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Layer 1 Switches: Key Functions and Technologies

By Yossi Maish | Co-founder, Lepton Systems

· Test Lab automation

Physical layer switches have been around for some time, but recently have been in the spotlight as the final piece to 100% test lab automation. If the physical layer is not automated, the benefits of automation are diluted by having to manually configure the infrastructure of the lab. Let's explore this often-overlooked, but key contributor to automating a test lab environment.

What is a Layer 1 switch?

A physical layer switch, or Layer 1(L1) switch, operates at the physical layer of the OSI (Open System Interconnection) model. The easiest way to think of a Layer 1 switch is an an electronic, programmable patch panel. It simply establishes the physical connection between ports. The connection is established using software commands and thus, allows test topologies to be automatically or remotely configured.

A Layer 1 switch does not read, manipulate or use packet/frame headers to route the data. Layer 1 switches are fully transparent to the data and typically have very low latency. Completely transparent connections between ports are important in testing environments as this allows the tests to be as accurate as if there were a patch cord between the devices.

How is that different from Layer 2?

In contrast, an Ethernet switch operates at Layer 2 or 3 in the OSI model and connects inputs and output ports by reading packet or frame headers and routes data based on the location designated in the header.

In some cases Ethernet switches are used for interconnecting devices into a physical topology. The characteristics of Layer 2/3 switch operation, however, can affect test results, rendering them inappropriate in many test environments. For example, a Layer 2/3 switch will filter bad packets or fragments, add/delete idle characters to compensate for differing input/output clock timing, or discard control frames. This would make it impossible to compare input to output data streams when testing. Layer 2/3 switches can also be costly to scale, and don't provide the flexibility of Layer 1 switches to support a mix of media types or media conversion.

Layer 1 switch technologies

There are several different technologies of Layer 1 switches: Beam steering (all optical or OOO), mechanical switching, and optical-electrical-optical (OEO) switching. Let's take a brief look a the characteristics of each.

Beam steering switches connect to the physical infrastructure using single mode fiber, and the switch fabric operates at the optical level. Because the fabric switch uses sophisticated architecture, it can be more expensive than other technologies.

  • Protocol and data rate agnostic
  • Single mode fiber media only (more costly to implement)
  • Clustering switches is not practical due to accumulated insertion loss
  • Connection speed is fast
  • High insertion loss (up to 4dB/16k of fiber)
  • Port flapping feature is slow and limited
  • 1 to n mapping (for generating traffic from one to many ports) is not possible
  • Optical power monitoring at the port level is optional

Mechanical switches use software controlled robotic devices to connect fiber ports to each other (think robotic patch panel) providing a purely physical connection. Operating at the pure physical level, the architecture is economical.

  • Protocol and data rate agnostic
  • Single mode or multimode fiber media
  • Clustering switches is awkward due to hard-wired inter-chassis connections
  • Connection is slow (1+ minute for a full-duplex connection depending on matrix size)
  • Mechanical parts require periodic maintenance and have a lower MTBF than other technoloogies
  • Connections to the switch are LC or SC, low cost, connectors
  • Low insertion loss
  • Very low latency
  • Port flapping feature is not offered
  • 1 to n mapping (for generating traffic from one to many ports) is not possible
  • Optical power monitoring at the port level is optional

Optical-electrical-optical switches offer software controlled connection of either fiber or copper interfaces. If the input is optical, the switch converts optical data to electrical data through to be mapped through the switch fabric and reconverted to optical at the output port. This technology can be more expensive than mechanical technologies but offer value-added features that can offset that difference.

  • <1GB to 128GB suppport, varies with manufacturer
  • Flexible media varies with manufacturer: single mode, multimode, AOC, DAC, PSM4 
  • Protocol agnostic
  • Clustering is possible and straightforward
  • Connection time is fast
  • No insertion loss
  • Latency is <50ns
  • Port flapping is flexible: using programmable start/stop and duration settings
  • Full wire-speed unicast, multicast,
  • Full wire-speed unicast, multicast and broadcast mapping options available
  • Port-level diagnostics of RX power, TX power, los of signal and loss of lock, varies with manufacturer

Other value-added benefits of Layer 1 switches

Depending on the technology, Layer 1 switches offer benefits to test lab automation environments in addition to mapping at the physical layer.

  • Full wire-speed unicast, multicast and broadcast mapping options are possible with OEO switches. This allows duplication of any incoming data to any number of output ports for testing multiple devices from a single test set or output, to reduce testing time and equipment requirements.
  • Simulation of cable breaks, called port flapping, can be performed using an OEO or OOO switch. In OOO switches the feature is rudimentary, but in OEO switches the duration, interval and repetition can be defined in software making the test very specific.
  • Diagnostics at the port level are available and vary in sophistication among technologies and manufacturers.
  • Mechanical switches offer latching connections that remain connected even if power is lost.

Scalability and blocking is always a consideration when choosing the technology of a Layer 1 switch. Smaller, lower rate L1 switches provide and-to-any mappings without blocking, and are relatively inexpensive. As port and data rate requirements increase, however, strictly non-blocking switch fabrics become restrictively expensive and some tolerance to blocking will need to be defined. To achieve increased port requirements, clustering multiple switches becomes necessary.

Automation software is only half of the solution

Without deploying Layer 1 switches, test lab automation software will only automate the actual testing being performed. Configuring the test topology will still be a labor-intense endeavor. Full CAPEX and OPEX benefits of test lab automation can only be realized with the implementation of Layer 1 switches

It may be necessary to deploy multiple technologies of Layer 1 switches. This allows better support of a variety of data rates and interface types reflected in the test environment. In addition, feature and function differences may dictate deploying several technologies for addressing the variety of needs such as, Fibre Channel support, port flapping, or multicast mapping capabilities. Economic factors may also come into play. Certain types of ports may be more cost effective in a particular technology.

Whatever the form factor or technology, the capabilities of a Layer 1 switch eliminate the final roadblock to 100% test lab automation.

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