With the greater acceptance of ATM and the threat of Gigabit Ethernet of the horizon, IT Managers are faced with choosing between these high-speed network technologies
LANs were introduced as a low-cost, timesaving technology, enabling the sharing of data and network-attached devices. Their popularity gave birth to distributed client/server computing, and host-based networks shifted to peer-to-peer models. As a result, these LANs have rapidly evolved into support systems that are critical to communications within organisations. In the future, the expansion of these communications functions will make networks the most valuable asset within an organisation.
Today, as network demand increases, users are experiencing problems such as delays in data transmission, interruptions in service, difficulties in relocating workstations and limited flexibility. Because of these problems, few networks are able to implement emerging multimedia applications, such as workstation-based videoconferencing or video on demand. The problems stem from the basic limitations of shared-media networks connected by bridging and routing technologies. These networks have neither the throughput capability nor the performance characteristics to solve the problems.
Most desktop users are on Ethernet or token-ring LANs and are limited to throughput of 10 or 16 Mbit/s respectively. Users share the LAN's bandwidth, which means that the aggregate throughput approaches 10 or 16 Mbit/s, but also that most users can use only a fraction of the available bandwidth during normal operations. Bridges and routers can both interconnect LAN segments, forming campus LANs and segment LANs, enabling multiprotocol networks and networking.
Most LAN traffic is governed by the IP, IPX, SNA and NetBIOS protocols. Using routers to segment networks improves performance, security and broadcast traffic control, while at the same time it allows routing of IP and IPX traffic between subnets, off the campus and outside the enterprise. For the SNA and NetBIOS protocols, routers bridge and filter the traffic.
Management of such networks is labour-intensive, resulting in high ownership costs. Although bridges and routers improve network performance, networks running current LAN applications experience significant communication bottlenecks across server connections and between workstations in different LAN segments. Even with faster connections, the latency of routers and bridges is high enough that it does not support the emerging, isochronous (time-sensitive) applications mentioned previously.
Alternative SolutionsThree alternative technologies are fast LANs, LAN switching and Asynchronous Transfer Mode (ATM) switching. Each of these technologies provides greater bandwidth and enhanced performance and supports current applications. Each technology has advantages and disadvantages.
Fast LANsFast, shared-LAN technologies include Fiber Distributed Data Interface (FDDI), 100 Carrier Sense Multiple Access with Collision Detection Ethernet (100 CSMA/CD), and 100 Voice Grade AnyLAN (100VG AnyLAN). Each of these solutions provides 100 Mbit/s of shared bandwidth. Increasing available bandwidth by a factor of six to 10 might significantly improve network performance through data bottlenecks such as server-to-workgroup connections. Fast LANs are also widely used as replacements for router and bridge network backbones.
However, fast LAN solutions also have disadvantages that potentially limit their long-term suitability:
( They require replacement of cabling and hardware and are thus expensive
( They do not support the technologies and applications being deployed with ATM
( The technology is not scalable for either the desktop or backbone, and this limits available bandwidth
( The shared nature of these technologies deters widespread use of isochronous multimedia applications even with the increased bandwidth
( They provide no clear migration path to ATM
If a lack of shared bandwidth is the major problem, the fast LAN's increase in available and aggregate bandwidth is a good short-term solution. This technology is especially applicable for server connections and small campus backbones using current applications.
LAN switches provide dedicated, packet-switched connections between their ports. Switches and bridges are similar, but switches have higher aggregate bandwidth and lower latency. Networks based on switches offer dramatic performance improvements.
Shared and dedicated LAN switches segment LANs, as do routers and bridges, thus increasing shared bandwidth. Dedicated LAN switches provide dedicated desktop connections for either current 10 and 16-Mbit/s LANs or the new, 100-Mbit/s, fast LAN protocols, and they support full-duplex operation.
A LAN switched network, with dedicated connections to end users' workstations, provides high bandwidth and can support time-sensitive, multimedia applications. LAN switching is inexpensive and presents few migration obstacles. Workgroup LAN switches can tie to high-speed backbones with fat pipes (high-bandwidth connections to second-level switches) providing ATM or fast LAN connections.
Switched networks can form logical LANs overlaying physical LANs. These virtual LANs dramatically change the role of routers in the campus. Today, most traffic passes through routers, resulting in congestion and latency. Virtual LANs allow router function to move from the network center to the periphery. In this new role, routers route traffic only between subnets, off the campus, and outside the enterprise, but not within the virtual LAN.
Advantages to LAN switching include:
( Simplified network and workstation management
( Improved latency with less reliance on routers
( Inexpensive, dedicated bandwidth
( Compatibility with existing LANs, complementary with an ATM backbone
However, LAN switching does not correct all the problems, which include those listed here:
( The technology is not scalable for either the desktop or backbone, and this limits available bandwidth
( No mechanism ensures that traffic has priority in the network, thus limiting the deployment of new multimedia applications
( Bridging between workgroup switches and the backbone induces latency
( Data loss can occur due to the lack of large buffers and standard flow-control mechanisms if switches are used in the backbone
( No formal method limits disruptive broadcast traffic; however, vendors are developing a Broadcast Manager product to eliminate this problem
( 100-Mbit/s switched LANs approach the cost of ATM and do not have the same levels of isochronous performance
LAN switching is an excellent migration technology for desktop and server connections, providing additional performance and improved manageability. It is not necessarily suitable for use in large campus backbones, but is well suited for small ones.
Asynchronous Transfer Mode (ATM) networks are being implemented across desktops, backbones, and wide area networks (WANs). The cell-switching technology that is the basis for ATM provides high-speed, scalable bandwidth with excellent network performance. ATM integrates mixed isochronous traffic (image, video, voice, and audio) with traditional data.
ATM has all the advantages of LAN switching plus:
( Scalable, reserved, on-demand bandwidth
( Excellent network performance, minimised routing function and seamless LAN-to-WAN integration
( Compatibility with existing LANs and applications such as LAN Emulation and Classical IP
( Reduced ownership costs as a result of simplified network management
Moreover, as an emerging technology, ATM can be expensive. However, the introduction of established 25-Mbit/s ATM as a standard dramatically lessens the impact. Inexpensive, 25-Mbit/s ATM NICs, priced below £250, provide superior bandwidth and performance against LAN-switched token ring or Ethernet. As ATM is widely deployed, the per-port cost of 25-Mbit/s ATM solutions will continue to decrease, approaching switched-Ethernet prices.
ATM will eventually replace the network infrastructure including hubs, concentrators, repeaters, and NICs. However, staged implementation enables applications and enhanced performance as needed, and existing infrastructure might only need to add ATM connectivity function.
For desktop and server connections, users can use 25 and 100-Mbit/s ATM NICs at industry-leading prices. Companies offer broad operating system and bus support for these NICs. The 25-Mbit/s NICs connect to the campus backbone switch, using an Intelligent Switching Hub, via an ATM Workgroup Concentrator. A stackable, 25-Mbit/s ATM workgroup switch will join the ATM Workgroup Concentrator later this year.
Using campus switches, based on scalable Switch-on-a-Chip, offers performances of 10-Gbit/s switching, and the campus backbone switch interconnects the campus with multiple, high-speed connections. Shared and switched LANs can connect to the ATM network using an ATM LAN Bridge or LAN switches. Current offerings are enabled for LAN Emulation or Classical IP; Native ATM was planned for 1996 support.
To manage the ATM network, companies use management software to add full virtual LAN function with a Virtual LAN Server. The LAN switch and the ATM campus backbone switch plan added distributed routing function in 1996 (users can continue to use current routers with ATM Forum-compliant ATM connections). In the WAN, Networking BroadBand Services (NBBS), implemented on the broadband switches, offers excellent quality-of-service function.
industry migration to ATM with the broadest, enterprise-wide, end-to-end solutions available. ATM technologies include scalable Switch-on-a-Chip, NBBS, and high-performance NICs. These technologies enable users to build the most cost-effective, long-term solutions for their networking requirements.
The solution to user requirements is to provide workstations with dedicated media connections into high-speed, ATM switched networks.
These networks will provide data throughput that is not possible with shared media, and eliminates the performance delays associated with router and bridged-based technologies, and lowers cost through simplifying network management.
Migration from current LANs to switched networks will be staged for most users.
Many vendors offer an integrated migration path from current LANs to ATM. Users can implement fast LAN or switched-LAN solutions as stepping-stones to ATM. Current and anticipated application and performance requirements will determine when customers should implement desktop ATM. If the need is not likely to exist in the next three years, users should implement LAN switching with an eventual migration to full ATM. A typical migration path for a router-segmented Ethernet LAN follows:
( Install a switched campus backbone (either ATM or switched, fast LAN technology)
( Connect servers to the campus backbone via fat pipes (either 100 or 155-Mbit/s ATM or fast LAN connections)
( Install dedicated workgroup switches to provide dedicated connections (25-Mbit/s ATM, 10-Mbit/s Ethernet, or 16-Mbit/s token ring). For current workgroups, use LAN switches and for new workgroups and new workstations or new facilities, use ATM. Users requiring higher speed connections can use higher speed ATM (100 or 155-Mbit/s) or switched, fast LAN connections
( Enable virtual LANs and move the routers to the network periphery
( As high-bandwidth WAN services develop, complete ATM deployment by connecting to Service Provider Equipment (SPE) ATM or Customer Premise Equipment (CPE) private ATM networks
Each user's problems are unique and that the best solution for upgrading LAN performance depends upon many factors such as current network, applications, topology, protocols, migration constraints, and immediacy. Choosing a campus network strategy provides multiple solutions for the desktop, server, and backbone networking environments and enables users to utilise networking for competitive advantage.
(c)International Business Machines Corporation, 1999
Compiled by Paul Phillips
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