell included both remote offices and host offices. Host
`offices connect with each other and the tandem switches to create the main
`structure. Approximately 3 remote offices connect to each host office in a
`parent node with three child node structure. Remember, a host office
`switches traffic for remotes connected to them, so for the purposes of our
`analysis, a cluster of one host and three connecting remotes constitutes a
`single node.
`• Bandwidth capacities in the links of the network were not modeled, largely
`due to time constraints.
`• When creating Nano Bell’s network in 2005, leased lines from Mini Bell were
`assumed to connect disconnected host offices via the most direct path.
`
`
`Call scenarios and the networks to analyze
`
`
`Although collecting data proved somewhat difficult for this project, we still wanted
`to represent as many call scenarios as possible with the network data we received. The table
`below illustrates the possible call scenarios.
`
`
`Table 1: Call scenarios represented by phone company interactions
`
`
`Various call scenarios, combined with the data sets we received, led us to run a
`network analysis on five networks:
`1. Nano Bell’s network in 2005
`2. Nano Bell’s projected network in 2010
`3. Mini Bell’s interconnections of tandem switches and central offices
`4. Mini (#3) combined with Nano 2005 (#1)
`5. Mini (#3) combined with Nano 2010 (#2)
`
`
`
`
`
`
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`
`O11r reasoning behind these five networks is as follows. Today (after 1999), all
`companies are allowed to offer both local and long distance calls. The question becomes
`whose wires are used to make the call? Smaller companies, like Nano Bell, have a limited
`number of wires that they control. Thus, they can usually provide local and intrastate calls on
`their own network, b11t they must rely on Mini and Maxi Bell companies to provide interstate
`long—distance services. Since we were able to obtain detailed data for the Nano Bell network,
`they became o11r focus of in-network calls. Our Mini Bell is large enough to control a large
`network that allows them to provide interstate long-distance calls, but our data is sparse at
`best. However, all companies still must interact when the two callers have different service
`providers. Hence, we needed at least a portion of Nlini Bell’s network to represent the
`interconnection of Nano Bell and Mini Bell for inter-network calls.
`
`O11r initial focus was on Nano Bell’s current and future networks. Looking at the
`2005 network shown in Figure 6, Nano Bell currently has many branches extending away
`from a core that is connected by a few rings. This is indicative of its current network
`arrangement, where some of its isolated nodes are connected by lines leased from Mini Bell.
`W’ithout complete contIol of its own network, Nano Bell focuses on connecting all of its
`cities as directly to a tandem switch as possible. By 2010, however, Nano Bell plans to
`connect all of its host offices via interconnected rings of do11ble—fiber connections witho11t
`leasing any fiber from Mini Bell. This redundant, fully intercomlected ring stxucture is easily
`visible in the 2010 graph in Figure 6. The benefit of the additional capital necessary to create
`these doubly—linked rings comes in terms of network robustness. Table 2 shows the network
`parameters for both 2005 and 2010 Nano Bell networks. In a later section, we will discuss
`the simulations we ran to illustrate l1ow robust the 2010 network is in comparison to the
`2005 network, but for now, we’ll illustrate conceptually why the doubly—linked ring structure
`is more robust.
`
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`
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`A Tandem Switch
`9 Host Office
`
`Figure 6 Nano Bell Networks: Current (2005) and Future (2010)
`
`Figure 7 below illustrates that a singly-linked fiber ring (aka. collapsed rings) can
`have a host office isolated with the failure of only two links. The link failures are represented
`by the little X’s, whereas the X over the H1 node represents the fact that the node is now
`isolated from the rest of the network. By physically separating two rings that connect the
`same set of nodes, two link failures is no longer enough to isolate the host office, as shown
`
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`
`on the right-hand side of Figure 7. It now takes the failure of four links before a host node is
`isolated from the network. So from the viewpoint of a single node, intuitively the node can
`sustain double the number of link failures when the number of links attached to the node
`doubles. However, our interest is in the network as a whole. By connecting all nodes with
`this doubly-linked ring structure (a.k.a. separated rings), what is the increase in the number
`of link failures the network can withstand before it becomes disconnected? Through
`simulation, we will show later that the factor of link failures the network can withstand is
`more than doubled when all nodes are connected via doubly-linked rings.
`
`
`Figure 7 Robustness in redundant fiber rings
`
`
`Figure 8 shows Mini Bell’s network. In the center are the fully-connected tandem
`switches (Class 4 switches). There are four types of tandem switches: Access, Local, Toll and
`911. Access tandems are the equal access tandems used by other carriers for reaching local
`customers. Local tandems are for routing the intrastate local calls. Toll tandems are for the
`long-distance traffic, whereas 911 tandems are for routing emergency access calls. Connected
`to the tandems are the clusters of central offices (Class 5 switches). As we can see, Mini
`Bell’s network appears much more hierarchical. Our conjecture, based on domain
`knowledge, is that there are three reasons for this. First, it is difficult and costly to change
`the legacy network they have inherited from AT&T. Second, the regulatory obligation of
`equal access that forces them to share the Access tandems with other carriers makes
`coordination of any upgrade difficult. Finally, there is the fact that Mini Bell built its network
`with low bandwidth voice communication in mind, whereas the new companies such as
`Nano Bell have engineered their network to handle high bandwidth voice and data tr