ATM Backbone Switches

Cisco Systems LightStream 1010
The LightStream 1010 checks in at the top of the midrange performing ATM switches. In our tests, the LightStream 1010 topped out at 135 calls per second and had an average good-throughput PDU rate of 83 percent--third highest of the devices we tested.

The LightStream 1010 has a generous 5-Gbps backplane and can hold up to eight half-width adapter cards, called PAMs (Port Adapter Modules). The LightStream 1010 supports up to 32 OC-3 ports or eight OC-12 ports; it also supports ATM over T1, T1 circuit emulation and ATM 25 adapters. But while the LightStream 1010 is rich in software features, it lacks a redundant switching engine, which we feel is essential for any mission-critical ATM backbone. Of course, ATM technology enables you to achieve complete fault tolerance through use of multiple switches, but you'll pay a hefty price for the additional ports in your network.

LightStream 1010's hardware features include optional dual load-balancing power supplies, PC Card flash memory support, a field-serviceable fan tray and an upgradeable processor feature card. Currently, this card supports per-VC statistics, multicast cell replication, ABR congestion control, traffic policing and EPD/PPD support. The upgradeable module ensures that future ATM features can be added to existing equipment.

The real strength of the Cisco solution lies in the company's IOS (Internetwork Operating System) software. The switch we tested included support for multiple-peer-group PNNI routing, redundant ATM services such as LES/BUS and LECS redundancy, and output pacing. The Cisco LightStream 1010 was the only switch we tested whose fully redundant LECS could interoperate with a standard ATM Forum LANE network. This feature alone is reason enough to consider the LightStream 1010 as a serious contender, because an ATM network without a functional LECS is virtu ally useless. The LightStream 1010 can also serve as a Classical IP ARP (Address Resolution Protocol) server.

The LightStream 1010 has 65,536 cell buffers that are dynamically managed by the switch processor. Buffers are assigned to up to four queues per port for every priority class; the next version of the software will feature per-VC queuing. In addition, the switch can support up to 32,000 concurrent VCCs, which is more than adequate for today's ATM networks. Overall, the LightStream 1010 offers a uniquely redundant services package and is worthy of serious consideration for any major backbone purchase.

Xylan Corp. OmniSwitch Omni9
Xylan's OmniSwitch was the biggest ATM backbone switch we tested. The OmniSwitch Omni9 we tested had nine slots, eight of which can be used for anything from Ethernet, FDDI, Token-Ring and frame relay to OC-3 and OC-12 interfaces. Up to 56 OC-3 or 14 OC-12 interfaces can be configured in an emulated LAN. In addition to its eight available slots, the OmniSwitch Omni9 sports a gigantic 13.2-Gbps cell switching backplane.

However, in our tests, the OmniSwitch performed very poorly. Its performance was well below the level we consider acceptable. Our benchmarks revealed the OmniSwitch could sustain only 17 calls per second, and EPD/PPD had no impact on performance during our tests. Xylan engineers indicated that a much-improved version of the software, capable of sustaining about 100 calls per second, is in the works. The EPD/PPD performance problems apparently arose because EPD/PPD wasn't turning on properly in the switching fabric, even though we enabled the software option on the switch. We were unable to resolve either the signaling performance problem or the EPD/PPD problem during our testing.

If Xylan gets its performance act together, this switch could be a real contender. It was the only one we tested that featured frame and cell switching backplanes in a single box, though this feature is found in competing products, such as Cisco's Catalyst 5500 and 3Com's CoreBuilder 7000, which were not included in these tests.

The switch has a fully redundant switching fabric, as well as hot-swappable load-sharing power supplies. Unlike the other switches tested, the OmniSwitch lacks internal support for LANE services; instead, it relies on an external server, the Omni-MSS, starting at about $18,000. Omni-MSS features redundant LES/BUS support, redundant (though proprietary) LECS support, and a unique feature known as broadcast filtering. The broadcast filtering mechanism in MSS lets you build very large ELANs because it broadcasts frames only to the appropriate destination switch.