Overview: OC-12 ATM Line Card

This chapter describes the OC-12 ATM line cards and contains the following sections:

Line Card Overview

The 1-port OC-12 ATM line card is a half-slot single-port line card used witht he Cisco 7304 router. The OC-12 ATM line card provides an ATM over SONET interface at OC-12c speeds (622.08 Mbps) . The OC-12 receives and transmits ATM cells on the physical interface while transmitting and receiving packets from the 4 Gigabits serial links via the backplane.

Asynchronous Transfer Mode Overview

Asynchronous Transfer Mode (ATM) uses cell-switching and multiplexing technology that combines the benefits of circuit switching (constant transmission delay and guaranteed capacity) with those of packet switching (flexibility and efficiency for intermittent traffic).

ATM is a connection-oriented environment. All traffic to or from an ATM network is prefaced with a virtual path identifier (VPI) and virtual channel identifier (VCI). A VPI/VCI pair is considered a single virtual circuit (VC). Each VC is a private connection to another node on the ATM network. It is treated as a point-to-point mechanism to another router or host and is capable of supporting bidirectional traffic.

Each ATM node is required to establish a separate connection to every other node in the ATM network with which it must communicate. All such connections are established using a permanent virtual circuit (PVC) or a switched virtual circuit (SVC) with an ATM signaling mechanism. This signaling is based on the ATM Forum User-Network Interface (UNI) Specification V3.0.

Each VC is considered a complete and separate link to a destination node. Users can encapsulate data across the connection as they see fit. The ATM network disregards the contents of the data. The only requirement is that data be sent to the OC-12 ATM card in the specific ATM adaptation layer (AAL) format.

An AAL defines the conversion of user information into cells. The AAL segments upper-layer information into cells at the transmitter and reassembles them at the receiver. ATM adaptation layer 5 (AAL5), one of four AALs recommended by the International Telecommunication Union Telecommunication Standardization Sector, supports data communications.

An ATM connection transfers raw bits of information to a destination router or host. The ATM router takes the common part convergence sublayer (CPCS) frame, carves it up into 53-byte cells, and sends these cells to the destination router or host for reassembly. 48 bytes of each cell are used for the CPCS data; the remaining 5 bytes are used for cell routing. The 5-byte cell header contains the destination VPI/VCI, payload type, cell loss priority (CLP), and header error control.

Unlike a LAN, which is connectionless, ATM requires certain features to provide a LAN environment to the users. One such feature is broadcast capability. Protocols wanting to broadcast packets to all stations in a subnet must be allowed to do so with a single call to Layer 2. In order to support broadcasting, the router allows you to specify a particular VC as a broadcast VC. When the protocol passes a packet with a broadcast address to the ATM driver, the packet is duplicated and sent to each VC marked as a broadcast VC. This method is known as pseudo broadcasting.

Encapsulation Method Support

SONET/SDH Overview

Synchronous Optical NETwork (SONET) is an American National Standards Institute (ANSI) standard (T1.1051988) for optical digital transmission at hierarchical rates from 51.840 Mbps (STS-1c) to 622.080 Mbps (STS-12c) and greater. SDH is the international standard for optical digital transmission at hierarchical rates from 155.520 Mbps (STM-1) to 2.488 Gbps (STM-16c) and greater.

SONET is an octet-synchronous multiplex scheme that defines a family of standard rates and formats. The available information bandwidth is 600.768 Mbps, which is the STS-3c/STM-4 Synchronous Payload Envelope (SPE), the payload portion of the SONET frame into which the octet-oriented user data is mapped. (Octet boundaries are aligned with the SPE octet boundaries.)

The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) defines a series of SDH transmission rates beginning at 155.520 Mbps as follows:


SONET¹	SDH Equivalent
STS-3c	STM-1
STS-12c²	STM-4c
STS-48c	STM-16c

¹ANSI-defined SONET specifications.

²Currently supported by the OC-12 ATM line card.

Despite the name, SONET is not limited to optical links. Electrical specifications have been defined for CATV 75-ohm coaxial cable. Transmission rates are integer multiples of 51.840 Mbps, which can be used to carry T3/E3 bit-synchronous signals.

Features

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Field-replaceable SFP optics modules with single- mode shor- reach, intermediate-reach, long-reach, and multimode options

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Dedicated I2C master port allowing the route processor (RP) to identify the type of the optical transceiver

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4094 simultaneous AAL5 frame reassemblies and segmentations for average packet size of 200 bytes

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Supports AAL5 data transport, F4 operation, administration, and maintenance (OAM) cells for loopback

The OC-12 ATM line card supports the following protocols, services, and ATM-specific software:

Interface Specifications

The physical layer interface for the OC-12 ATM line card is Optical Carrier-3 (OC-12c, the specification for SONET STS-12c and SDH STM-4c transmission rates). The OC-12 ATM line card provides a single 622.080-Mbps packet OC-12 network interface for the Cisco 7304 router.

The OC-12 ATM line card uses small form-factor pluggable (SFP) modules with LC-type fiber connectors. The SFP is an input/output (I/O) device used to connect the OC-12 to the network. (For more information on the optical fiber cables, see "Cables and Connectors".)

Packet data is transported using Point-to-Point Protocol (PPP) and is mapped into the STS-12c/STM-4c frame (RFC-1619).

Fiber-Optic Transmission Specifications

This section describes the SONET specifications for fiber-optic transmissions, defines the power budget, and helps you approximate the power margin for multimode and single-mode transmissions. This section includes the following subsections:

SONET Distance Limitations

The SONET specification for fiber-optic transmission defines two types of fiber: single mode and multimode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode fiber allows only one mode of light to propagate through the fiber, whereas multimode fiber allows multiple modes of light to propagate through the fiber. Because multiple modes of light propagating through the fiber travel different distances depending on the entry angles, causing them to arrive at the destination at different times (a phenomenon called modal dispersion), single-mode fiber is capable of higher bandwidth and greater cable run distances than multimode fiber.

The typical maximum distances for single-mode and multimode transmissions, as defined by SONET, are in Table 1-1.

Note If the distance between two connected stations is greater than the maximum distances listed, significant signal loss can result, making transmission unreliable.

Table 1-1 OC-12 Optical Parameters
Transceiver Type¹	Transmit Power	Maximum Power to Receiver²	Receiver Sensitivity	Loss Budgets	Nominal Distance Between Stations
Single-mode³ long reach	-5 dB to 0 dBm at 1280-1335 nm⁴	-10 dBm	-34 dBm	10 to 28 dB	Up to 25 mi. (40 km)
Single-mode⁵ intermediate reach	-15 dBm to -8 dBm at 1280-1335 nm 4	-8 dBm	-31 dBm	0 to 12 dB	Up to 9 mi. (15 km)
Multimode⁶ short reach	62.5 um fiber (-20 to -14 dBm) 50 um fiber (-24 to -14 dBm) at 1270-1380 nm 4	-14 dBm	-26 dBm	62.5 um fiber 0 to 6dB 50 um fiber 0 to 2 dB	Up to 500 meters

¹This table gives nominal OC-12 optical parameters.

²This value represents the maximum power to which any receiver can be exposed.

³Complies with Bellcore GR-253-CORE Long Reach Specification (LR-1).

⁴Nominal wavelength is 1310 nm.

⁵Complies with Bellcore GR-253-CORE Intermediate Reach Specification (IR-1).

⁶Complies with Complies with ATM Forum Technical committee 622.08 Physical Layer Specification af-phy-0046.000.

To calculate link losses and dispersion losses for your application, refer to the following specifications and documents:

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ITU-T Recommendation G.957 Optical Interfaces for Equipment and Systems Relating to the Synchronous Digital Hierarchy

Power Budget

To design an efficient optical data link, evaluate the power budget. The power budget is the amount of light available to overcome attenuation in the optical link and to exceed the minimum power that the receiver requires to operate within its specifications. Proper operation of an optical data link depends on modulated light reaching the receiver with enough power to be correctly demodulated.

Attenuation, caused by the passive media components (cables, cable splices, and connectors), is common to both multimode and single-mode transmission.

The following variables reduce the power of the signal (light) transmitted to the receiver in multimode transmission:

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Chromatic dispersion (spreading of the signal in time because of the different speeds of light wavelengths)

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Modal dispersion (spreading of the signal in time because of the different propagation modes in the fiber)

Attenuation is significantly lower for optical fiber than for other media. For multimode transmission, chromatic dispersion and modal dispersion reduce the available power of the system by the combined dispersion penalty. The power lost over the data link is the sum of the component, dispersion, and modal losses.

Table 1-2 lists the factors of attenuation and dispersion for typical optical fiber cable.

Table 1-2 Typical Fiber-Optic Link Attenuation and Dispersion Limits
Limits	Single Mode	Multimode
Attenuation	0.5 dB/km	1.0 dB/km
Dispersion	No limit	500 MHz/km¹

¹The product of bandwidth and distance must be less than 500 MHz/km.

Approximating the OC-12 ATM Line Card Power Margin

The LED used for a multimode transmission light source creates multiple propagation paths of light, each with a different path length and time requirement to cross the optical fiber, causing signal dispersion (smear). Higher-order mode loss (HOL) results from light from the LED entering the fiber and being radiated into the fiber cladding. A worst-case estimate of power margin (PM) for multimode transmissions assumes minimum transmitter power (PT), maximum link loss (LL), and minimum receiver sensitivity (PR). The worst-case analysis provides a margin of error; not all of the parts of an actual system will operate at the worst-case levels.

The power budget (PB) is the maximum possible amount of power transmitted. The following equation lists the calculation of the power budget:

The power margin (PM) calculation is derived from the power budget minus the link loss, as follows:

Table 1-3 lists the factors that contribute to link loss and the estimate of the link loss value attributable to those factors.

Table 1-3 Link Loss Factors and Values
Link Loss Factor	Estimate of Link Loss Value
Higher-order mode losses	0.5 dB
Clock recovery module	1 dB
Modal and chromatic dispersion	Dependent on fiber and wavelength used
Connector	0.5 dB
Splice	0.5 dB
Fiber attenuation	1 dB/km

After you calculate the power budget minus the data link loss, the result should be greater than zero. Circuits with results that are less than zero may have insufficient power to operate the receiver.

The SONET specification requires that the signal must meet the worst-case parameters listed in Table 1-4.

Table 1-4 OC-12 ATM Line Card SONET Signal Requirements
	Single Mode (SML)	Single Mode (SMI)	Multimode
PT	-5 dBm	-15 dBm	-20 dBm
PR	-34 dBm	-31 dBm	-30 dBm
PB	29 dBm	16 dBm	10 dB

Multimode Power Budget Example with Sufficient Power for Transmission

The following is a sample multimode power budget calculated based on the following variables:

PB = 10 dB - 3 km (1.0 dB/km) - 4 (0.5 dB) - 3 (0.5 dB) - 0.5 dB (HOL) - 1 dB (CRM)

The positive value of 2 dB indicates that this link would have sufficient power for transmission.

Single-mode Transmission

The singlemode signal source is an injection laser diode. Single-mode transmission is useful for longer distances because there is a single transmission path within the fiber, and smear does not occur. In addition, chromatic dispersion is also reduced because laser light is essentially monochromatic.

The receiver for single-mode intermediate reach (SMI) cannot be overloaded by the SMI transmitter and does not require a minimum fiber cable length or loss. The maximum receive power for single-mode long reach (SML) is -10 dBm, and the maximum transmit power is 0 dBm. The SML receiver can, therefore, be overloaded when short lengths of fiber are used. Overloading the receiver will not damage the receiver but can cause unreliable operation. To prevent overloading an SML receiver connected with short fiber links, insert a minimum 10-dB attenuator on the link between any single-mode long-reach transmitter and the receiver.

SONET Single-Mode Power Budget Example

The following example of a single-mode power budget assumes two buildings, 8 kilometers apart, connected through a patch panel in an intervening building with a total of 12 connectors.

The value of 6 dB indicates that this link would have sufficient power for transmission and does not exceed the maximum receiver input power.

Using Statistics to Estimate Link Loss and Power Budget

Statistical models more accurately determine the power budget than standard worst-case methods. Determining the link loss with statistical methods requires accurate knowledge of variations in the data link components. Statistical power budget analysis is beyond the scope of this document. For further information, refer to ITU-T standards and your equipment specifications.

The following publications contain information on determining attenuation and power budget:

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T1E1.2/92-020R2 ANSI, the Draft American National Standard for Telecommunications entitled Broadband ISDN Customer Installation Interfaces: Physical Layer Specification.

LEDs

The OC-12 ATM line card faceplate has one LED that indicates line card status and one LED that indicates interface status. (See Figure 1-2.)

After system initialization, the STATUS LED goes on to indicate that power is received and that the OC-12 ATM line card is enabled for operation.

The following conditions must all be met before the OC-12 ATM line card is enabled:

If any one of these conditions is not met, or if the initialization fails, the STATUS LED does not go on.

Table 1-5 OC-12 ATM Line Card LEDs
LED Label	Color	State	Function
STATUS	Green/Yellow	Green	Indicates line card is online.
		Yellow	Indicates line card bootstrapping is in progress.
		Off	Indicates line card is offline and deactivated.
OIR	Green	On	Line card is ready to be removed in CLI-controlled OIR.
OIR	Green	Off	Line card is online.
CARRIER/ALARM	Green/Yellow	Green	Indicates that a valid SONET signal has been detected with no alarm conditions.
		Yellow	Indicates that an alarm condition is present. See Table 1-6 for alarm definitions.
		Off	No valid SONET signal has been detected.
ACTIVE/ LOOPBACK	Green/Yellow	Green	Indicates that the port has been configured and is enabled.
		Yellow	Indicates that the port is in diagnostic loopback mode.
		Off	Indicates that the port has not been configured.

Alarms

A yellow CARRIER/ALARM LED indicates the presence of an alarm condition. Table 1-6 lists the alarm conditions that might be present. Use the show controllers command to show the specific alarm condition.

Table 1-6 Alarm Definitions
Alarm	Definition
Section
LOF	Loss of Frame
LOS	Loss of Signal
BIP(B1)	Bit Interleaved Parity N
Line
AIS	Alarm Indication Signal
RDI	Remote Defect Indication
BIP(B2)	Bit Interleaved Parity N
Path
AIS	Alarm Indication Signal
RDI	Remote Defect Indication
BIP(B3)	Bit Interleaved Parity N
LOP	Loss of Pointer

Small Form-Factor Pluggable Modules

The OC-12 ATM line card uses a single optical small form-factor pluggable (SFP) module to connect to the network. The five types of SFP modules are listed in Table 1-7.

Table 1-7 SFP Modules Used with the OC-12 ATM Line Card
Product ID	Type	Distance	Mbps
SFP-OC12-SR	Single-mode short range	1310 nm	622
SFP-OC12-IR1	Single-mode intermediate range	1310 nm	622
SFP-OC12-LR1	Single-mode long range (LR-1)	1310 nm	622
SFP-OC12-LR2	Single-mode long range (LR-2)	1550 nm	622
SFP-OC12-MM	Multimode	1310 nm	622

Table 1-8 SFP Module Transmit Power, Receive Power, and Power Budget
SFP Module	Transmit Power		Receive Sensitivity
SFP Module	Minimum	Maximum	Minimum	Maximum
SFP-OC12-SR	-5 dBm	-8 dBm	N/A	-23 dBm
SFP-OC12-IR1	-15 dBm	-8 dBm	-8 dBm	-28 dBm
SFP-OC12-LR1	-5 dBm	0 dBm	-10 dBm	-34 dBm
SFP-OC12-LR2	-5 dBm	-14 dBm	-8 dBm	-23 dBm
SFP-OC12-MM	-20 dBm	-14 dBm	-8 dBm	-23 dBm

Table 1-9 SFP Module Specifications
Specification	Description
Dimensions (H x W x D)	0.03 in x 0.53 in x 2.22 in (8.5 mm x 13.4mm x 56.5 mm)
Connectors	Multimode fiber-optic: LC-type connector Single-mode fiber-optic: LC-type connector
Wavelength (average)	SFP-OC12-SR—1310 nm SFP-OC12-IR1—1310 nm SFP-OC12-LR1—1310 nm SFP-OC12-LR2—1550 nm SFP-OC12-MM—1310 nm
Cabling distance (maximum)	SFP-OC12-SR—1.2 miles (2 km) SFP-OC12-IR1—9 miles (15 km) SFP-OC12-LR1—25 miles (40 km) SFP-OC12-LR2—50 miles (80 km) SFP-OC12-MM—1.2 miles (2 km)
Operating temperature range	32 to 122 degrees F (0 to 50 degrees C)
Storage temperature range	-40 to 185 degrees F (-40 to 85 degrees C)

Cables and Connectors

Note For maximum cable lengths between stations, see Table 1-1. Single-mode optical fiber cables for the OC-12 ATM line card are not available from Cisco Systems; they are available from commercial cable vendors.

Warning Invisible laser radiation may be emitted from disconnected fibers or connectors. Do not stare into beams or view directly with optical instruments. Statement 1051

Note The optical fiber connectors must be free of dust, oil, or other contaminants. See Inspection and Cleaning Procedures for Fiber-Optic Connections at the following URL:

http://www.cisco.com/warp/public/127/cleanfiber2.html

Line Card Slot Numbering

In a Cisco 7304 router, slots are numbered beginning with slot 0 (a dual-width slot reserved for the NSE-100) on the bottom, and continuing from the lower left to the upper right through slot 5. The OC-12 ATM line card can be installed in slots 2 through 5. Figure 1-5 shows a Cisco 7304 with an OC-12 ATM line card installed in slot 4.

Line Card Interface Address

Interface addresses specify the actual physical location of each interface on a router. The address is composed of a two-part number in the format slot-number/interface-port-number. See Table 1-10.

The OC-12 maintains the same interface address regardless of whether other cards are installed or removed from the router. However, if you move the OC-12 to a different slot in the router, the first number in the interface address changes to reflect the new slot number.

Table 1-10 Identifying Interface Addresses
Platform	Interface Address Format	Numbers	Syntax
Cisco 7304 router	Slot-number/interface-port-number	Slot—0 through 5¹ Interface port number—0	4/0

¹Slot 0 and slot 1 are reserved for the network services engine (NSE).

The interface address of the interface for the OC-12 in slot 4 is 4/0 (slot 4 and interface 0). If the OC-12 was in slot 2, this same interface would be numbered 2/0 (slot 2 and interface 0).

Table Of Contents

Overview: 1-Port OC-12 ATM Line Card