The key thing to remember about MPLS is that it’s a technique, not a service — so it can be used to deliver anything from IP VPNs to Metro Ethernet services. So although carriers build MPLS backbones, the services that users buy may not be called MPLS. They could be called anything from IP VPN to Metro Ethernet — or whatever the carriers’ marketing departments dream up next.

The fundamental concept behind MPLS is that of labeling packets. In a traditional routed IP network, each router makes an independent forwarding decision for each packet based solely on the packet’s network-layer header. Thus, every time a packet arrives at a router, the router has to “think through” where to send the packet next.

With MPLS, the first time the packet enters a network; it’s assigned to a specific forwarding equivalence class (MPLS path through the network), indicated by appending a short bit sequence (the MPLS label) to the packet. Each router in the network has a table indicating how to handle packets of a specific FEC (forwarding equivalence class) type, so once the packet has entered the network, routers don’t need to perform header analysis. Instead, subsequent routers use the label as an index into a table that provides them with the appropriate MPLS tunnel for that packet.

This gives the MPLS network the ability to handle packets with particular characteristics in a consistent fashion. Packets carrying real-time traffic, such as voice or video, can easily be mapped to low-latency routes across the network. The key architectural point with all this is that the labels provide a way to “attach” additional information to each packet so the best routing decision can be made without any delays.

Layer 2 or Layer 3?

There’s been some of confusion over the years about whether MPLS is a Layer 2 or Layer 3 protocol, but MPLS doesn’t fit neatly into the OSI seven-layer hierarchy. In fact, one of the key benefits of MPLS is that it separates forwarding mechanisms from the underlying data-link service. MPLS can be used to create forwarding tables for ATM or frame relay switches or for plain old IP routers by appending MPLS tags to IP packets.

The bottom line is that network operators can use MPLS to deliver a wide variety of services. The two most popular implementations of MPLS are layer 3 BGP/MPLS-VPNs (based on RFC 2547) and Layer 2 (or pseudowire) VPNs.

RFC 2547 VPNs have been implemented by most of the major service providers, including AT&T, Verizon, BT and many others. The fundamental characteristic of a 2547 VPN is that traffic is isolated into MPLS-VPNs as it enters the network.

Interior routers have no knowledge of IP information beyond the label-only base forwarding decisions on the MPLS label. BGP is used by edge routers to exchange knowledge of VPNs, thus enabling service providers to isolate traffic from multiple customers or even the Internet over a shared backbone.

There are several flavors of layer 2 MPLS services, but what they have in common is that a Layer 2 packet (or ATM cell or frame relay frame) is encased in an MPLS header and forwarded through the MPLS core. When it reaches the other side, the packet’s labels are removed, and the packet that arrives at the ultimate destination exactly where it entered the MPLS network. Thus, Layer 2 MPLS services effectively extend services such as Ethernet or frame relay across an IP WAN.

What are the different types of MPLS?

The version of MPLS that’s generally used to encapsulate connection-oriented frame relay and ATM services is called pseudo Wire Edge to Edge Emulation (PWE3). PWE3 defines point-to-point tunnels across the MPLS backbone, and thus works well for circuit-oriented networking protocols. PWE3 can also be used to support connectionless LAN protocols, but only for point-to-point circuits.

For connectionless protocols (mostly Ethernet) there’s a different specification, called virtual private LAN service (VPLS). VPLS addresses some of the specific challenges with extending Ethernet across the metropolitan area or WAN, most notably scalability and multipoint circuits. Another emerging spec is the ITU’s transport-MPLS (T-MPLS), which is designed to simplify deployment of Ethernet services

PBT is based on using existing IEEE 802.1 VLAN tags to deliver Ethernet services across a provider network. PBT competes head-to-head with T-MPLS, however VPLS and T-MPLS appear to be the standards of choice for most carriers.

Finally, a variant of MPLS called Generalized Multiprotocol Label Switching (GMPLS) gives routers the ability intelligently signal the optical layer, enabling providers to establish, change or tear down optical links in real time. Thus, service providers can signal the provisioning of “optical wavelength” services using MPLS.

Related posts:

1. New Switch/Router Platform Search
2. Network Upgrade Update

Four new POP switches being configured

We have completed installation of our new switches at five of our POPs (points-of-presence, i.e. towers or buildings that customers connect to). We have fourteen left to go and we’re doing four per week. This upgrade process is a massive undertaking and we’re trying to have everything done by the middle of summer. Here’s how we’re splitting it out:

Physical switch upgrades

We plan to have these done by the middle of March. This is a plug-and-play process, but it takes a lot of man-hours to prep the switches and visit all those sites in the middle of the night. We spent a lot of lab time perfecting our plan and the ones we’ve done so far have gone almost flawlessly, so we’re pretty happy with the process.

Upgrade to MPLS

Once all the switches are in place we will begin rolling out MPLS on customer transport circuits. This will be a customer-by-customer process that will take place over several days. We will schedule maintenance windows, but there should be very little customer impact. Once this process is completed all of our layer-2 transport services will use MPLS for encapsulation. I believe the initial change over to MPLS should be done by the middle of April

Microwave Backhaul Upgrades

I can’t decide if the switches or the backhauls are more exciting. I love the changes that the switches bring, but once we’re done with the backhaul upgrades we’ll be able to provide multi-hundred megabit connections to the majority of our network. You want a 200Mbps internet pipe in Wilsonville? OK, no problem. How ’bout a 300Mbps transport circuit from Newberg to Gresham? Done! We’ll be able to do it all on the backhauls that we’ll already have in place!

The backhaul upgrades require far more labor than the switches. Our tower crews have to hoist heavy antennas up the towers, run new cabling, aim the links, and then we finally have to integrate them into the network without causing any service disruptions. We’re experts at all of these things, but they take time. We aggressively plan to upgrade 3-5 backhauls per month and we hope to be done in August.

Related posts:

1. New Switch/Router Platform Search
2. MPLS-in a nutshell

After an intense research and interview process we finally have made a decision. Our new platform will use MPLS to pass all layer-2 transport traffic rather than the current VLAN-based system. This new architecture will provide significant improvements in scalability and efficiency to the network.

Image courtesy of Wordle

Related posts:

1. MPLS-in a nutshell
2. Network Upgrade Update

We describe our service as “Fixed Wireless” to help clarify what we do, but there are still many misconceptions that people have about it. This mini-series will explore the top 4 misconceptions that people have of Freewire’s Fixed Wireless service.

Misconception #1: “It’s like Clear”

Clear (the mobile WiMAX provider operated by Clearwire) utilizes 802.16e mobile WiMAX equipment to provide services to homes and very small businesses. Their network is similar to ours in many ways, but the way they use it to provision services is very different. Here’s a comparison:

Line-Of-Sight

Clear uses short-distance, non-line-of-sight RF links from their towers to the customer radio. We use medium to long distance, direct line-of-sight RF links. The huge difference here is the line-of-sight connection. Having clean RF line-of-sight allows us to guarantee performance with an SLA. Our signals don’t have to deal with swaying trees and cars driving in front of the antennas.

Mobile vs. Fixed

Clear uses indoor and mobile customer radios. We only use outdoor fixed customer radios. Using fixed equipment mounted outdoors gives us a much better signal-to-noise ratio, allowing greater throughput. Since our radios are not mobile we can be sure of clean line-of-sight and consistent performance.

Asymmetric vs. Symmetric

Clear’s WiMAX equipment is designed and configured for asymmetric upload and download speeds, resulting in slower uploads than downloads. We design and configure our equipment for symmetric speeds, resulting in the same performance on upload traffic that you see on download traffic.

Oversubscription

Clear oversubscribes much more heavily. The oversubscription levels on cable, DSL, and other non-SLA networks (including Clear’s) are much higher than our oversubscription levels. Yes, we oversubscribe, every internet provider does. If you ever see a company saying “we don’t oversubscribe” it’s either a blatant lie or a partial truth. We strive to oversubscribe only to the point where our systems are efficiently utilized, not to the point of reduced speed (like the non-SLA providers)

Tech Support

Clear’s tech-support phone-line is only available 9AM-10PM. Our tech-support is 24/7/365.

Service Level Agreement

Clear’s service is not backed by an SLA (that I am aware of). When they go down, or when the speed is slow, their customers have no financial recourse. Our service is supported by an amazing SLA that guarantees significant payment to our customers if our network has problems. It’s the best SLA I’ve ever seen, and I’m confident that we can deliver on it (I wrote it and designed the network, so I should know).

Freewire Is Built For Business

As you can see, there is just no comparison. While Clear and Freewire use similar architectures, the service and usage methodology is completely different.

If you’re looking for service at home, call our friends at Whiz to Coho or Upward Access, and if they can’t serve you then call Clear.

If you want mobile or nomadic service, call Clear. They have spent significant amounts of money and effort establishing an impressive network to be able to provide true mobile WiMAX service.

Finally, if you’re looking for service at your business, then call us. There is no other wireless provider in the region that can provide better performance and reliability for your business needs.

In part 2 of this series I will discuss the second common misconception—”It’s not secure, just like WiFi”. You’ll learn how our service differs from WiFi and what measures we have in place to keep the network secure.

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