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A network TAP (Test Access Point) is a simple device that connects directly lớn the cabling infrastructure to split or copy packets for use in analysis, security or general network management. Although the term “Tap” predates the networking industry by decades, the IT industry has generally adopted the term to lớn mean demo Access Point.

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Network TAPs Overview

Since a network TAP provides the most effective means to copy actual traffic running across a system, the remainder of this paper is dedicated lớn TAP types, usage và functionality. It should be noted that TAPs are available for a wide variety of network speeds and cable types. Instead of two switches or routers connecting directly lớn each other, the network TAP sits between the two endpoint devices connected directly to lớn each of them. Then traffic is seen & copied, providing visibility into the networked traffic. See Figure 1.

TAPs are straightforward devices that run for years và are generally placed in secured locations. Once the traffic is tapped, the copy can be used for any sort of monitoring, security, or analytical use. Thus, TAPs are a key component of any visibility system.

Figure 1: Direct cabling vs. TAP cabling

Types of Network TAPs và How They Work

There are many different types of TAPs. The two primary types of network TAPs are Passive TAPs & Active TAPs.

Passive TAPs

A passive TAP requires no power nguồn of its own and does not actively interact with other components of the network. It uses an optical splitter khổng lồ create a copy of the signal và is sometimes referred to as a “photonic” TAP. Most passive TAPs have no moving parts, are highly reliable và do not require configuration.

A Typical TAP Installation Involves:

1. Placing the TAP on a shelf or in a rack

2. Connecting the cables

3. Verifying everything is working

It is really that simple. If the TAP fails to lớn work, there is probably a cabling issue or a bad connection. Vì chưng be aware that installing or replacing a TAP in an existing environment does bring down the links while the cables are reconnected. So TAP installations are typically scheduled during pre-defined maintenance windows, or during the network architecture design phase, prior to lớn running live traffic.

Optical fiber sends light from a transceiver through a thin glass cable lớn a receiver on the other end. Instead of connecting directly to lớn each other, each of the two endpoint nodes (switches, routers, database, etc) are connected lớn network ports on the TAP. These special ports are physically wired in pairs such that traffic continually passes through them. In addition to lớn the network ports are monitoring ports. The monitoring ports send out complete copies of the traffic seen, as shown in Figure 2.

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Figure 2: TAP diagram showing logical flow

Unlike network ports with both TX (transmit) & RX (receive) traffic, monitoring ports are unidirectional and only send traffic. They have no ability lớn receive traffic & never pass traffic back into the system. You will notice there are two monitoring ports in the diagram. Since each network port both sends & receives traffic, a 10Gb links could have 20Gb running across it. If all this traffic were put into one monitor cable, the links could quickly be oversubscribed. By running two separate monitor links, oversubscription is eliminated. The monitored traffic is thus separated into two transmit (TX-only) signals, one copy from endpoint A (Switch X) and one copy from endpoint B (Switch Y).

As depicted in Figure 2, a passive network optical TAP leverages a simple internal design. The external connectors lead khổng lồ sets of glass fibers, splitters và more glass fibers leading back to lớn the external connectors. Each splitter has one fiber coming in and two going out.

Optical Splitter Types

Internal to lớn the TAP, between the network port pairs, lies a small piece of hardware called an optical splitter. The splitter does exactly as the name implies; it splits an optical stream into two paths. A portion of the light continues onto its original destination; the second path is directed to lớn a monitor port. A traditional method lớn split the light is khổng lồ fuse (or melt) two cables together such that a portion of the light is funneled off to lớn the secondary stream. This giải pháp công nghệ is called Fused Biconical Taper (FBT) and is shown in Figure 3. The concept is similar to lớn when a river hits a fork. A portion of the water continues in the original direction while the rest takes an alternative path. Both forks of the river continue lớn flow downstream. Like water, light is also directional. As a result, the FBT tends to lớn pass the traffic one way. FBTs tend lớn be low cost và work well for lower-speed cable plants.

Figure 3: Fused Biconical Taper (FBT)
Figure 4: Thin Film splitter công nghệ

A second splitter type uses Thin Film technology. The concept here is similar to shining a flashlight through a clear glass window. Although the majority of the light continues through the window, a portion of the light is reflected back as it hits the glass. If angled properly, a semipermeable membrane cutting across the fiber will copy a portion of the optical signal lớn the monitor port, as shown in Figure 4. Thin Film’s reflective công nghệ tends khổng lồ have a lower loss rate when working with high-speed links, such as 100Gb where hot spots tend lớn occur due to lớn uneven light distribution across the fiber. The FBT slice sees only the portion of the light where it is fused. Thin Film is more evenly distributed because it sees the reflected light across the entire diameter of the cable.

Specialized 40Gb BiDi TAP

Thin Film is also preferred for TAP bidirectional links, such as 40Gb Cisco BiDi, because multiple wavelengths can be reflected simultaneously lớn break out each lambda (or wavelength) of light. Cisco BiDi leverages 40Gb giải pháp công nghệ using standard LC-based cabling lớn minimize the overall costs of deploying 40Gb links. This is a growing trend, especially with regard to Cisco leaf/spine configurations. See Figure 5 for an example of how reflective công nghệ is used within this highly-specialized passive TAP.

Figure 5: Thin Film used in a bidirectional implementation

Split Ratios

Regardless of the method used, the passive splitter physically diverts a portion of the light from its original source. The proportional giới thiệu of light for each path is known as the split ration. The split ratio is written as a combination of two percentages. The first number is designated as the network percentage, the second number is the monitor percentage. They always showroom up khổng lồ 100 percent. For example, a common split ratio for traditional 1Gb short-range links is 70/30; where seventy percent of the light continues lớn the network and thirty percent is allocated lớn the monitor port.

The concept is to allocate more light khổng lồ the network to lớn reduce the risk of dropping network traffic. Speeds such as 10Gb, 40Gb và 100Gb have different technical requirements and tend lớn use more of an even split ratio such as 50/50 or 60/40. The most common split ratio deployed in networks today tends lớn be 50/50, provided the proper light levels are available. When light levels are marginal, the safe option is khổng lồ move khổng lồ better optics offering higher safety margins.

oered.org tests every TAP manufactured và provides the actual tested loss values with each oered.org-branded TAP shipped. In addition, oered.org data sheets for TAPs describe the maximum acceptable network and monitor loss values (including connections) for each split ratio are as follows: