everal reasons. The trick is to achieve this benefit while preserving some of the key features of routing.
One aspect that makes switching attractive is that it
saves money because you retain your desktop investment. You can still use the same Ethernet or Token Ring adapter cards that have been sitting in your desktop systems all the while. You simply swap out your existing hubs and install switched hubs.
A second advantage to adding switching technology to a network is that everyone gets more bandwidth. Rather than all users on the same LAN segment sharing 4, 10, or 16 Mbps (depending on the type of LAN that they are connected to), each user can get up to the full segment of bandwidth dedicated to their connection.
Additionally, backbones that employ switching technology are easier to manage. That's because switches typically replace routers in such environments. And switches, which are essentially multiport bridges, are much easier to manage than routers.
But since switches are essentially multiport bridges, they do not typically perform routing at the network layer--something you might want to take advantage of. For example, you might want to
contain traffic from applications running over chatty protocols to specific segments of a network. Or you might want to route between subnetworks.
ATM Steps Up
One way of deploying switching in an organization that seems to be taking hold is to use Ethernet or Token Ring switching in the workgroup and asynchronous transfer mode
(ATM) switching
in the backbone. This lets an organization retain its desktop environment and offers a switched, high-speed backbone.
The drawback of using this combination is that truly taking advantage of ATM in such a situation requires some work. There are basically two issues to deal with: One has to do with the way packets are routed, and the other deals with gaining access to ATM services.
One approach to carrying LAN traffic over ATM is LAN emulation (LANE). With LANE, LAN packets are converted to ATM cells so they can pass over the backbone. LANE is pretty much the simplest way to merge LAN and ATM networks.
The downside to LANE is that it requires routers to handle traffic that must pass between different subnetworks.
Often companies use switches and routers together, and the routers confine the traffic to portions of the network and let only the appropriate traffic pass to other subnetworks. A more interesting approach is to include the routing functions with the switch. A number of switch vendors' products allow you to sort and filter based on network-layer information within each packet. However, one limitation to such an approach is that you cannot take advantage of ATM's quality of service (QoS) feature.
Quality Is Job One
In a pure ATM network, QoS lets an administrator associate a QoS level with different types of traffic. The idea behind QoS is that you can give some types of traffic priority over other types.
For example, suppose you have a network that supports videoconferencing and large file transfers for an engineering group using CAD/CAM and also carri
es transaction-processing-based traffic from telemarketers taking phone orders from customers. If there are no priority levels, the engineers might clog the network when sending many files at once.
That wouldn't do wonders for the videoconferencing application, where delays produce noticeable jumps from frame to frame. And customers placing orders over the phone won't be happy if it takes the telemarketing folks a long time to confirm that each item is in stock. With QoS, a manager can set the levels so that the transaction-processing traffic gets the highest priority, the videoconferencing traffic the next-highest priority, and file transfer the lowest priority.
There are several ways to give traditional LAN applications access to ATM QoS features. But no one way is right for everyone. The most direct way is to write ATM APIs that directly link an application to ATM services. The main problem with ATM APIs is that they are proprietary and are often tied to the OS under which the application is
running.
Ipsilon Networks, a start-up in Palo Alto, California, has come up with an interesting solution to the problem of getting LAN traffic over ATM and tapping ATM's strengths. The company has developed a routing protocol that delivers the features of traditional routing while leveraging ATM speeds.
Ipsilon sells a switch that runs the protocol. But other vendors will license the protocol from the company. Already, Ipsilon and Digital Equipment have announced a strategic technical alliance where Ipsilon's technology will be implemented on Digital's GIGAswitch/ATM switch and LAN switches.
Ipsilon's approach is to put an IP stack on an ATM switch. The company's software uses an intelligent classification scheme whereby the switch dynamically determines when to switch and when to route. The determination is based on the nature of the traffic.
Traffic entering the switch is checked by an IP switch controller, the intelligent routing software. The IP switch controller performs what
Ipsilon calls
flow classification
. The flow is defined as a sequence of like IP packets that are sent from one node to one destination address.
When longer-lasting flows, such as file transfers, multimedia traffic, and video streams, are noticed, the switch shifts into a cut-through switching mode of operation, where the switching is done in the ATM hardware. This is an efficient way to transfer the data between nodes. "The `flow-labeled IP' approach gives you a blazingly fast router/switch combo," says analyst Ron Jeffries of Jeffries Research.
The Ipsilon architecture lets IP switches forward packets at speeds up to 5.3 million packets per second, according to the company. Short-lived traffic is moved using traditional store-and-forward techniques, where decisions are made using software. Basically, it's much slower to move data this way than by using cut-through, hardware-based switching.
The main disadvantage to Ipsilon's approach is that it's proprietary. But many industry a
nalysts don't think this will deter users from taking advantage of the technology. "This is exactly what users are looking for," comments Fred McClimans, principal at Decisys, a consultancy based in Sterling, Virginia.
A secondary disadvantage to Ipsilon's approach is that the backbone must be IP-only. However, this is not a problem for many users. For example, 85 percent of Digital's installed customer base has pure IP backbones, according to William Maro, the company's vice president of network product engineering.
More Than One
For networks that have multiple protocols, there are other solutions. The ATM Forum has developed a specification called Multi-Protocol over ATM (MPOA), a routing protocol for ATM networks.
MPOA defines
IP and IPX routing for ATM switches. Switches that run the protocol do not require separate routers in order to connect different subnetworks.
MPOA builds on the Forum's LAN emulation standard, which specifie
s at the media access control (MAC) layer (OSI layer 2) how legacy LAN protocols traverse ATM networks. MPOA adds layer 3 support and support for ATM's QoS.
The specification, expected to be completed later this year, has three components. First, there's edge devices that are LAN switches with ATM interfaces; they have software that processes packets at layer 3 but forwards packets at layer 2.
Next, route servers run traditional routing protocols, such as RIP (Routing Information Protocol) or OSPF (open shortest path first), and calculate routing paths. The route servers and edge devices split the functions of a traditional router.
The last component of MPOA is the Internet Address Summarization Groups, which are ranges of network-layer address groups. Here's how MPOA works: An edge device checks incoming packets. If a device receives a packet with a destination address that the edge device already knows, the packet is forwarded directly to the appropriate edge device. But if the edge dev
ice doesn't know the destination address, it must be looked up and the most appropriate route calculated before the packet is passed on.
ATM might eventually work its way all the way down to the desktop. But until it does, users must compromise with solutions such as MPOA or Ipsilon's when it comes to efficiently moving LAN traffic over ATM backbones.
Where to Find
Digital Equipment Corp.
Maynard, MA
Phone: (800) 344-4825
Internet:
http://www.digital.com
Life in the Fast LANE
