The MGX 8240 Private Line Service Gateway offers a high density ATM Circuit Emulation Service (CES) access concentration solution for T1, Fractional T1, and DS0 leased line service providers.

Used in conjunction with an ATM core network, the MGX 8240 chassis provides a cost effective private line transport service. The MGX 8240 provides integration of voice service and private lines, and transfers traffic onto a common ATM backbone.

The MGX 8240 can be provisioned and managed remotely from a central management and alarm station, thus decreasing the operations and equipment costs and provisioning time. Because each private line is provisioned through software, new circuits can be added quickly, with point-and-click operations.

The MGX 8240 performs the following:

Provides complete CES (structured and unstructured).
Minimizes the cost and time for provisioningT1 and Fractional T1 private lines. The cost per port is dramatically lower than when using traditional TDM/Digital Cross Connect equipment.
Each PSM card supports up to 2016 circuits— 3(DS3s) x 28(DS1/DS3) x 24(DS0/DS1).
Provides efficient transport.
Supports mixed voice/data traffic on a single private access line without requiring any change to the cards.
Eliminates the need for digital cross-connect ports, since all traffic is handled by the ATM switch fabric.

A TM/CES and Service Interworking

Carriers transfer certain types of constant bit-rate (CBR) or "circuit" traffic over ATM networks. To support CBR traffic, packet-oriented ATM-based networks must emulate circuit characteristics. A CBR circuit transported using Circuit Emulation Service (CES) over ATM is comparable to the current Plesichronous Digital Hierarchy (PDH) technology.

Figure 1-1 provides a reference model for CES. It contains two ATM CES Interworking Functions (IWFs) connected to an ATM network via physical interfaces defined in the ATM Forum UNI Specification. The CES Interworking Functions (IWFs) are also connected to standard CBR circuits (for example, DS1).

Figure 1-1: CES Reference Model

The two IWFs extend the constant bit-rate circuit to which they are connected across the ATM network. This is transparent to the terminating equipment of the CBR circuit. This means that the ATM portion of the connection retains its bit integrity (analog signal loss cannot be inserted and voice echo control cannot be performed). Thus for facilities intended to carry voice or multimedia services, any required echo control must be performed by the terminal equipment or before the ATM CES IWF is encountered. Using AAL 1 over ATM constant bit-rate virtual channels is a simple, general, and effective method of addressing this type of application.

Private Line Service

The MGX 8240 provides private line transport over the ATM network. Figure 1-2 shows the Private Line Transport Service.

Figure 1-2: Private Line Transport Service

Service Modes

The following section discusses the types of service modes for the MGX 8240.

DS1 CES Structured Service

Nx64 service emulates a point-to-point Fractional DS1. The service is accessed via 1.544 Mbit/s DSX-1. The 24 timeslots available at the DSX-1 interface are carried across the ATM network and reproduced as output.

Note DS0s may be combined in contiguous or non-contiguous slots up to the full DS1.

DS1 CES Unstructured Service

Many applications use DS1 interfaces, making use of the entire bandwidth. DS1 unstructured CBR service is intended to emulate a point-to-point DS1 circuit. The service is accessed via 1.544 Mbit/s DSX-1. The service is defined as a "clear channel pipe," transparently carrying the full data stream (DS1 at 1.544 Mbit/s).

For Synchronous Residual Time Stamp (SRTS), the end-user timing source interface signals are not necessarily traceable to a primary reference source (PRS). For Synchronous, the end-user timing source interface signals must be traceable to PRS.

Note Framing formats other than standard SF or ESF formats cannot be supported by all Plesichronous Digital Hierarchy (PDH)/Synchronous Digital Hierarchy (SDH) installed equipment. If an exchange carrier offers CES service for such non-standard framing formats, the carrier may have difficulty in maintaining the service interface due to the lack of facility support for operations and maintenance functions such as performance monitoring, facility loopbacks, and so forth.

If SF or ESF framing is used, the DS1 unstructured service also provides an optional feature that allows non-intrusive performance monitoring of the link.

Circuit Emulation Service Connection

A CES connection can reside on either a full DS1 (unstructured or structured 24 x DS0), fractional DS1 (2 to 24 DS0s), or single DS0. Each line card or PSM can support three DS3s for a transfer up to 2016 circuits.

1 DS0 to 24 DS0s can be supported only using structured mode. Once a connection is built, it cannot be modified between structured and unstructured.

Clocking

Each Processor Service Module (PSM) card has multiple sources for deriving the card's timing for physical layer clocking (see Figure 1-3). On the Internal Management Card Input/Output card (IMC I/O), two BITS clock inputs (labeled BITS A and BITS B) connect to each PSM card. In addition to the BITS sources, the network SONET timing can also be used for system (card) timing. Network SONET timing can be used as a timing source only for the card where the SONET port resides. Separate cards cannot share a single SONET timing source.

Figure 1-3: Clocking Diagram

Note Each PSM card has its own oscillator which drives its timing from the BITS A, BITS B, or local network SONET port.

Synchronization of clocking is critical to forwarding DS1 circuits through the network. Non-synchronized clocks in a TDM network can result in frame slips and loss of data.

To prevent data loss within the network, it is important to keep the clocks of the two TDM end points synchronized with the same reference clock.

There are two sources from which end devices receive timing:

1. Building Integrated Timing Supply (BITS) clocks (BITS A and BITS B)

2. Receive clock of the network SONET ports

Two BITS timing inputs exist on the MGX 8240—BITS A and BITS B. Either of these timing inputs can be configured as the primary or secondary timing reference. Each card can be configured as BITS A, BITS B, or network SONET port.

The MGX 8240 supports definition of a primary and secondary clock on the network. Any method of clocking, BITS (A or B) or network SONET receive clock, can back up the other. The operator can configure any of these as the primary or secondary source. The active timing source is used to time all transmitted synchronous signals.The MGX 8240automatically switches to an alternate timing reference when the primary reference fails. The automatic switch to the alternate timing source occurs within 50 ms.

All provisioned references, both primary and secondary, are continuously monitored for failures. When the primary reference fails, the switch uses the secondary reference. When all timing references fail, the MGX 8240 switches to its free-running internal oscillator.

The MGX 8240 allows users to provision the switching mode between references as revertive and non-revertive. The default mode for switching between references is non-revertive. For revertive switching, automatic restoration from secondary to primary occurs within two seconds after the clearing of the reference failure.

The following status information is available via theVirtual Command Line Interface (VCLI):

Clock mode (normal and free-run).
Time of most recent reference switch.
Indication of a forced state (such as override of revertive switching).
Input reference that is in active use.
Condition of input reference source (normal, LOS, AIS).

The VCLI provides a command to switch to a selected input if it is available. If the reference is in a failed condition, this request is not honored.

Redundancy

The MGX 8240 card-level redundancy enables one PSM card to serve as a backup for a configurable number of primary cards. This group of cards, called a redundancy group, can contain from one to four primary cards. The cards in a redundancy group must be placed in adjacent slots.

The MGX 8240 can support up to four redundancy groups. Whenever there is failure in any of the primary cards that a backup is protecting, the backup card automatically takes over for the failed primary card-this is known as a 'SwitchOver.'

Note Following a SwitchOver, the remaining cards served by the switched-in backup become unprotected. In other words, if another primary card in the redundancy group failed, all service on that primary card would be lost.

A backup can be made to take over for a primary—this 'Forced SwitchOver' can be useful in several instances. For instance, if the redundancy software fails to detect a failure and initiate a SwitchOver, the operator can force a SwitchOver. Also, a Forced SwitchOver is useful during maintenance, such as software upgrade on a card. During maintenance, the operator can do a Forced Backup and thus prevent an interruption to service.

SwitchOver is not auto-revertive. That is, the newly switched-in backup card does not revert back to being a backup once the original primary card has been repaired or replaced. Re-assigning the roles requires operator intervention via an external management interface. This operation is called a 'Forced SwitchBack.'

The backup card must be the left-most card within any single Redundancy Group. The configuration of M (number of Redundancy Groups) and N (number of Primary cards in each group) is done via management station interfaces. Redundancy Groups cannot be overlapped. All cards in a Redundancy Group must be placed side-by-side.

All cards in a redundancy group must be the same type, and all of them must have the same software release.

The cards of the MGX 8240 can be grouped into up to 7 separate redundancy groups. For example, if an operator configured conservatively, s/he could have seven separate 1:1 redundancy groups, and a slot left over that could be an unprotected primary. Figuring aggressively, an operator could have four groups of 1:4 redundancy.

Note Redundancy is not used when the MGX 8240 is to be deployed in its most port-dense configuration (45 channelized DS3s). In this case, all 15 slots are populated with unprotected cards.

IMC Redundancy and Switchovers

Two IMC cards are used in each MGX 8240 switch to provide 1:1 redundancy. In the initial configuration, the top IMC is primary, with the lower card standing by as a backup. As with PSM cards, switchovers happen automatically if the primary card fails, or a user can force a switchover.

Whenever there is an IMC card switchover, all the active telnet and FTP sessions must be re-established. Otherwise, the current telnet or FTP session will be terminated, and all telnet sessions will hang.

After a switchover, roles are reversed. When the lower card becomes primary, it stays primary until the next switchover—the original primary/backup status (primary on top, secondary on bottom) is not automatically reset at startup. On switchover, it takes about 15 seconds for the card to become reachable by management stations.

PSM Card Redundancy

Card-level redundancy is configured from the management station. When cards are inserted into the MGX 8240 chassis and service-turnup is performed via CLI, a card defaults to a Redundancy Role of none, and its Redundancy Operational State is set to non-redundant, meaning that it is unprotected. Cards are protected and unprotected (made redundant) via the management station as described below.

Note The management station should maintain a non-volatile database containing the redundancy configuration of the MGX 8240 chassis as a whole.

A redundancy group contains one backup card with up to four primary cards. To configure a card to protect (back up) a group of primary cards, the operator uses the management station to configure the Redundancy Role of that card to bebackup. Thereafter, cards can be added to the backup's redundancy group (and hence made redundant), by using the management station to set those cards' Redundancy Roles to primary.

Cards in a redundancy group must be adjacent to each other and contiguous. The backup card will be the card on the left, with up to four primary cards to its right.

The network management does not limit how many cards can be added to a redundancy group. Users must know that, if more than four primary cards are put in a redundancy group, only the first four cards added will be protected.

Note Primary cards are reachable via in-band and/or out-of-band management; backup cards are only reachable via out-of-band management.

Network Management

All MGX 8240 PSM cards can be managed by Cisco Wan Manager (CWM), by the VCLI , and by a menu-driven interface (CMDR). For more information on the CWM, see "Ordering Documentation" section. The VCLI is used to configure specific switches, but it cannot manage the entire network at once. The CMDR is used primarily for the First Day of Service (FDOS). It provides the user with a specific set of screens focused on the initial setup of the switch. A limited number of troubleshooting commands are also supported.

This CLI can be accessed using one of the following ways:

Serial RS232 port
Telnetting to out-of-band management (ethernet) IP address with Port number n*1000 +4=n004 (see Table 1-1).

The MGX 8240 supports out-of-band network management, providing connections between the MGX 8240 and a network management station.

Users can access the MGX 8240 PSM cards using IP via an Ethernet connection through IMC (one IP address for the entire chassis).

Table 1-1: IP Addresses

Service	IP Address	Port Number
SNMP	IMC IP address	(n*1000 + 1)
Telnet	IMC IP Address	(n*1000 +2)
FTP	IMC IP Address	(n*1000 +3)
CLI FDOS	IMC IP Address	(n*1000 +4)

Virtual Command Line Interface

The Virtual Command Line Interface (VCLI) is used for configuring MGX 8240 cards, and it runs on an external workstation. It is an SNMP-based network management package.

The Telnet connection is to the CLI and FDOS menus. The FTP connection allows updates of system binary or configuration files over the network. SNMP packets carry binary queries, configuration settings, replies and asynchronous alarms. These are SNMP V1.0 compatible Protocol Data Units (PDUs).

Cisco WAN Manager

The primary management tool for the MGX 8240 is Cisco WAN Manager (CWM). For information on CWM, see documentation for Cisco Wan Manager.

Integrated Bit Error Rate Tester

The MGX 8240 has three integral Bit Error Rate Testers (BERTs) on each PSM card. BERT testing is done at startup as port of diagnostics on cards. Users can also perform BERT testing manually on a DS1 or CES connection through the VCLI. DS1 BERT can only test the TDM side; connection BERT can test either the TDM or ATM side. All three BERTs can run tests on separate DS1s simultaneously, but each BERT tester can test only one DS1 at a time. Data collected during these tests is stored on the switch, and can be accessed via SNMP.

A loopback from the PBX switch to the MGX 8240 should be done before turning on the integral BERTs on the MGX 8240 (see Figure 1-4).

Figure 1-4: Using Integral BERT to Loopback to TDM Switch

CES Connection BERT

BERT testing can also be performed over a CES Connection. If the CES connection is configured for structured CES, then BERT testing on the connection can be used to perform DS0 or NxDS0 BERT testing. BERT testing will not work properly for structured CES connections that are configured in CAS mode.

If the CES Connection BERT is being performed by looping back the BERT pattern, either the far end of the DS1 port, or the individual DS0s (in the case of NxDS0) must be looped back externally.

During a CES Connection BERT test towards the ATM side of the connection, the local CES endpoint interworking function will not generate or receive ATM cells.

OAM Loopback Delay Measurement

With the OAM Loopback Delay feature, you can measure round trip OAM cell delays through ATM networks. Precise time measurement of OAM cells is performed as follows:

1. The OAM cell generator records the time of the loopback OAM cell creation.

2. The OAM cell generator reads the time of the looped back OAM cell return.

3. The software calculates the OAM Loopback Delay based on the time stamps in procedures 1 and 2, above.

Automatic Protection Switching

Automatic Protection Switching (APS) provides a redundant SONET physical link for the SONET ATM trunk port (see Figure 1-5).

Figure 1-5: Redundant Trunking

To support APS between a MGX 8240 PSM card and an ATM switch, two separate lines provide connection with one line acting as the working line and the second line as the redundant (protection) line. For the PSM cards, the OC3-1 is called the working line, and OC3-2 is called the protection line. The same traffic is sent over both lines, but only one line is in use at any given time.

Two different APS architectures are defined for SONET trunks: 1+1 and 1:n. The MGX 8240 supports the APS 1+1, non-revertive, unidirectional or bidirectional model. With the 1+1 architecture, the signal is bridged and both links receive the same signal simultaneously. Should the primary link go down, the signal is automatically transferred to the backup or protection line.

The MGX 8240 supports unidirectional or bidirectional APS. When the MGX 8240 is operating in bidirectional APS, only one of the two SONET port directions (TX or RX) needs to fail for the signal to be transferred to the backup (protection line). When it is operating in unidirectional mode, switching of the working channel could happen independently. That is, one switch could receive the signal from the working line, while the other switch could receive the signal from the protection line. Once line conditions change and the working line becomes viable again, the signal can be transferred back to the working line. Because the MGX 8240 supports non-revertive APS, the signal must be transferred back to the working line manually through user configuration.

Note Only one ATM logical port is supported per PSM card. It resides on the active SONET trunk. Should a trunk carrying the ATM logical port fail, it automatically switches to the active trunk.

Note Both ends of a connection must be configured either as bi-directional or as uni-directional.

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