Preparing for Installation
Before installing a Juniper Networks router, several steps must be taken to prepare the installation site. These include installing the rack that will house the router; ensuring adequate clearance for router air flow and maintenance access; providing appropriate cables for power, grounding , and network connections; and calculating the router's power budget and power margin. Rack RequirementsJuniper Networks routers can be installed into several types of racks, including standard 19-inch equipment racks, telco four-post racks, and center-mount racks. The rack used must be tall enough to hold the router or routers, and strong enough to support the combined weight of the installed equipment. There must be enough space around the router and the rack for proper airflow and to allow servicing of the router. Table 4.1 shows the size and weight of each router and the amount of clearance required. Table 4.1. Router Physical Specifications and Clearance Requirements
Power and Grounding Cable RequirementsThe proper cables are required to connect the router to the power source and to earth ground. Table 4.2 lists the AC power cable requirements for those routers using AC power, and Figure 4.1 shows the AC plug types. Table 4.3 lists the DC power and grounding cable requirements. Figure 4.1. AC Plug Types Table 4.2. AC Power Cord Requirements
Table 4.3. DC Power and Grounding Cable Specifications
When planning the wiring for a site, distance limitations for signaling, radio frequency interference, and electromagnetic interference must be taken into consideration. If wires are installed improperly, they can emit radio interference. In addition, potential damage from lightning strikes increases if wires exceed recommended distances, or if wires pass between buildings . The electromagnetic pulse (EMP) caused by lightning can damage unshielded conductors and destroy electronic devices. Network Cable RequirementsAppropriate cables of the proper type and length are required for each network interface connection, and for the management device connections. Table 4.4 lists the cable requirements for each type of connection. Table 4.4. Network Cable Specifications
Site Wiring GuidelinesProper site wiring involves consideration of distance limitations for signaling, radio frequency interference (RFI), and electromagnetic interference (EMI). Using twisted-pair cable with a good distribution of grounding conductors, and keeping cable lengths within recommended distances, minimizes the risk of RFI. If recommended distances are exceeded, a high-quality twisted-pair cable with one ground conductor for each data signal, when applicable , is recommended. Strong EMI could destroy the signal drivers and receivers in the router and could conduct power surges over the lines into the equipment, resulting in an electrical hazard . To minimize the risk of damage from EMI, it is important to provide a properly grounded and shielded environment and to use electrical surge suppression devices. Fiber- Optic Connection GuidelinesFiber-optic interfaces use one of two types of fiber connection: multimode or single-mode. Multimode fiber is large enough in diameter to allow rays of light to internally reflect or bounce off the inner walls of the fiber. Light sources on interfaces with multimode optics are typically LEDs, which are not coherent light sources. An LED sprays varying wavelengths of light into multimode fiber, which reflects the light at different angles. Light rays travel in jagged lines through a multimode fiber, causing signal dispersion. When light traveling in the fiber core radiates into the fiber cladding, higher-order mode loss (HOL) results. All these factors limit the transmission distance of multimode fiber compared to single-mode fiber. Multimode fiber has an approximate maximum transmission distance of up to 1.5 miles (2 kilometers). Significant signal loss, causing unreliable transmission, can occur at greater distances.
Single-mode fiber is so small in diameter that there is not enough room for the rays of light passing through it to reflect internally through more than one layer. Light sources on interfaces with single-mode optics are lasers, which generate light rays in a single wavelength and which travel in a straight line, directly through the single-mode fiber. Single-mode transmission is useful for longer distances and is capable of higher bandwidth than multimode fiber. However, it is more expensive. The maximum distance between transponders is determined by fiber loss, chromatic dispersion, transmitter power, and receiver sensitivity. Table 4.6 on page 103 lists the factors that contribute to link loss. Attenuation and DispersionA functional optical data link depends on modulated light reaching the receiver with enough power to be correctly demodulated. Attenuation is the reduction in power of the light signal as it is transmitted. Attenuation is caused by passive media components , such as cables, cable splices, and connectors. While attenuation is significantly lower for optical fiber than for other media, it still occurs in both multimode and single-mode transmission. An efficient optical data link must have enough light available to overcome attenuation. Dispersion is the spreading of the signal in time. The following two types of dispersion can affect an optical data link:
For multimode transmission, modal dispersion, rather than chromatic dispersion or attenuation, usually limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a factor. However, at higher bit rates and over longer distances, chromatic dispersion, rather than modal dispersion, limits maximum link length. An efficient optical data link must have enough light to exceed the minimum power that the receiver requires to operate within its specifications. In addition, the total dispersion must be less than the limits specified in Telecordia GR-253-CORE Section 4.3 and ITU G.957 for the corresponding type of link. When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty in the power budget. The optical power budget must allow for the sum of component attenuation, power penalties (including those from dispersion), and a safety margin for unexpected losses. Power Budget and Power MarginThe power budget ( P B ) is the maximum possible amount of power that can be transmitted over the link. It is calculated as a worst-case analysis to provide a margin of error, although all the parts of an actual system do not operate at the worst-case levels. The worst-case estimate of power budget ( P B ) is calculated assuming minimum transmitter power ( P T ) and minimum receiver sensitivity ( P R ). Table 4.5 lists equations for calculating the power budget for SONET/SDH PIC interfaces. Table 4.5. Sample Power Budget Calculation for SONET/SDH PIC Interfaces
The power budget is used to calculate the power margin ( P M ), which estimates the amount of power available for the link after subtracting attenuation or link loss ( LL ) from the power budget. A worst-case estimate of P M assumes maximum LL : P M = P B LL A P M greater than zero indicates that the power budget is sufficient to operate the receiver. Table 4.6 lists the factors that contribute to link loss and estimates the link-loss value attributable to those factors. The following example calculates a multimode power margin with the length of multimode link at 2 kilometers, with 5 connectors, 2 splices, a higher-order loss, and a clock recovery module: P M = P B LL P M = 13 dB 2 km (1.0 dB/km) 5 (0.5 dB) 2 (0.5 dB) 0.5 dB (HOL) 1 dB (CRM) P M = 13 dB 2 dB 2.5 dB 1 dB 0.5 dB 1 dB P M = 6 dB The following example calculates the single-mode fiber power budget for two sites that are 8 kilometers apart, connected with single-mode SONET cable with 7 connectors: P M = P B LL P M = 13 dB 8 km (0.5 dB/km) 7 (0.5 dB) P M = 13 dB 4 dB 3.5 dB P M = 5.5 dB The calculated value of 5.5 dB indicates that this link has sufficient power for transmission and does not exceed the maximum receiver input power. Table 4.6. Link-Loss Estimation
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EAN: 2147483647
Pages: 185