Table of Contents

Physical Description

BPX Enclosure
Node Cooling
Node DC Powering
Optional AC Power Supply Assembly
Card Shelf Configuration

Functional Description

ATM

Physical Layer
ATM Layer

IPX and IGX Trunk Interfaces to ATM

BPX Addressing Mode
Simple Addressing Mode

Cloud Addressing Mode
FastPacket Adaptation to ATM

BPX Major Groups

General Description

This chapter contains an overall physical and functional description of the BPX. The physical description includes the BPX enclosure, power, and cooling subsystems. The functional description includes an overview of BPX operation.

Physical Description

Functional Description

BPX Major Groups

Physical Description

The BPX is supplied as a stand-alone assembly. It may be utilized as a stand-alone ATM switch, or it may be integrated at customer sites with one or more narrowband IPX nodes, multi-band IGX nodes, AXIS shelves, and other access devices to provide network access to broadband backbone network links for narrowband traffic. Cisco StrataCom and CPE service interface equipment can also be co-located with the BPX and connect to its ATM service interfaces.

BPX Enclosure

The BPX enclosure is a self-contained chassis which may be rack mounted in any standard
19 inch rack or enclosure with adequate ventilation. It contains a single shelf which provides fifteen slots for vertically mounting the BPX cards front and rear. See Figure 2-1 which illustrate the front view of the BPX Shelf.

At the front of the enclosure (Figure 2-1) are 15 slots for mounting the BPX front cards. Once inserted, the cards are locked in place by the air intake grille at the bottom of the enclosure. A mechanical latch on the air intake grille must be released by using a screwdriver and the grille must be tilted forward in order to remove or insert cards.

At the rear of the enclosure (Figure 2-2) is another series of card slots for mounting the rear plug-in cards. These are held in place with two thumbscrews, top and bottom. A mid-plane, located between the two sets of plug-in cards, is used for interconnect and is visible only when the cards are removed.

To provide proper cooling, it is essential that blank faceplates be installed in all unused slots. Failure to do so will degrade node cooling and circuit card damage will result. The blank faceplates also provide RFI shielding.

Figure 2-1: BPX Cabinet Exterior Front View

BPX Cabinet Exterior Rear View

Node Cooling

A fan assembly, with three six-inch 48 VDC fans is mounted on a tray at the rear of the BPX shelf (Figure 2-2). Air for cooling the cards is drawn through an air intake grille located at the bottom in the front of the enclosure. Air passes up between the vertically-mounted cards and exhausts at the top, rear of the chassis. All unused slots in the front are filled with blank faceplates to properly channel airflow.

Node DC Powering

The primary power for a BPX node is -48 VDC which is bused across the backplane for use by all card slots. DC-to-DC converters on each card convert the 48V to lower voltages for use by the card. The 48 VDC input connects directly to the DC Power Entry Module (PEM). The DC Power Entry Module () provides a circuit breaker and line filter for the DC input.

Nodes may be equipped with either a single PEM or dual PEMs for redundancy. They are mounted at the back of the node below the backplane. A conduit hookup box or an insulated cover plate is provided for terminating conduit or wire at the DC power input. It is recommended that the source of DC for the node be redundant and separately fused.

Optional AC Power Supply Assembly

For applications requiring operation from an AC power source, an optional AC Power Supply Assembly and shelf is available. It provides a source of -48 VDC from 208/240 VAC input. A shelf, separate from the BPX shelf, houses one or two AC Power Supplies and mounts directly below the node cabinet. This provides a secure enclosure for the power supply assemblies (supplies cannot be removed without the use of tools).

Two of these supplies are usually operated in parallel for fail-safe redundant operation. The front of the AC Power Supplies for the BPX includes two green LEDs to indicate correct range of the AC input and the DC output for each individual supply (Figure 2-4).

Card Shelf Configuration

There are fifteen vertical slots in the front of the BPX enclosure to hold plug-in cards (Figure 2-5). The middle two slots, slots number 7 and number 8, are used for the primary and secondary Broadband Controller Cards (BCC). The right-most slot, number 15, is used to hold the single Alarm/Status Monitor Card. The other twelve slots, number 1 through number 6 and number 8 through number 14, can be used for the Network Interface and Service Interface cards.

Functional Description

ATM

ATM transmits broadband information using fixed length, relatively small, 53-byte cells which are suitable for carrying both constant rate data (for example, voice and video) as well as bursty data.

ATM evolved from the Broadband Integrated Services Digital Network (B-ISDN) standard, which in turn is an extension of ISDN. ISDN defines service and interfaces for public telecommunications networks. B-ISDN utilizes a 7-layer reference model similar to the Open Systems Interconnection (OSI) 7-layer architecture. ATM redefines the lower three levels as shown in Figure 2-6. These are the Physical Layer, the ATM layer, and the ATM Adaptation Layer (AAL).

Physical Layer

The physical layer is divided into two parts, the Transmission Convergence sub-layer and the Physical Medium sub-layer.

The Physical Medium sub-layer (PMD) handles processing specific to a particular physical layer, such as transmission rate, clock extractions, etc.

The Transmission Convergence sub-layer (TC) extracts the information content from the physical layer data format. This includes HEC generation and checking, extraction of cells from the data stream, processing of idle cells, etc.

ATM Layer

The ATM layer processes ATM cells. The ATM cell consists of a 5-byte header and a 48-byte payload. The header contains the ATM cell address and other management information Figure 2-7.

There are two basic header types defined by the standards committees, a UNI header and a NNI header; both are quite similar. Cisco has expanded on these header types to provide additional features beyond those proposed for basic ATM service. Usage of each of the various cell header types is described as follows:

• The UNI header (Figure 2-8) must be specified for each User-to-Network Interface. A UNI is any interface between a user device, such as an ATM router, and an ATM network.
  
  
• The NNI header (Figure 2-9) must be specified for each Network-to-Network Interface. This is used, for example, at the interface between a user's private ATM network and a service provider's public ATM network.
  
  
• The STI header (Figure 2-10) is an extension of these two header types and is a Cisco StrataCom Interface. This header type is used between Cisco StrataCom nodes to provide advanced network features, including ForeSight, that improve performance, efficiency, and congestion control.
  
  

Figure 2-8: UNI Header

NNI Header

STI Header

The most important fields in all three ATM cell header types are the Virtual Path Identifier (VPI) and a Virtual Circuit Identifier (VCI). The VPI identifies the route (path) to be taken by the ATM cell while the VCI identifies the circuit or connection number on that path. The VPI and VCI are translated at each ATM switch, they are unique only for a given physical link.

In the STI header (Figure 2-10), the Payload Class is used to indicate various classes of service and BPX queues, for example, Opticlass, the enhanced class of service feature of the BPX. The ForeSight Forward Congestion Indication, the F bit, is used by ForeSight for congestion status.

The Cell Loss Priority (CLP) bit follows the PTI bits in all header types. When set, it indicates that the cell is subject to discard if congestion is encountered in the network. For Frame Relay connections, depending on mapping considerations, the frame Discard Eligibility status is carried by the CLP bit in the ATM Cell. The CLP bit is also set at the ingress to the network for all cells carrying user data transmitted above the minimum rate guaranteed to the user.

ATM Cell Addressing

Each ATM cell contains a two-part address, VPI/VCI, in the cell header. This address uniquely identifies an individual ATM virtual connection on a physical interface. VCI bits are used to identify the individual circuit or connection. Multiple virtual circuits that traverse the same physical layer connection between nodes are grouped together in a virtual path. The virtual path address is given by the VPI bits. The Virtual Path can be viewed as a trunk that carries multiple circuits all routed the same between switches

The purpose of the ATM Adaptation Layer (AAL) is to receive the data from the various sources or applications and convert, or adapt, it to 48-byte segments that will fit into the payload of an ATM cell. Since ATM benefits from its ability to accommodate data from various sources with differing characteristics, the Adaptation Layer must be flexible.

Traffic from the various sources have been categorized by the standards committees into four general classifications, Class A through Class D, as indicated in Table 2-1. This categorization is somewhat preliminary and initial developments have indicated that it may be desirable to have more than these initial four classes of service.

Initially, four different adaptation layers (AAL1 through AAL4) were envisioned for the four classes of traffic. However, since AAL3 and AAL4 both could carry Class C as well as Class D traffic and since the differences between AAL3 and AAL4 were so slight, the two have been combined into one AAL3/4.

The ATM Adaptation Layer consists of two sub-layers (Figure 2-6):

• Convergence Sub-Layer (CS)
  
  
• Segmentation and Reassembly Sub-Layer (SAR)
  
  

Data is received from the various applications layers by the Convergence Sub-Layer and mapped into the Segmentation and Reassembly Sub-Layer. User information, typically of variable length, is packetized into data packets called Convergence Sublayer Protocol Data Units (CS-PDUs). Depending on the Adaptation Layer, these variable length CS-PDUs will have a short header, trailer, a small amount of padding, and may have a checksum.

The Segmentation and Reassembly Sub-Layer receives the CS-PDUs from the Convergence Sub-Layer and segments them into one or more 48-byte SAR-PDUs, which can be carried in the 48-byte ATM information payload bucket. The SAR-PDU maps directly into the 48-byte payload of the ATM cell transmitted by the Physical Layer. Figure 2-11 illustrates an example of the Adaptation Process.

IPX and IGX Trunk Interfaces to ATM

The IPX/IGX connect to a BPX or other ATM switch via an AIT/BTM T3 or E3 trunk. The AIT(IPX) or BTM (IGX) can operate in several different addressing modes selected by the user (Table 2-12 and Figure 2-13). To allow the IPX or IGX to be used in mixed networks with other ATM switches, there are two other addressing modes available, Cloud Addressing Mode (CAM) and Simple Addressing Mode (SAM).

BPX Addressing Mode

In the BPX Addressing Mode (BAM), used for all Cisco StrataCom networks, the system software determines VPI and VCI values for each connection that is added to the network. The user enters the beginning and end points of the connection and the software automatically programs routing tables in each node that will carry the connection to translate the VPI/VCI address. The user does not need to enter anything more. This mode uses the STI header format and can support all of the optional Cisco StrataCom features.

Simple Addressing Mode

In the Simple Addressing Mode (SAM), the user must manually program the path whole address, both VPI and VCI values.

Cloud Addressing Mode

The Cloud Addressing Mode (CAM) is used in mixed networks where the virtual path addresses are programmed by the user and the switch decodes the VCI address. Both CAM and SAM utilize the UNI header type.

FastPacket Adaptation to ATM

A specialized adaptation that is of particular interest to users of Cisco equipment is the adaptation of IPX FastPackets to ATM cells. There are a large number of narrowband IPX networks currently in existence that are efficiently carrying voice, video, data, and Frame Relay. A means must be provided to allow these networks to grow by providing a migration path to broadband.

With the Simple Gateway protocol, the AIT card in the IPX (or BTM in the IGX) loads 24-byte FastPacket cells into ATM cells in ways that are consistent with each application. (Each of the two FastPacket cells loaded into the ATM Cell is loaded in its entirety, including the FastPacket header.) For example, two FastPackets can be loaded into one ATM cell provided they both have the same destination. This adaptation is performed by the IPX AIT card or the IGX BTM card.

Starting with Release 8.1, with the Complex Gateway capability, the FRSM card in the AXIS, the AIT card in the IPX (or BTM card in the IGX) streams the Frame Relay data into ATM cells, cell after cell, until the frame has been completely transmitted. Since only the data from the FastPacket is loaded, the Complex Gateway is an efficient mechanism. Also, discard eligibility information carried by the Frame Relay bit is mapped to the ATM cell CLP bit, and vice versa. See Chapter 13 for further information on Frame Relay to ATM interworking. A comparison of the simple gateway and complex gateway formats is shown in Figure 2-14.

BPX Major Groups

There are four major groups in the BPX. These major groups are listed in Table 2-2.

• Common Core
  
  
• Network Interface
  
  
• Service Interface
  
  
• Power Supplies
  
  

Table 2-2 lists these groups and their components along with a brief description of each.

Posted: Mon Aug 19 18:09:18 PDT 2002
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