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Issue of January 2004 

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Multiservice Metro Networks

Moving towards more flexible architectures

To interconnect its offices at different locations, Indian enterprises lease capacity and infrastructure from service providers. But Multiservice Metro networks are more beneficial than traditional SDH networks says Rajiv Ramaswami, CTO, Optical Networking Group, Cisco Systems. He spoke to Brian Pereira about this upcoming trend.

Why have Multiservice Metro networks suddenly gained significance in the enterprise?

The trend (especially in the US) is that more enterprises are choosing Ethernet over Optical instead of traditional leased lines. They either build their own networks using leased capacity from a service provider or get a customized managed service offering from a service provider.

What are the benefits of Multiservice Metro networks over traditional SDH networks?

There are a couple of reasons why Multiservice Metro networks are favorable to enterprises.

Firstly, these networks support native data and storage protocols such as Ethernet, and Fiber Channel, which are widely used in enterprises. Ethernet service offers granularity of bandwidth. If you buy an E1 line you get 2 Mbps bandwidth. With traditional SDH networks, if one wanted more bandwidth, it meant buying in multiples of 2 Mbps. So you'd buy 2 E1 lines (for 4 Mbps). But what if someone wanted 2.5 Mbps?

So (with traditional SDH networks) you don't have this continuity of bandwidth. But with Metro Ethernet networks, if the service provider offers an Ethernet plug—it's good enough for 100 Mbps. And it can be sliced from 1 to 100, so there's very fine granularity. Then one can also get bandwidth on the fly, without changing the pipe. So instead of buying a 2 Mbps private line or a leased line, you'll be able to buy 1.5 Mbps bandwidth. The ability to have granularity and also be able to change the granularity, is an advantage of Ethernet-based metro networks.

The other major advantage is that on the Ethernet plug you can offer a host of services, like Ethernet private line, simple Layer 2 VPN, Layer 2 VPN with QoS (with a guaranteed bandwidth), or a service with Layer 3-aware SLA (such as Voice over IP). So it presents significant benefits to both service providers and enterprises. Service providers can offer a range of services (they can charge differently for different services), and enterprises have more choice in getting a service tailored for their needs.

Ethernet private line is being offered in India today, but more advanced services (like Layer 2 VPN with QoS) will be provided soon.

Cisco sells equipment that supports all these services today on an optical platform.

Other vendors support TDM services and mostly offer Ethernet private lines.

An enterprise customer should be able to buy more bandwidth for the same price. For the customer, connectivity costs are going down so they get lower cost per megabit. Also, customers can buy more flexible services, with reliable SLAs for each of the services.

Can you explain why Storage is a key driver for the adoption of Metro Ethernet networks?

Most storage is going to be networked, and the amount of storage on the network is going to go up.

Storage applications that support Business Continuance may not have taken off in India in a big way. People are now focused on having multiple data centers. There's a whole range of applications.

At the low-end, small businesses just need to create back-ups. But at the high end, the banking and financial institutions need to have synchronous mirroring and mirrored data centers. The data that's captured needs to be mirrored in real-time locally, and at a remote site (as in stock exchange transactions).

What's your technique for transporting Ethernet and Storage protocols? What are the emerging trends in this area?

Multi-service provisioning platforms are optical transport boxes that can transport Ethernet and storage protocols efficiently in addition to being able to deliver the traditional leased line services. Within our optical boxes we incorporate packet multiplexing and a QoS engine. This is the same engine that sits inside our routers. This enables us to offer the new services we talked about, as well as reduce the cost of the transport infrastructure, compared to a purely circuit-switched SDH infrastructure.

Finally, we have to make SDH more efficient to carry data. There's more innovation in this area, like more efficient multiplexing mechanisms. SDH deals with time slots. Each time slot offers a certain chunk of bandwidth. Virtual concatenation enables you to utilize the time slots more efficiently.

The core of the network is realized using IP/MPLS routers. Now the SDH boxes at the edge have to talk to the routers, particularly with respect to QoS. The QoS that we have in the SDH boxes is exactly the same as that in the routers. The software (Cisco IOS) in our routers and optical boxes is exactly the same.

On the storage side, the top companies have built their own DWDM networks as disaster recovery is really crucial to them. This is for very high capacity connections (10 gigabits or more). Then there are the companies in the mid-tier. They will lease capacity from a service provider. On this SDH box we opt for a native fiber channel plug. You can take a fiber channel fabric switch and connect it to an SDH box. This is yet to happen here although it is happening in the US.

So the service provider can sell you a fiber channel service instead of selling you a private line. We are one of the few companies that can offer native fiber channel on an SDH box. Though fiber channel and storage is a niche or high-end of the market, every business will have Ethernet and IP. So fiber channel over Ethernet/IP is what will really take off here.

The other trend coming in is managed services. Enterprises are outsourcing the management of its network infrastructure, end-to-end. We want to exploit the trend from moving away from private line or TDM services to packet-oriented services.

Which technology are you in favor of: SDH or DWDM? What are the limitations of each, and how are these being addressed?

We do both. They are complementary technologies catering to different capacity requirements. Our optical box is called MSPP (Multi Service Provisioning Platform).

SDH is a very mature technology. The main limitation for SDH has to do with how efficiently it could transport data. So it's a matter of optimizing SDH to carry data.

On the WDM front, some of the things that are being addressed here are the flexibility in the platforms. Right now we are talking about doing the multi-service platform and the WDM, all in one. Earlier there were standalone platforms (separate box for WDM, separate box for SDH.)

The second thing is being flexible with the distances. Previously WDM was optimized for specific distances. Now you are seeing a broader spectrum of applications. Earlier you had to buy one box for 50 Kms and another for 500 Kms. Now you get modular platforms that can operate over a range of distances (up to 1,000 Kms for a metro system).

How have the new systems introduced flexibility in the way the network is engineered?

Earlier with WDM you had to accurately predict the capacity, the channels to be dropped, the lambdas to be dropped etc. But this could never be accurately predicted. The older systems were very rigid.

The new systems have that capability. We are moving towards more flexible architectures where you don't have to make that decision up front. You can add whatever channels you want even later. Flexible Optical add-drop multiplexers make this possible.

WDM has multiple wavelengths. So for every wavelength you needed a different line card. For 32 wavelengths you need 32 different line cards. This is difficult to manage. Then you need to keep another set of spare line cards. The solution to this is Tunable Lasers—lasers that can operate at multiple wavelengths. This dramatically reduces all these costs. Today we sell 4 and 8 channel tunable lasers. So I only need to stock 4 or 8 part numbers instead of stocking 32 different part numbers.

Our routers and switches come with pluggable optical interfaces. So you buy a switch or line card that does not have the optical transceiver on it. When you want to enable ports you plug in the optical transceivers. The advantages are a lower up-front cost, a more modular solution, and the ability to customize the card by plugging in different types of tranceivers (different distances, coarse or dense WDM).We recently introduced a DWDM pluggable.

Brian Pereira can be reached at

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