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Allocation of optimal discovery slots in IEEE 802.3av networks
Int. J. Electron. Commun. (AEÜ) 66 (2012) 211–213
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.de/aeue
Allocation of optimal discovery slots in IEEE 802.3av networks Milan Bjelica a,∗ , Ana Peric´ b a b
School of Electrical Engineering (ETF), Communications Department, Bulevar kralja Aleksandra 73, RS-11000 Belgrade, Serbia MDS informatiˇcki inˇzenjering, Bulevar Milutina Milankovi´ca 7d, RS-11070 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 31 March 2011 Accepted 8 July 2011
a b s t r a c t In this letter, we consider ONU registration procedure in symmetrical 10 Gb/s IEEE 802.3av networks. Through series of computer simulations we state the optimization guidelines that lead to the reduction of message collisions and the increase of link utilization. © 2010 Elsevier GmbH. All rights reserved.
Keywords: Communication networks Collision probability Computer simulation Optical fiber networks
1. Introduction Passive optical networks (PONs) are considered as cost-effective mean of providing large scale broadband access. These networks consist of a headend or optical line terminal (OLT) which is connected to a number of user terminals known as optical network units (ONUs). The latest standard in this field, IEEE 802.3av describes PONs capable of delivering up to 10 Gb/s on both uplink and downlink [1]. In these networks, downlink transmission is based on wavelength division multiple access, where separate optical carrier is assigned to each ONU. On the uplink, Ethernet-based time division multiple access is applied. To avoid collisions, each ONU is assigned a distinct time slot in which it can transmit. However, the collisions may occur during the so-called discovery phase, when ONUs try to announce their presence to the OLT and register themselves to the network. In this letter, we analyze the discovery procedure in IEEE 802.3av networks. The analytical treatise of older standard IEEE 802.3ah for 1 Gb/s PONs is given in [2] for equidistant ONUs, while [3] gives more general result. Besides the fact that we consider newer standard with different protocol parameters, we also apply somewhat different methodology. From our on-field experience, the analytical solution from [3] is bulky and virtually unsuitable for engineering applications. A more recent paper [4] regards the 802.3av PONs, but within the scope different from ours. We continue the work started
∗ Corresponding author. ´ E-mail addresses: [email protected] (M. Bjelica), [email protected] (A. Peric). 1434-8411/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2011.07.003
in [5] and use computer simulations to propose ad hoc optimization guidelines, which would reduce message collision probability and improve link utilization.
2. Discovery procedure Discovery is the process through which newly connected or previously off-line ONUs (e.g. for the reason of energy saving) announce their presence to the OLT and register to the network. Discovery is a three-way handshake. To give chance to the unregistered ONUs to register, the OLT periodically opens the discovery windows. An unregistered ONU waits for some random time within its discovery slot and then issues REGISTER REQ message. Upon its successful reception, the OLT replies with confirmation message and schedules the observed ONU for access to the network. Discovery is inherently a contention-based procedure. Since ONUs cannot communicate directly to each other it is possible that more than one of them may attempt to register at the same time, causing their REGISTER REQ messages to collide at the OLT input. The random delay (or wait) time is needed to avoid the persistent collisions which would otherwise occur between the ONUs with (approximately) equal round-trip delays (i.e. those that are equidistant from the OLT). The examples of successful and unsuccessful registration are shown in Fig. 1. It is interesting to notice a similarity between PON discovery procedure and contention in CSMA/CA; however, they perform significantly different because of different delay mechanisms applied [4]. Those ONUs whose REGISTER REQ messages have collided remain unregistered and invisible to the OLT; they must wait for the
212
M. Bjelica, A. Peri´c / Int. J. Electron. Commun. (AEÜ) 66 (2012) 211–213 Table 1 Components of REGISTER REQ transmit time with references to IEEE 802.3(av) specification.
Fig. 1. (a) Successful and (b) unsuccessful ONU registration.
next discovery window and then try to register. The performance of ONU discovery and registration in terms of link utilization is closely related to the collision probability, which depends on the discovery window duration. 3. Methodology Instead of applying strictly analytical approach, we chose to derive the experimental results by simulating the discovery process in several typical network deployments. While this is an ad hoc approach, we expect its results to be more informative and useful to the network designers. Let us recall Fig. 1. Let the round-trip (two-way propagation) delays corresponding to the observed ONUs i and j be ti and tj , and their random wait times wi and wj , respectively. Let the total time needed to transmit a REGISTER REQ message with necessary protocol overhead be T. Two REGISTER REQ messages will collide if following condition holds true:
(ti + wi ) − tj + wj ≤ T.
(1)
In the network of N ≥ 2 ONUs, this condition applies to each pair of them. T must account not only for Ethernet frame duration with REGISTER REQ as its payload, but also for guard intervals needed to complete the transients on both transmitter and receiver side [1]: T = Ton + Treceiver
settling
+ TCDR + Tcode
group align
+|SoF| + |REGISTER REQ| + Toff + |IPG|.
+ |PREAMBLE| (2)
Designation
Value
Reference
Ton Treceiver settling TCDR Tcode group align PREAMBLE SoF REGISTER REQ Toff IPG
<512 ns 800 ns <400 ns 0 5.6 ns (7 B) 0.8 ns (1 B) 51.2 ns (64 B) <512 ns 9.6 ns (96 b)
60.7.13.1.1 60.7.13.2.1 76.4.2.1 36.3.2.4, 75.7.14 3.2.1 3.2.2 77.3.6.3 60.7.13.11.1 Table 4.2
The values of individual components are given in Table 1. In the worst case we obtain T = 2.2912 s. We ran series of Monte Carlo simulations to estimate the probability of successful ONU registration in the first attempt, POK in the network with N ∈ [2, 64] ONUs. The independent tries included random generation of registration messages by the ONUs and their reception by the OLT; these were repeated until either POK was estimated with 90% of confidence with relative error not greater than 2% [6], or maximum number of 106 runs was achieved. We considered three deployment scenarios. In the first one, the ONUs are equidistant from the OLT, so ti = tj for all pairs (i, j). For the reasons of convenience we adopt ti = 0 which does not influence the generality of our findings. In the remaining two scenarios, the round-trip delays are uniformly distributed on [0, tmax ], with tmax equal to 10 s in the second case or 200 s in the third one. For silica glass fiber with refraction index ≈1.5, these values correspond to the fiber spans of 1 km and 20 km, respectively. While the first scenario can be regarded as pessimistic because numerous collisions are expected and the third one as optimistic but with large fiber span (which could occur only under the extremely rural deployment), we find the “clustered” scenario as the most likely to appear in the real world, as it corresponds to the urban PON deployment. The value of random delay applied by the observed ONU is uniformly distributed on [0, wmax ], with wmax ∈ [1, 500] s. The duration of corresponding discovery slot is then wmax + T and the minimal duration of OLT discovery window is tmax + wmax + T . The estimated probability of successful registration, POK was then used to calculate the registration efficiency U, defined as the ratio of the time effectively needed to transmit the non-collided REGISTER REQ messages to the duration of discovery window: U=
T NPOK . tmax + wmax + T
Fig. 2. Registration efficiency for equidistant (left), clustered (center) and randomly distanced (right) ONUs.
(3)
M. Bjelica, A. Peri´c / Int. J. Electron. Commun. (AEÜ) 66 (2012) 211–213
Fig. 3. Optimal number of ONUs that could be registered within a given maximal wait time.
213
consequently minimize the total time needed for discovery and registration, which then means that more time would be available for the exchange of user data. Closer inspection of Fig. 2 reveals that for both equidistant and clustered scenarios this optimization could be two-way: (i) it is possible to determine optimal number of ONUs that could be served within a given maximal random wait time, or (ii) given a number of ONUs, it is possible to determine optimal maximal wait time. Since utilization surfaces are not symmetrical regarding to N and wmax , these optima will not match. The optimization space for random scenario is more constrained. The saddle-shaped efficiency surface suggests that the only possibility here is to determine optimal number of ONUs that could be served within a given maximal random wait time; approach (ii) would lead to the result wopt = 1 s. These considerations are illustrated in Figs. 3 and 4. The previous observations are expected and can be easily explained as follows. For all deployment scenarios, given a discovery slot duration, the registration efficiency will increase with the increase of the number of ONUs, reach its maximum and then start decreasing. The initial increase is due to the reduced idle periods on the link, which occur when there are few ONUs to register. While their number increases, the contention becomes more apparent and the growing number of collisions causes the link utilization to decrease. When considering equidistant and clustered scenarios, for a given number of ONUs the registration efficiency will firstly increase with the prolongation of discovery slot, which is caused by the reduced number of collisions, and then will start to decrease because of the unnecessarily long link idle periods. In the optimistic case of randomly spaced ONUs, fiber span is so large that the propagation delay range (tmax = 200 s) alone is enough for almost all ONUs to successfully register in the first attempt. Any additional wait time would bring little benefit to the registration, but would unnecessarily keep the link idle. 5. Conclusion
Fig. 4. Optimal maximal wait time for a given number of ONUs.
4. Results Fig. 2 shows the dependence of registration efficiency (U) upon the number of ONUs (N) and the maximal wait time (wmax ). As it can be easily seen, the pessimistic and the optimistic configurations behave significantly different, while the realistic scenario performs almost identically as the pessimistic one. Let us note that registration efficiency is defined in [2,3] as the ratio of the number of successful registrations (NPOK ) to the discovery window duration. We find this definition both dimensionally incorrect and obfuscatory; the result it returns cannot be straightforwardly interpreted as it can easily be greater than 1. Moreover, we were also repeatedly unable to reproduce the results presented in the cited works by using it – our results were larger by the factor of 106 s−1 . The results from Fig. 2 can be used to state the optimization criteria for each scenario, such to maximize the link utilization during the discovery phase. This maximization of the number of ONUs that were successfully registered in the first attempt would
In this letter, we studied the performances of registration procedure in passive optical networks compliant to the new standard IEEE 802.3av. We examined three deployment scenarios through series of Monte Carlo simulations, with registration efficiency (expressed as link utilization) as performance metric. We used the obtained results to derive ad hoc optimization guidelines. The fact that the realistic (clustered) deployment scenario behaved almost identically as the pessimistic one (equidistant ONUs) means that in practical considerations it is justified and sufficient to restrict to the simplified case, instead of using bulky mathematical expressions. References [1] IEEE standard for information technology 802.3av. IEEE, October; 2009. [2] Kramer G. Ethernet passive optical networks. McGraw-Hill; 2005. [3] Bhatia S, Bartoˇs R. Closed-form expression for the collision probability in the IEEE Ethernet Passive Optical Network registration scheme. Journal of Optical Networking 2006;5:1–14. [4] Hajduczenia M, da Silva H. Comparison of collision avoidance mechanisms for the discovery process in xPON. Journal of Optical Networking 2009;8:317–36. [5] Bjelica M. Performance analysis of registration procedure in IEEE 802.3av networks. In: Proc. ECCSC. 2010. p. 47–50. [6] Law A. Simulation modeling and analysis. McGraw-Hill; 2006.
Contents lists available at ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.de/aeue
Allocation of optimal discovery slots in IEEE 802.3av networks Milan Bjelica a,∗ , Ana Peric´ b a b
School of Electrical Engineering (ETF), Communications Department, Bulevar kralja Aleksandra 73, RS-11000 Belgrade, Serbia MDS informatiˇcki inˇzenjering, Bulevar Milutina Milankovi´ca 7d, RS-11070 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 31 March 2011 Accepted 8 July 2011
a b s t r a c t In this letter, we consider ONU registration procedure in symmetrical 10 Gb/s IEEE 802.3av networks. Through series of computer simulations we state the optimization guidelines that lead to the reduction of message collisions and the increase of link utilization. © 2010 Elsevier GmbH. All rights reserved.
Keywords: Communication networks Collision probability Computer simulation Optical fiber networks
1. Introduction Passive optical networks (PONs) are considered as cost-effective mean of providing large scale broadband access. These networks consist of a headend or optical line terminal (OLT) which is connected to a number of user terminals known as optical network units (ONUs). The latest standard in this field, IEEE 802.3av describes PONs capable of delivering up to 10 Gb/s on both uplink and downlink [1]. In these networks, downlink transmission is based on wavelength division multiple access, where separate optical carrier is assigned to each ONU. On the uplink, Ethernet-based time division multiple access is applied. To avoid collisions, each ONU is assigned a distinct time slot in which it can transmit. However, the collisions may occur during the so-called discovery phase, when ONUs try to announce their presence to the OLT and register themselves to the network. In this letter, we analyze the discovery procedure in IEEE 802.3av networks. The analytical treatise of older standard IEEE 802.3ah for 1 Gb/s PONs is given in [2] for equidistant ONUs, while [3] gives more general result. Besides the fact that we consider newer standard with different protocol parameters, we also apply somewhat different methodology. From our on-field experience, the analytical solution from [3] is bulky and virtually unsuitable for engineering applications. A more recent paper [4] regards the 802.3av PONs, but within the scope different from ours. We continue the work started
∗ Corresponding author. ´ E-mail addresses: [email protected] (M. Bjelica), [email protected] (A. Peric). 1434-8411/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2011.07.003
in [5] and use computer simulations to propose ad hoc optimization guidelines, which would reduce message collision probability and improve link utilization.
2. Discovery procedure Discovery is the process through which newly connected or previously off-line ONUs (e.g. for the reason of energy saving) announce their presence to the OLT and register to the network. Discovery is a three-way handshake. To give chance to the unregistered ONUs to register, the OLT periodically opens the discovery windows. An unregistered ONU waits for some random time within its discovery slot and then issues REGISTER REQ message. Upon its successful reception, the OLT replies with confirmation message and schedules the observed ONU for access to the network. Discovery is inherently a contention-based procedure. Since ONUs cannot communicate directly to each other it is possible that more than one of them may attempt to register at the same time, causing their REGISTER REQ messages to collide at the OLT input. The random delay (or wait) time is needed to avoid the persistent collisions which would otherwise occur between the ONUs with (approximately) equal round-trip delays (i.e. those that are equidistant from the OLT). The examples of successful and unsuccessful registration are shown in Fig. 1. It is interesting to notice a similarity between PON discovery procedure and contention in CSMA/CA; however, they perform significantly different because of different delay mechanisms applied [4]. Those ONUs whose REGISTER REQ messages have collided remain unregistered and invisible to the OLT; they must wait for the
212
M. Bjelica, A. Peri´c / Int. J. Electron. Commun. (AEÜ) 66 (2012) 211–213 Table 1 Components of REGISTER REQ transmit time with references to IEEE 802.3(av) specification.
Fig. 1. (a) Successful and (b) unsuccessful ONU registration.
next discovery window and then try to register. The performance of ONU discovery and registration in terms of link utilization is closely related to the collision probability, which depends on the discovery window duration. 3. Methodology Instead of applying strictly analytical approach, we chose to derive the experimental results by simulating the discovery process in several typical network deployments. While this is an ad hoc approach, we expect its results to be more informative and useful to the network designers. Let us recall Fig. 1. Let the round-trip (two-way propagation) delays corresponding to the observed ONUs i and j be ti and tj , and their random wait times wi and wj , respectively. Let the total time needed to transmit a REGISTER REQ message with necessary protocol overhead be T. Two REGISTER REQ messages will collide if following condition holds true:
(ti + wi ) − tj + wj ≤ T.
(1)
In the network of N ≥ 2 ONUs, this condition applies to each pair of them. T must account not only for Ethernet frame duration with REGISTER REQ as its payload, but also for guard intervals needed to complete the transients on both transmitter and receiver side [1]: T = Ton + Treceiver
settling
+ TCDR + Tcode
group align
+|SoF| + |REGISTER REQ| + Toff + |IPG|.
+ |PREAMBLE| (2)
Designation
Value
Reference
Ton Treceiver settling TCDR Tcode group align PREAMBLE SoF REGISTER REQ Toff IPG
<512 ns 800 ns <400 ns 0 5.6 ns (7 B) 0.8 ns (1 B) 51.2 ns (64 B) <512 ns 9.6 ns (96 b)
60.7.13.1.1 60.7.13.2.1 76.4.2.1 36.3.2.4, 75.7.14 3.2.1 3.2.2 77.3.6.3 60.7.13.11.1 Table 4.2
The values of individual components are given in Table 1. In the worst case we obtain T = 2.2912 s. We ran series of Monte Carlo simulations to estimate the probability of successful ONU registration in the first attempt, POK in the network with N ∈ [2, 64] ONUs. The independent tries included random generation of registration messages by the ONUs and their reception by the OLT; these were repeated until either POK was estimated with 90% of confidence with relative error not greater than 2% [6], or maximum number of 106 runs was achieved. We considered three deployment scenarios. In the first one, the ONUs are equidistant from the OLT, so ti = tj for all pairs (i, j). For the reasons of convenience we adopt ti = 0 which does not influence the generality of our findings. In the remaining two scenarios, the round-trip delays are uniformly distributed on [0, tmax ], with tmax equal to 10 s in the second case or 200 s in the third one. For silica glass fiber with refraction index ≈1.5, these values correspond to the fiber spans of 1 km and 20 km, respectively. While the first scenario can be regarded as pessimistic because numerous collisions are expected and the third one as optimistic but with large fiber span (which could occur only under the extremely rural deployment), we find the “clustered” scenario as the most likely to appear in the real world, as it corresponds to the urban PON deployment. The value of random delay applied by the observed ONU is uniformly distributed on [0, wmax ], with wmax ∈ [1, 500] s. The duration of corresponding discovery slot is then wmax + T and the minimal duration of OLT discovery window is tmax + wmax + T . The estimated probability of successful registration, POK was then used to calculate the registration efficiency U, defined as the ratio of the time effectively needed to transmit the non-collided REGISTER REQ messages to the duration of discovery window: U=
T NPOK . tmax + wmax + T
Fig. 2. Registration efficiency for equidistant (left), clustered (center) and randomly distanced (right) ONUs.
(3)
M. Bjelica, A. Peri´c / Int. J. Electron. Commun. (AEÜ) 66 (2012) 211–213
Fig. 3. Optimal number of ONUs that could be registered within a given maximal wait time.
213
consequently minimize the total time needed for discovery and registration, which then means that more time would be available for the exchange of user data. Closer inspection of Fig. 2 reveals that for both equidistant and clustered scenarios this optimization could be two-way: (i) it is possible to determine optimal number of ONUs that could be served within a given maximal random wait time, or (ii) given a number of ONUs, it is possible to determine optimal maximal wait time. Since utilization surfaces are not symmetrical regarding to N and wmax , these optima will not match. The optimization space for random scenario is more constrained. The saddle-shaped efficiency surface suggests that the only possibility here is to determine optimal number of ONUs that could be served within a given maximal random wait time; approach (ii) would lead to the result wopt = 1 s. These considerations are illustrated in Figs. 3 and 4. The previous observations are expected and can be easily explained as follows. For all deployment scenarios, given a discovery slot duration, the registration efficiency will increase with the increase of the number of ONUs, reach its maximum and then start decreasing. The initial increase is due to the reduced idle periods on the link, which occur when there are few ONUs to register. While their number increases, the contention becomes more apparent and the growing number of collisions causes the link utilization to decrease. When considering equidistant and clustered scenarios, for a given number of ONUs the registration efficiency will firstly increase with the prolongation of discovery slot, which is caused by the reduced number of collisions, and then will start to decrease because of the unnecessarily long link idle periods. In the optimistic case of randomly spaced ONUs, fiber span is so large that the propagation delay range (tmax = 200 s) alone is enough for almost all ONUs to successfully register in the first attempt. Any additional wait time would bring little benefit to the registration, but would unnecessarily keep the link idle. 5. Conclusion
Fig. 4. Optimal maximal wait time for a given number of ONUs.
4. Results Fig. 2 shows the dependence of registration efficiency (U) upon the number of ONUs (N) and the maximal wait time (wmax ). As it can be easily seen, the pessimistic and the optimistic configurations behave significantly different, while the realistic scenario performs almost identically as the pessimistic one. Let us note that registration efficiency is defined in [2,3] as the ratio of the number of successful registrations (NPOK ) to the discovery window duration. We find this definition both dimensionally incorrect and obfuscatory; the result it returns cannot be straightforwardly interpreted as it can easily be greater than 1. Moreover, we were also repeatedly unable to reproduce the results presented in the cited works by using it – our results were larger by the factor of 106 s−1 . The results from Fig. 2 can be used to state the optimization criteria for each scenario, such to maximize the link utilization during the discovery phase. This maximization of the number of ONUs that were successfully registered in the first attempt would
In this letter, we studied the performances of registration procedure in passive optical networks compliant to the new standard IEEE 802.3av. We examined three deployment scenarios through series of Monte Carlo simulations, with registration efficiency (expressed as link utilization) as performance metric. We used the obtained results to derive ad hoc optimization guidelines. The fact that the realistic (clustered) deployment scenario behaved almost identically as the pessimistic one (equidistant ONUs) means that in practical considerations it is justified and sufficient to restrict to the simplified case, instead of using bulky mathematical expressions. References [1] IEEE standard for information technology 802.3av. IEEE, October; 2009. [2] Kramer G. Ethernet passive optical networks. McGraw-Hill; 2005. [3] Bhatia S, Bartoˇs R. Closed-form expression for the collision probability in the IEEE Ethernet Passive Optical Network registration scheme. Journal of Optical Networking 2006;5:1–14. [4] Hajduczenia M, da Silva H. Comparison of collision avoidance mechanisms for the discovery process in xPON. Journal of Optical Networking 2009;8:317–36. [5] Bjelica M. Performance analysis of registration procedure in IEEE 802.3av networks. In: Proc. ECCSC. 2010. p. 47–50. [6] Law A. Simulation modeling and analysis. McGraw-Hill; 2006.