VoIP on MPLS

While MPLS technology has been around for several years, businesses are now taking advantage of service provider offerings and beginning their own corporate implementations. It's a good idea for network architects and engineers to get a head start on the technology.

Multi-protocol Label Switching (MPLS) is a standards-approved technology for speeding up network traffic flow and making it easier to manage. In addition to moving traffic faster overall, MPLS is flexible, fast, cost efficient and allows for network segmentation. While MPLS technology has been around for several years, businesses are now taking advantage of service provider offerings and beginning their own corporate implementations....

It's a good idea for network architects and engineers to get a head start on the technology.

MPLS

This "tunnel-less" or connectionless method of creating a virtual private network can be difficult to understand because of the lack of a point-to-point connection.

Definition: Multiprotocol Label Switching (MPLS) is a standards-approved technology for speeding up network traffic flow and making it easier to manage. MPLS involves setting up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to. MPLS is called multiprotocol because it works with the Internet Protocol (IP), Asynchronous Transport Mode (ATM), and frame relay network protocols.

With reference to the standard model for a network (the Open Systems Interconnection, or OSI model), MPLS allows most packets to be forwarded at the layer 2 (switching) level rather than at the layer 3 (routing) level. In addition to moving traffic faster overall, MPLS makes it easy to manage a network for quality of service (QoS). For these reasons, the technique is expected to be readily adopted as networks begin to carry more and different mixtures of traffic. (Whatis.com)

In a tip from Iftekhar Hussain on SearchNetworking.com, an MPLS-based network is described as consisting of "...routers and switches interconnected via transport facitilies such as fiber links. Customers connect to the backbone (core) network through multiservice edge (MSE) routers. The backbone comprises the core routers that provide high-speed transport and connectivity between the MSE routers. An MSE router contains different types of line cards and physical interfaces to provide Layer 2 and Layer 3 services, including ATM, FR, Ethernet, and IP/MPLS VPNs."

In the incoming direction, line cards receive packets from external interfaces and forward them to the switching fabric. In the outgoing direction, line cards receive packets from the switching fabric and forward them to the outgoing interfaces. The switching fabric, the heart of the router, is used for switching packets between line cards.

The IP/MPLS control-plane software, the brain of a router, resides in the control processor card. The phrase IP/MPLS control plane refers to the set of tasks performed by IP routing and MPLS signaling protocols. IP routing protocols are used to advertise network topology, exchange routing information, and calculate forwarding paths between routers within (intra) and between (inter) network routing domains.

Examples of IP routing protocols include Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), and Border Gateway Protocol (BGP). MPLS signaling protocols are used to establish, maintain, and release label-switched paths (LSP). Examples of MPLS signaling protocols include BGP, Label Distribution Protocol (LDP), and Resource Reservation Protocol (RSVP). The IP control plane may also contain tunneling protocols such as Layer 2 Tunneling Protocol (L2TP) and Generic Routing Encapsulation (GRE).
-- Iftekar Hussain, "Understanding MPLS network components"

The MPLS-VPLS Resource Center offers a similar definition of the protocol that details a basic step-by-step path of how a packet travels on a MPLS network.

Further MPLS reading resources:
In his tip, "Migrating to MPLS, part 2", Dan Downer explains a typical MPLS site and the common features that can be deployed in a variety of environments.

What is the relevance of MPLS in regards to QoS for e-commerce? VoIP guru and VoIP expert on SearchVoIP.com's Ask the Expert section, Carrie Higbie highlights MPLS Resource Center's excellent MPLS compiliation, with particular emphasis on important vocabulary.

Pros and Cons of an MPLS network

Like any significant business decision, a number of qualifying factors usually drive a potential migration to MPLS. Several common reasons are:

  • Converged services capabilities (voice, video, data).
  • Any-to-any connectivity without the high cost of individual circuits.
  • Advanced features for ingress and egress routing policies (load sharing, policy routing).
  • Secure flexibility of adding future businesses and partners (multiple VPN support).
  • Circuit consolidation (frame, T-X, ATM).

-- Doug Downer, "Migrating to MPLS"

MPLS Quality of Service
QoS on an MPLS backbone is used to provide predictable, guaranteed performance metrics required to transport real time and mission critical traffic. The providers have an overall QoS architecture that is used to deliver a subset of QoS services to each customer. -- Robbie Harrell, "MPLS QoS models"

MPLS supports the same QoS capabilities as IP. These mechanisms are IP Precedence, Committed Access Rate (CAR), Random Early Detection (RED), Weighted RED, Weighted Fair Queuing (WFQ), Class-based WFQ, and Priority Queuing. Proprietary and non-standard QoS mechanisms can also be supported but are not guaranteed to interoperate with other vendors.

Since MPLS also supports reservation of Layer 2 resources, MPLS can deliver finely grained quality of service, much in the same manner as ATM and Frame Relay. DiffServ can support up to 64 classes while the MPLS shim label supports up to 8 classes. -- MPLS Resource Center

Further reading on QoS for MPLS:
QoS for IP/MPLS Networks, Chapter 2
QoS for IP/MPLS Networks by Santiago Alvarez is a practical guide that will help you facilitate the design, deployment, and operation of QoS using Cisco IOS Software and Cisco IOS XR Software. The book provides a thorough explanation of the technology behind MPLS QoS and related technologies, including the different design options you can use to build an MPLS network with strict performance requirements. http://searchnetworking.techtarget.com/generic/0,295582,sid7_gci1193836,00.html

Carrier MPLS support for VoIP
Implementing MPLS can help enterprises to achieve QoS for reliable voice transmissions. This article discusses how today's network transport carriers provide support for VoIP with their MPLS offerings. It will also explain the best way to leverage a carrier's MPLS services to support the deployment of real-time services.

MPLS and ATM 

Definition: ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.

Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).
-- Whatis.com

MPLS is useful as an enabler for VoIP because it provides Asynchronous Transfer Mode (ATM)-like capabilities with an IP network. Unlike the expensive ATM links that would be required to support VoIP, MPLS provides guaranteed services utilizing IP quality of service on the carrier's backbone. This service and the ability to converge VoIP onto the data network present a tremendous opportunity to reduce operational costs and consolidate infrastructures.
-- Robbie Harrell, "Carrier MPLS support for VoIP"

MPLSRC.com explains the role of ATM for MPLS, "MPLS brings the traffic engineering capabilities of ATM to packet-based network. It works by tagging IP packets with "labels" that specify a route and priority. It combines the scalability and flexibility of routing with performance and traffic management of layer 2 switching. It can run over nearly any transport medium (ATM, FR, POS, Ethernet...) instead of being tied to a specific layer-2 encapsulation. As it uses IP for its addressing, it uses common routing/signaling protocols (OSPF, IS-IS, RSVP...). But MPLS was not designed to replace ATM though the practical reality of the dominance of IP-based protocols coupled with MPLS's inherent flexibility has led many service providers to migrate their ATM networks to one based on MPLS."

"MPLS can co-exist with ATM switches and eliminate complexity by mapping IP addressing and routing information directly into ATM switching tables. The MPLS label-swapping paradigm is the same mechanism that ATM switches use to forward ATM cells. For ATM-LSR the label swapping function is performed by the ATM forwarding component. Label information is carried in the ATM Header, specifically the VCI and VPI fields. MPLS provides the control component for IP on both the ATM switches and routers. For ATM switches PNNI, ATM ARP Server, and NHRP Server are replaced with MPLS for IP services. The ATM fowarding plane (i.e 53-byte cells) are preserved. PNNI may still be used on ATM switches to provide ATM services for non-MPLS ports. Therefore, an IP+ATM switch delivers the best of both worlds; ATM for fast switching and IP protocols for IP services all in a single switch."
-- MPLS Resource Center.

Further reading on the pros and cons of MPLS:
MPLS -- what voice managers need to know
Multiprotocol Label Switching (MPLS) is a rapidly growing connectivity choice for enterprises using VoIP. In this tip, Tom Lancaster briefly discusses some of the basic issues voice managers will encounter learn what they need to know about MPLS.

Interconnecting MPLS clouds
MPLS providers are constrained by the scope of their MPLS backbones. Similar to the early days of Frame and ATM when providers could only offer services out of the locations that were interconnected on their backbones, MPLS today faces the same restrictions. In this tip, Robbie Harrell explains some of the issues that face MPLS from gaining wide usage.

What are the advantages and disadvantages of MPLS?
VoIP expert, Carrie Higbie, summarizes advantages and disadvantages of MPLS for one company looking to expand the communications solution to remote offices spread across a country.

Is MPLS really necessary? What will MPLS do for your VoIP network?

"If we wanted to get the quality that we wanted, we had to go with MPLS," said IFC CIO Jim Norman. "We wanted reliability, convenience and efficiency." MPLS provides the solution one company needed. "VoIP, MPLS converge financial firm's network". In this article, a financial services company successfully rolled out VoIP on a converged MPLS network. Norman said he's noticed a productivity boost as well, because the phones and PCs communicate. "He's also been able to generate comprehensive call reporting logs and track calls from every location."
-- Jim Norman, from "VoIP, MPLS converge financial firm's network"

"MPLS transport options" by Robbie Harrell offers a list of important considerations once the necessity for a new telephony systems. Learn options for MPLS transport and the criteria you should consider when deciding among them in this tip. Author Robbie Harrell also takes a technical look at how carrier transport architecture affects the access links that connect to the MPLS backbone on your network.

Choosing your MPLS transport 
Why is the choice of MPLS transport important? As mentioned above, you can use ATM, frame and Ethernet, as well as private line, Broadband DSL and others. The choice of one or the other depends on multiple factors, and an analysis is required to determine the best fit.

  1. First of all, it makes sense -- if possible -- to utilize the same WAN technology you use today. This may not be possible, but it should be the first criterion analyzed. Using the same transport alleviates problems such as having to change interfaces on routers and avoids migration from one technology to another for local access. If the routers do not support the interface type, complete router upgrades may be required, at significant cost.
  2. A second factor is bandwidth. The fact that a customer is migrating to MPLS makes feasible the converging of voice, video and data onto a single WAN backbone. Voice and video will increase the bandwidth requirements significantly. It is a good idea to provision the right size of access transport or to use a transport with flexible upgrade paths to higher bandwidth.
  3. Another factor is carrier reach. ATM and frame are widely available almost anywhere in the world, and the costs are well known. With MPLS, you can get the ATM and frame local loops, but the MPLS PoP that these terminate on may be hundreds of miles away from your location. Ethernet may not be available in some areas. The costs of backhauling one access over another should be considered. Most carriers recommend one access only, although some may support multiple access types for customers.

-- Robbie Harrell, from "MPLS transport options"

Further reading on the necessity of MPLS:
While MPLS, as a technology, itself is mature, commercial MPLS offerings are still in the ramp-up stage, according to Forrester's Pierce. For instance, few providers have fully meshed MPLS backbones, which means that IP packets may be forced to make several hops to reach their destination. This can affect the network's overall ability to recover and route around problems and delivery times. In a white paper from CIO Decisions, "The Next-Gen Network," Elisabeth Horwitt elaborates on MPLS outpacing frame relay in prioritizing traffic, increasing bandwidth and, often, cost. Just don't underestimate what's involved in deploying it.

Selecting an MPLS provider: Key questions to ask

A MPLS VPN that needs to be implementated, for most businesses, means contracting with a service provider. Learn about the important aspects to consider and what you should demand from a service provider before you embark on an MPLS migration.

Podium: MPLS: Bringing Brand Mojo to All Company Sites
MPLS networks are boosting communications among enterprises workers -- increasing productivity and creativity among employees.

Further reading on how MPLS can improve voice communications:

Carrier MPLS support for VoIP
Implementing MPLS can help enterprises to achieve QoS for reliable voice transmissions. This article discusses how today's network transport carriers provide support for VoIP with their MPLS offerings. It will also explain the best way to leverage a carrier's MPLS services to support the deployment of real-time services.

Is a MPLS network really secure?

VoIP and MPLS: Making it work for your company
MPLS-based networks and VoIP are two of the hottest topics for networking professionals worldwide. Virtually all enterprises are either evaluating VoIP and/or MPLS or have begun deploying the technologies, however many enterprises have experienced serious difficulty in deploying these solutions successfully.

This webinar from Fluke Networks will highlight how pre-assessment and post-deployment visibility can help enterprises understand the impacts of VoIP and MPLS and how to deploy successfully. -- Webtorials.com

Understanding Private IP/MPLS-based Networks
As an enterprise, you must decide if the benefits associated with private IP - improved disaster recovery, application prioritization, a fully-meshed infrastructure and reduced complexity - should be the catalyst for migrating from a frame relay or ATM network. Join Fluke Networks for an in-depth discussion on how to optimize the benefits associated with private IP, while limiting the challenges of migrating.

MPLS VPNs: What's Next (webinar)
MPLS VPNs create a robust platform for converged services allowing for cost-effective, any-to-any connectivity. As convergence moves to the next stage, distinctions between wireless and wired networks are blurring, and collaboration is becoming part of the holistic networking environment. Attend this webcast to learn how the network is becoming more intelligent and application-aware, reaching toward a dynamic, unified IT environment that will be more cost effective, agile, and high-performing.

The lines between wired and wireless networks are quickly blurring with MPLS VPNs providing a robust platform for converged services. Attend this webcast to learn more about how networks are becoming more cost effective, agile and high performing.

This was first published in April 2007
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