The Switched Network Revolution
By Bob Tabke
Ask network managers to list the top three problems they face, and you will hear about bottlenecks, the increasing difficulty of supporting distributed computing, and a concern about the looming needs of upcoming bandwidth-hungry applications. So how do you eliminate these problems? Everyone agrees that switched networks are the answer.
Frank Dzubeck of Communications Network Architectures says, "1995 is the year that LAN switching installations begin. They will go in quickly. I don't think there are going to be any more shared LANs in the business environment by the year 2000 (unless they are in the outer reaches of the galaxy). It no longer makes sense to put in shared Ethernet or Token Ring at the workgroup level. The reason is that per-port costs for switching are very low, and you get a full 10 Mbits/sec in a switched Ethernet LAN compared to maybe only 1 Mbit/sec if you are sharing the LAN with a bunch of other users. Today, switching is a no-brainer."
If your networks are like most, usage is growing. An increasing number of users and new bandwidth-hungry applications have resulted in large increases in the amount of data being transferred. This increase will only accelerate over time. Traditional shared-media networks are becoming more and more congested by this new traffic, and they cannot meet the transmission delay and continuous bandwidth requirements of upcoming real-time multimedia traffic (particularly digitized audio and full-motion video). The audio tends to breakup, and the video appears jerky. The solution to such problems is a network that can provide dedicated bandwidth when and where you need it.
One alternative is to stay with router-based networks and just make them larger by adding parallel connections. FDDI is the fastest shared media available today for interconnecting LANs, but it is limited to 100 Mbits/sec. When you hit that limitation, you can add another FDDI ring. But the shared media characteristic and the 100 Mbits/sec limitation make this a short-term solution to backbone bottlenecks. Adding routers just complicates network management even more. Configuration management and changes get more complex as more routers and LAN segments are added. This increases support costs and the potential for errors not the direction most of us would like to go.
Only switched networks provide a long-term solution a fact that LAN and WAN vendors both agree upon. As Dr. Daniel Abensour, IBM's program director of ATM market development says, "Centralized routers is an idea whose time is gone--switching is the way to go. Even centralized routers will not be able to handle increasing data traffic," he continues, "especially when joined by other delay-sensitive traffic, such as audio and video." Switched networks provide the scalability and dedicated bandwidth that shared-media LANs cannot provide. Plus, they simplify the management of large, growing networks.
Campus Switching
A LAN switch can be thought of as a very fast (same speed as the LAN media), low-latency multiport bridge (latency refers to the time it takes a frame to go through the LAN switch). One advantage of Ethernet and Token-Ring switches is that they can be used to add capacity to a network without changing workstation adapters or applications. You can create separate shared LAN segments off the switch with one or more workstations on each segment. LAN resources such as servers can be attached directly to the switch. Thus, the switch connects each segment directly to the servers eliminating the sharing of bandwidth on the LAN.
LAN and ATM switches can also replace layer 3 routers, so that a virtual LAN can be created. For network managers, life gets much simpler with virtual LANs. Workstation moves and changes can be made without your having to reconfigure the workstations. This is possible because of the virtual LAN management software and the integration of bridging and routing functions in switches. The management software can automatically reconfigure the network to place a workstation or server on the proper subnet regardless of where it is physically located.
LAN emulation provides the appearance of an Ethernet or Token-Ring LAN to a computer attached to an ATM network. This means that applications on the computer do not have to change in order to exploit the advantages of ATM. The ATM Forum has finalized the LAN emulation specification, which is a standard way for ATM-attached hosts, bridges, routers, and LAN switches to communicate with each other.
Migration to Switched Networks
If you are faced with LAN congestion, what are the steps to migrate to the switched model? Robert Zimmer, an IBM networking program manager, suggests, "First determine what you want the network to look like when you are done. Then create a plan to get there. As you build the new network, you must provide interconnectivity to the old one users on the new network must talk to people on the old one. Be sure to design the interconnection point between the two networks so it does not become a bottleneck when half the users are on each network."
Here is an example of how a typical installation might migrate from a shared-media network to a switch-based network. The starting point is a routed campus with three buildings and a collapsed backbone. The routers in the basement of each building perform three functions:
- Interface the LANs to the outside world (T1, frame relay, or leased lines)
- Interconnect the LANs in each of the buildings over an FDDI ring
- Play the collapsed backbone role in each building (interconnection of the LANs on each floor)
What are some of the problems with this configuration? If you are a network manager, this situation might look familiar. You get congestion on the LANs because of the use of shared media (Ethernet or Token Ring). There is also likely to be congestion in the routers because their processors have a lot of work to do and performance is limited. The backbone can also become congested. Moves, adds, and changes are very costly. In this example, we will solve these problems in several phases, as follows. These phases can overlap, and some can even be skipped on the way to an end-to-end switching environment.
Figure 1
Phase 1: Introduce LAN switching (see Figure 1).
LAN switching is the most cost-effective approach for new installations today, because you can selectively upgrade existing LANs to switching in order to relieve bottlenecks. By installing LAN switches (either Ethernet or Token Ring), you can have many more LAN segments, all the way to the point of having one workstation per segment connected directly to the switch. This relieves the congestion on the shared-media LANs. The advantage of this approach is that you can retain the current subnetting, the LAN adapters, and the router.
Figure 2
Phase 2: Add ATM switching (see Figure 2).
The next step is to tie the switched LANs into an ATM backbone. Also, you can use high-speed (155 Mbits/sec) ATM links to connect the LAN switches on all the floors of the building to an ATM switch in the basement, increasing overall system performance. An ATM backbone can be easily scaled from 155 Mbits/sec up to many gigabits/sec.If you want to extend ATM to workstations at 25 Mbits/sec, you can use a workgroup or campus ATM switch. These ATM switches can be connected to each other by ATM links. Super servers can be connected directly to these switches to tremendously improve their performance. This approach is much better than using multiple shared-media network interface cards in the server you end up with less cost and more bandwidth.
According to Zimmer, "The beauty of using ATM within a switch-based network is that it can be used from 25 Mbits/sec all the way up to 622 Mbits/sec today, and 2.4 Gbits/sec and beyond in the future. You can scale up in bandwidth without having to change the technology or how you manage it. Going from 155 Mbit/sec ATM to 622 Mbit/sec ATM is simply a link upgrade--the same management, call setup, and so on."
Migration to ATM at the desktop can be done as requirements dictate. But if you are buying new PCs as part of a new installation, it is even easier to justify providing ATM to the desktop. Then you do not waste any investment in LAN adapters, assuming that you will need the speed and other benefits (such as the handling of audio and video) of ATM to the desktop in the next three to five years.
The router attaches to the workgroup ATM switch. To interface to the outside world, you can use a small wide-area ATM switch rather than the router. It would be connected to fractional T1, frame relay, or leased lines. The wide-area ATM switch does something no router can do it merges voice traffic from the PBX onto the same lines that carry the data traffic. This consolidation saves the cost of separate circuits relieving all the congestion on the campus backbone and providing end-to-end quality of service (guaranteed bandwidth).
Figure 3
Phase 3: Add end-to-end switching (see Figure 3).
In the next year, you will see expensive centralized routers phased out in favor of distributing functions in devices at the edge of the network, such as LAN switches, intelligent hubs, and adapters. The distributed functions will include routing, LAN emulation, broadcast management, bandwidth management, and directory services. This will provide the capability of exploiting new native applications, such as interactive multimedia, videoconferencing, and distance learning. Other benefits from distributing these functions include:- Higher performance: With more CPUs available to implement these functions, you will see faster connections and better response times.
- Increased availability: Distributed routing eliminates single points of failure (as with large centralized routers).
- Lower cost: Centralized routers are expensive compared to implementing the same functions as added software and hardware in LAN switches, hubs, and ATM adapters.
Vendors will differentiate themselves on the ability of their customers to apply this approach in a smooth and staged manner, as well as their ability to provide a very flexible implementation of related functions and end-to-end system solutions.
You can start implementing this scenario now. As Barbara Leonard, IBM's ATM market plan project manager, says, "If you are going to a switching model in the future, why invest any more in shared-bandwidth networks? Although the initial cost is slightly higher than the shared-media approach, it is justifiable by avoiding the cost of obsoleting and swapping out shared-media adapters, routers, bridges, and hubs."
Copyright © 1996 Bob Tabke. All Rights Reserved.
