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3.2.1. Data T1 Circuits
Data T1 circuits are used to connect the Tier 1 POPs to the Tier 2 POPs. Because there are only dial-in subscribers at the Tier 1 POPs, the number of data T1s is based on the traffic generated by dial-in subscribers (analog and ISDN) at the Tier 1 POPs. As discussed in the modem sizing section, the number of modems is based on the peak offered traffic by the dial-in users. Similarly, data T1 circuits are based on the peak offered traffic.
59 The discounts are specific to each company and product. For example, MCI’s discounts for a 60-month lease of a T1 range from 21% to 56%; Sprint’s discounts for a 60-month lease of a T1 range from 29% to 53%; and MCI’s discounts for a 60 month lease of a T3 range from 8% to 47%. Because only one set of discounts is included in the model, estimates must be made from these ranges.
INPUT PARAMETER BASELINE IT
Bulk transport discount (monthly cost in dollars) % %
0 - 25,000 10 10
25,001 - 100,000 20 20
100,001 - 200,000 25 25
200,001 - 350,000 35 35
350,001 - 500,000 45 45
500,001 + 50 50
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Because of assumed symmetry, determining the number of T1s needed for one Tier 1/Tier 2 connection is sufficient for all corresponding Tier 1/Tier 2 connections.
The following parameters, along with user profile parameters, which were previously described, are used to size T1 transport:
• % capacity at which to operate transport
• T1 circuit capacity
• % O-D pairs found on ISP’s network
• % Tier 1 traffic destined for Tier 2
• % Tier 1 traffic destined for backbone.
In order to account for the bursty nature of Internet traffic, the parameter % capacity at which to operate transport is used for sizing the number of transport circuits. Through experience, an ISP learns that once a link becomes more than a certain percent full packets start to be dropped and latency increases. The ISP thus determines a threshold that when traffic exceeds this threshold, on average, a new link needs to be added. This is the method used in the model to account for the bursty nature of Internet traffic.
It is assumed that all traffic on this network has a single origin and destination (i.e., effects of multicast and broadcast are negligible). The origins are assumed to be the end users on the ISP’s network; the destinations are other end users on the ISP’s network or on some other ISP’s network. The % O-D pairs found on ISP’s network parameter represents what percent of destinations of requests are found on this ISP’s network.
For traffic (requests) originating at a Tier 1 POP, the destinations can be at (1) the same Tier 1 POP, (2) the upstream Tier 2 POP (and any other Tier 1 POPs connected to this Tier 2 POP, and (3) some other Tier 2 POP on the backbone. The parameters % Tier 1 traffic destined for Tier 2 and % Tier 1 traffic destined for backbone are used to represent the respective destinations.
When sizing the T1 circuits that connect a Tier 1 POP to its upstream Tier 2 POP, one needs to consider cases 2 and 3 in the above paragraph because any traffic terminating at the Tier 1 POP will not be carried on the T1.
The following formula is used to determine the total amount of traffic that originates at a Tier 1 POP:
Tier 1 orig. traffic = # analog modems * avg bw per analog user + peak # ISDN users * avg bw per ISDN user
This following formulas are used to determine the magnitude of the various types of traffic originating at the Tier 1 POP:
Tier 1 traffic terminating at Tier 2 = Tier 1 orig. traffic * % O - D * % Tier 1 traffic destined for Tier 2
Tier 1 traffic terminating at backbne = Tier 1 orig. traffic * % O - D * % Tier 1 traffic destined for backbone
Tier 1 traffic terminating at NAP = Tier 1 orig. traffic * (1 - % O - D )
The effective traffic that is carried on the T1 is the sum of the above three equations:
eff. Tier 1 orig. traffic = Tier 1 tr. @ Tier 2 + Tier 1 tr. @ backbone + Tier 1 tr. @ NAP
The number of data T1s between each Tier 1 and Tier 2 POP is then:
# data T1s = eff. Tier 1 orig. traffic T1 capacity * % operating capacity roundup
60
Based on tariffs filed by telecommunications providers, each T1 has a fixed cost per month (non-distance sensitive) and a distance-based cost per month. The total monthly data T1 cost is determined by the following formula:
T1 cost = (1- discount) *(avg. Tier 1 Tier 2 dist.* $ per mile + fixed $ per mo.) * # T1s per POP * # Tier 1 POPs
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Because the Tier 1 POP connects to one of nine Tier 2 POPs, the % Tier 1 traffic destined for Tier 2 parameter is set to 11% (1/9) and the % Tier 1 traffic destined for backbone is set to 89% (8/9). This implies that no traffic remains at the Tier 1 POP (such as a dial-in user browsing a web server located at the same Tier 1 POP). Hence, this is a conservative estimate of the traffic distribution.
60 roundup is ignored if the round up lumpy quantities? parameter is set to N or n.
The values for the fixed and per-mile costs are based on tariffs of Sprint and MCI.61
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Data T1s connect the Tier 1 POPs to the Tier 2 POPs. The only subscribers at the Tier 1 POPs are the dial-in subscribers. Hence, the data T1 costs are split among the residential and business dial-in subscribers (analog and ISDN) according to the relative amount of total traffic they generate.