Home > Articles > Networking

  • Print
  • + Share This
This chapter is from the book

3.7 Comparative Advantages of Different Layers for Survivability

The layered view we have just worked through allows us to see that survivability measures at each layer are for the most part complimentary, not competitive. Physical layer measures are essential and service layer measures always help. And we should always have at least one technique implemented at the system or logical layers but there is really no need to employ both, especially if cost is considered. An important planning decision is thus whether to employ a system layer or a logical layer recovery scheme. Two of the main factors in this decision are flexibility and efficiency. With rings, or 1+1 diverse routing, there will be an investment of over 100% in redundant transmission capacity because (by definition) both diverse routes cannot be equally shortest routes. With logical layer mesh alternatives this may often be reduced to 50-70% redundancy. In addition, complete flexibility exists with an OXC-based (i.e., logical layer) implementation (1) to adapt the protection stance to changing demand patterns, (2) to evolve the entire protection strategy from one scheme to another and/or, (3) to implement prioritized protection service classes. A summary of other comparative aspects is offered in Table 3-7.

Table 3-7. Comparative Strengths and Weaknesses of Layers for Survivability

Attribute

Transmission System Layer

Logical Cross-Connection Layer

Services (or IP Transport) Layer

example:

BLSR Rings

Span Restoration

MPLS SBPP

Capacity Required

Highest

Middle

Least

Speed

Highest (~50 ms)

High (~ 100–300 ms typ.)

Slowest (seconds–minutes)

Certainty / predictability

Highest

High

Lower

Multiple Quality of Protection (QoP)

None

Easily supported on per path basis

Easily supported

Provisioning view (working)

Ring-constrained shortest path

Shortest path

Shortest path coordinated to be disjoint with protection

Provisioning view (survivability)

Inherent once routed

Checked upon shortly after routing

Coordinate protection sharing arrangements network-wide

Degradation characteristics (if restoration fails)

Abrupt and total outage

Abrupt on affected channels, may be partial

More graceful degradation; congestion not outage

Oversubscription strategies

No

SONET or WDM: no ATM VP: yes

Yes

Customer control

Least

Through VPN services

Most

Database and protocol dependencies

Least: a "hardwired" implementation

Little: event-driven protocols in firmware interacting on overhead bytes, network state is database

Highest–large databases of global network state, dissemination protocols, software dependent

Susceptibility to SRLG effects and fault escalation

Least, controlled during planning

Low, especially with adaptive distributed restoration

Highest vulnerability to SRLG effects and physical-to-logical fault expansion

Multi-Layer Protection: Containing the Inheritance of Dependencies

In thinking about the different layers where we can implement survivability, the issue of physical to logical fault multiplication is critical. Adequate knowledge of SRLG relationships may be extremely difficult to obtain (or maintain) if there are several steps of the emergence and inheritance of failure dependencies, to use the terms introduced in [OePu98]. At every layer of routing abstraction, new fault dependencies emerge and are inherited by all higher levels. The growth in complexity of determining physical diversity between paths as one goes higher up the hierarchy from physical toward service layers is conveyed in Figure 3-14, based on [OePu98]. Graph G shows the layout of cables which in this case involves some degree 1 nodes. As mentioned, a first step is to create a biconnected physical graph. Doing so in this example would remove some of the dependencies in G' emerging from G, but not all. Even when G is biconnected, dependencies between transmission systems are impossible to avoid as long as the systems are allowed to pass through nodes without terminating. They are especially frequent if least-cost routing of each system is desired. For example, we could close G with respect to stub-node 7 by adding a cable (7-8) but transmission systems (6-5) and (7-5) would likely remain dependent because span (5-7) in G is on both of their shortest routes. Observe also that node 11 is a junction in the cable graph but has no corresponding appearance in the higher level logical graphs. This is the classic case of a common duct (here, 5-11) creating dependency between what are otherwise viewed as separate transmission spans (6-5) and (7-5) in G'. When one routes lightpaths over theses transmission systems, still further dependencies emerge where lightpaths share transmission systems and the prior dependencies from the cables to systems layer are inherited.

03fig14.gifFigure 3-14. Illustrating the fault dependencies that emerge and are inherited by higher levels (adapted from [OePu98]).

The example in Figure 3-14 goes up only two layers above the cables and considers only eight top-layer nodes. In practice if the G'' shown is the lightpath service layer, then service paths at the STS-3c level routed over them have at least one or two more layers of dependency emergence and inheritance. The overwhelming impression, extrapolating from Figure 3-14 as a simple example, is that it may be difficult to give a robust assurance of full survivability against a cable cut if operating higher up in the hierarchy. Diverse STS-3c level paths would be able to protect the corresponding service against same-level failures or a single lightpath failure (one layer down), or perhaps also a single transmission system failure (two layers down). For example, STS-3c level 1+1 diversity can protect against an STS-3c interface port failure on the host router or against a lightpath failure (including access multiplexing) one layer below. But at three layers of reach-down (to the cables) it seems far less plausible that we would always be certain that STS-3c primary and backup paths would have no inherited dependencies.

In practice this a compelling reason to use protection strategies at the service layer and at either the system or logical layers. Diversity measures at one level can realistically be expected to protect against single failures with known dependencies one or two levels below, or at the same level, but it is probably unrealistic to expect a services layer diversity mapping to retain complete physical disjointness more than two layers below. One set of options to consider is rings, p-cycles, or mesh protection implemented with whole-fiber cross-connects directly over a biconnected physical cable graph, G. In this case there are no emergent dependencies to be inherited by higher layers since each cable span becomes a directly protected single-failure entity. This is simple, robust, and requires relatively low-cost devices for whole-fiber protection switching. It is not very fine-grained, however, and the devices used for protection have no secondary use such as for dynamic service provisioning (other than provisioning dark fiber services).

Alternately, logical layer measures implemented in G' using cross-connects for mesh-protection at the channel level are more agile, multi-purpose, and fine-grained in capacity-handling and only have to cope with one level of known dependencies, such as arise in G' from the cable junction node 11 in G. Using SBPP in G'' (instead of a protection scheme in G') is not infeasible, but we see a major complexity associated with this alternative because now we are two layers above the level at which physical faults occur—so the complete map of dependencies is far more complex. To go yet another layer up and rely on MPLS autoreprovisioning or MPLS-level SBPP, it becomes hard to imagine that we could support a claim of protection (implemented at that level) against failures stemming from the physical layer, because G is a full three levels below the MPLS layer. This all suggests a practical principle that the emergence and inheritance of SRLG-like effects may need to be "contained" by an appropriate protection arrangement every two layers. If followed, this leads to a strategy of choosing some basic "infrastructure" protection scheme at the system or logical layers and complimenting it (possibly only for high priority service paths) with an additional technique at the corresponding service layer itself. For example system level p-cycles and service level 1:1 APS would be one combination. Lightpath level SBPP complimented by MPLS layer p-cycles would be another viable combination, and so on. Note in this regard that even in an ideal "IP over WDM" network, the three layers (G, G', G'') in Figure 3-14 all still exist. Where the reduction of levels occurs in IP over WDM is actually in the levels above the lightpath layer.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020