This section focuses on the network interface second generation (NI-2) family of IP DSL switches. This family of Cisco devices is also called the Cisco 6000 series. Before the NI-2 module’s advanced functionality, the Cisco 6000 series were exclusively called DSL Access Multiplexers (DSLAMs). The NI-2 module provides FCAPS functionality (fault, configuration, accounting, performance, and security). This latest generation of DSLAMs allows Layer 3 intelligence for IP switching, as well as Layer 1 multiplexing.
In addition to management, the NI-2 card provides network uplink and downlink connectivity through the options described next. The NI-2 card allows linking of multiple peers through a technique called subtending (described later in this section), in which up to 12 other IP DSL switches share a single network uplink with fair and balanced queuing of traffic through that uplink from the linked devices.
Network connectivity includes IMA. Inverse multiplexing divides traffic from a single, large connection into carefully balanced smaller connections and then recombines the traffic onto a larger connection. As described next, IMA on the Cisco IP DSL switch makes efficient use of an existing T1/E1 infrastructure in the field, allowing the larger DSL network OC-3 or STm1 optical fiber connection to be carried over multiple T1s/E1s that are already installed.
DSL subscribers are connected directly to DSL modem cards in the DSLAM/IP-DSL switch.
These DSL modem cards have varying numbers of ports, currently either four or eight ports depending on the card type. These modem cards are also called line cards, because each port on a line card terminates a single DSL subscription line, the wire pair extending to the customer premises. This termination of the wire pair is the end of the OSI Layer 1 DSL connectivity.
The Cisco 6000 series of DSLAMs/IP-DSL switches has these characteristics:
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Depending on line card port density, up to 256 DSL subscriber ports VCCs supported by a single DSLAM•
3-Gbps point-to-point backplane•
ATM WAN interface OC-3c/STM1 (both single-mode intermediate reach and multimode fiber), DS-3/E3, or T1/E1 with IMA•
Network Equipment Building Systems (NEBS) Level 3-compliant (6015, 6160)•
European Telecommunications Standards Institute (ETSI)-compliant (6015, 6260)•
Compatible with high-density POTS splitter chassis (PSC)•
Multiprotocol Label Switching/Virtual Private Network (MPLS/VPN)•
IP Security Virtual Private Network (IPSec VPN)•
WebCache awareness to conserve WAN bandwidth and speed the user experience in web browsing•
Multicast for video entertainment and education•
Firewall for enhanced security•
ATM CoS and DSL QoS support for unspecified bit rate (UBR), available bit rate (ABR), variable bit rate real-time (VBRrt), variable bit rate nonreal-time (VBRnrt), and constant bit rate (CBR)Suppose that your network combines voice and data over the same wire pairs. As shown in Figure 4-2, ADSL lines are brought into the CO/exchange’s main distribution frame (MDF).
Figure 4-2 IP DSL Switch System Connections
The line is then jumpered over to a cable block that runs to the PSC. The POTS signal is then split out and run back to the MDF. The ADSL signal is then connected to the ADSL modem parts within the IP DSL switch.
The PSC, provided by companies such as ADC and Corning, separates voice from ADSL signals in advance of the IP DSL switch, thereby preserving emergency calling protection (such as 911 service in the U.S.). The PSC itself is passive, requiring no power, and therefore is unaffected by power loss. Although there are still some MDF-mounted PSCs, the past few years of product development have concentrated on rack-mounted types.
Other successful network arrangements with ADSL do not require use of the PSC. The most common non-PSC architecture makes use of dedicated wire pairs for DSL signals, physically isolating data traffic from the wire pair carrying voice traffic. The DSL provider may be a separate company, or it may be just a department of the voice provider. Symmetric DSL (SDSL) service does not allow line sharing with voice service. As you read in Chapter 1’s discussion of xDSL varieties, SDSL occupies all possible frequencies on the wire pair, even the low ones otherwise used by voice. Therefore, SDSL service does not require a PSC. The SDSL line cards on the IP DSL switch are called STU-Cs (Symmetric Transceiver Units-Central office). ADSL with its voice integration capability makes use of line cards also called Asymmetric Transceiver Units-Remote (ATU-Rs). Both of these card types can have eight ports, or more in the future.
Each Cisco 6160 supports up to 128 DSL modem ports (subscribers) with four-port cards and up to 256 subscribers with eight-port cards. Each Cisco 6260 supports up to 120 subscribers with four-port cards and 240 subscribers with eight-port line cards. The Cisco 6015 supports up
ATM
to 24 direct subscribers with four-port line cards and 48 subscribers with eight-port cards. For ADSL with voice integration on the same wire pairs, the PSCs are generally designed to match these figures closely and to fit compactly onto the same racks as the IP DSL switches themselves.
NI-2-based systems also support subtending. This feature allows up to 12 other chassis to be subtended to a single host DSLAM/IP-DSL switch system, aggregating the subtended systems through a single network uplink. For implementations not fully using the ATM bus and network uplink, subtending can be used to further lower the cost per subscriber.
Subtending is accomplished through the use of WAN interfaces. In a subtending arrangement, each chassis is connected by one WAN interface to the chassis above it in the subtending hier-archy or, if it is at the top of the hierhier-archy, to the network trunk. Each chassis is connected by one or more WAN interfaces to those below it in the hierarchy. The distance allowed between subtended nodes is determined by the WAN interface used, whether optical fiber or coaxial cable.
The current, second generation of Cisco Smart DSLAM follows the first generation of simple DSLAMs. Today the Cisco 6000 IP DSL switch series integrates IP routing with ATM switching at the network edge.
This IP DSL switch family provides an evolution path from the first-generation DSLAM with a UBR-oriented Internet access service model to a varied CoS family of service levels with individual ATM QoS. The service variety makes use of traffic management and policing, extensive-output queue buffering, and virtual path shaping. Switch linking (daisy-chaining multiple IP DSL switches so that they share a common uplink trunk—also called subtending) allows support of up to 3328 ports, allowing network providers to extract maximum value from each network trunk connection without creating the “parking lot” problem encountered with first-generation DSLAMs.
NI-2
The NI-2 is found in these Cisco systems: 6160, 6260, later 6130s, and 6015. (The Cisco 6130 is now end of life [EOL], but many are still deployed around the U.S.) The NI-2 includes the following features:
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Console/craft Cisco IOS software and Ethernet SNMP management•
Processing and nonvolatile storage resources (memory)•
Timing and redundancy control•
Alarm interface for the IP DSL switch itselfTwo WAN options also determine the subtending topology: the OC-3/STM1 creates a daisy-chain topology and the DS3/E3 WAN connection can be set up in a tree (pyramid) topology.
In order for each chassis in a subtended group to have fair access to the shared trunk, each chassis must have a unique ID number. The originating chassis places this ID number in the General Flow Control (GFC) field of the ATM header of each cell, and the ID number is used to forward cells up the tree toward the trunk in a fair manner. The daisy-chain scheme is shown in Figure 4-3.
Figure 4-3 Cisco NI-2 OC-3/STM1 Subtending: Daisy-Chain Topology
Two dual SC connectors are recessed into the NI-2 faceplate. The upper SC connector pair is an uplink; it can serve as either a network trunk port or, in a subtending arrangement, as the subtending interface to the chassis above this one in the hierarchy. The lower SC connector pair is for subtending only; it cannot serve as a trunk port. This means that a Cisco IP DSL switch equipped with the fiber-connected NI-2 has only one downlink for subtending (due to the circuitry size of the optical connections).
In a daisy-chain topology, the top chassis connects to a network trunk and to a subtended chassis. As you will learn in Chapter 6, “Cisco IOS Configurations,” the network trunk is designated Interface ATM 0/1. It is also called the uplink or northbound trunk, although its actual direction may be anywhere. The subtending trunk is designated Interface ATM 0/2 and is frequently referred to as the downlink or southbound trunk. The subtended chassis can connect to another subtended chassis, which can connect to another, and so on. Figure 4-4 shows the top half of the fiber-connected NI-2 card.
Figures 4-5 and 4-6 show the connectors and indicators on the lower half of the OC-3/STM1 NI-2 card.
Network
Trunk Top
Chassis 1st Subtended
Chassis 2nd Subtended Chassis
Up to 10 More Subtended Chassis
DSLAM DSLAM DSLAM
Figure 4-4 OC-3 NI-2 Card Faceplate (Top)
Figure 4-5 OC-3/STM1 NI-2 Card Faceplate (Bottom)
Figure 4-6 OC-3/STM1 NI-2 Card Faceplate (Bottom, Continued)
The components shown in these views are as follows:
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Console port—An RJ-48 connector that is an EIA/TIA-232 port configured as a DCE device. This is used for servicing by craft personnel. You will see in Chapter 6 that it is used briefly during the initial installation of the IP DSL switch to define basic settings.After the initial installation, maintenance and management are usually performed via Ethernet.
•
Auxiliary port—An RJ-48 connector that is an EIA/TIA-232 port configured as a DTE device. When used, this usually connects a dialup (analog) modem to the IP DSL switch for management access redundancy. This port provides modem signaling that the console does not have. In other words, if the Ethernet and trunk connections fail, the service provider can dial in remotely to begin servicing the IP DSL switch.•
Ethernet port—An RJ-45 10BASE-T connector that complies with Ethernet standards.It is used to connect the Cisco IP DSL switch to its management LAN. Management can be performed through Telnet sessions or through SNMP communications, as you will learn in later chapters.
Although several of the LEDs are common to both the coaxial versions of the NI-2 (DS-3, E3) and the fiber-optic versions (OC-3, STM1), there are important differences between the two card versions. The most important consideration for subtending is that there are two available downlinks (subtending links) on the coaxial version of the NI-2 card. As you will learn in depth in Chapter 6’s explanation of software configurations, the network trunk is designated Interface ATM 0/1, the first subtending trunk is designated Interface ATM 0/2, and the other subtending trunk is designated Interface ATM 0/3. This means that subtending can take the form of a pyramid, or tree, as shown in Figure 4-7.
In a tree topology, the top chassis connects to a network trunk and to two subtended chassis.
Those two chassis each connect to two more subtended chassis, which in turn connect to one or two more chassis.
As the coaxial NI-2 offers three coaxial connections, three identical sets of LEDs report the status of the trunk and subtending WAN interfaces. These LED sets are labeled TRNK 1, SBTD 2, and SBTD 3, as shown in Figure 4-8.
Both NI-2 cards’ connectivity versions, optical fiber and coaxial cable, permit module redun-dancy for the NI-2. Understanding NI-2 redunredun-dancy requires understanding two sets of terms.
The basic designation for two NI-2s describes their location in the chassis as seen from the front of the Cisco 6000 chassis. The NI-2 card on the left is always the primary card, and its twin on the right is always the secondary one.
Figure 4-7 Cisco NI-2 DS-3/E3 Subtending: Tree Topology
Either the primary (left side) or secondary (right side) NI-2 can be the active one. The terms active and standby refer to the status of the NI-2s rather than their placement in the chassis. The active (online) NI-2 can be either the primary card or the secondary card. Complementarily, the standby (inactive) NI-2 can be either the primary or secondary card. The standby unit’s boot process is suspended before completion, remaining at standby. There is no CLI access to the standby NI-2. On the standby unit, there is no access to or from Ethernet or console ports. Nor is there access to line cards. Alarms can be asserted, but they are reported via the active NI-2 card.
The active unit offers full access to line cards, full access to system buses, and full access to and from Ethernet and console ports. The active unit uses the Ethernet MAC address on the system I/O board (electrically erasable programmable read-only memory [EEPROM]).
ChassisTop
1 2
3 4 5 6
Network Trunk
Figure 4-8 Cisco NI-2 DS-3/E3 Card Faceplate (Top)
Initial CLI configuration is required after you install a new or replacement standby NI-2. Flash memory and configuration must be manually synchronized before you enable auto-sync. There is no auto-sync immediately after you install a standby card. After initial manual sync, and after sync is enabled, the standby NVRAM mirrors the active unit’s NVRAM. In this auto-synch configuration, the standby unit receives config updates from the active card.
CISCO SYSTEM
Automatic Protection Switching (APS) is a standard method of providing link redundancy on SONET/SDH interfaces, providing link redundancy for OC-3 and STM1 ports. Redundancy is available for both trunk and subtended fiber-optic ports. Switchover occurs in response to loss of signal (LOS) or loss of frame (LOF), which are SONET/SDH failure conditions. As per industry standards, there is a maximum 10-millisecond LOS/LOF detection interval, followed by a maximum 50-millisecond switchover. The switchover applies to the receive path only. Both NI-2 cards transmit the same data simultaneously. APS link redundancy provides for a single OC-3 link, shared by both primary and secondary NI cards. This is done via intercard buses only.
An optical Y-cable cannot be used on NI-2 OC-3 interfaces. As per telecommunications industry standards, the default status of link redundancy is nonrevertive. This means that the switched link does not revert to the former primary link without manual intervention. This prevents pre-mature switching to a still-faulty connection, which could result in a flapping interface.
There are two types of NI-2 redundancy—cold and hot. Cold redundancy is when the standby unit does not completely mirror the state of the active unit. In DSL applications, this means that line-trained rates are not maintained in both the active and standby units. A switchover requires that DSL lines be retrained. Cold redundancy may be planned when a single, regional spare backs up more than one IP DSL switch. The regional spare may be minimally configured for transportability.
Hot redundancy is the state in which the standby unit mirrors the state of the active unit. In DSL applications, this indicates that all line-trained rates are maintained as dynamic information in both the active and standby units. This allows very fast switchovers.
The switchover process is as follows:
Step 1 Standby detects a failure of the active unit, typically a loss of keepalive messages.
Step 2 Standby NI-2 resets and boots.
Step 3 Standby attempts the following functions as part of the sequence to online status:
Step 4 The system syncs the running config if possible.
Step 5 The system syncs Flash memories if possible.
Switchover can be prompted by any failure that interrupts IOS program execution. There is minimal verification of standby unit functionality before failover.
Manual switchover can be made via the command-line interface (CLI) prompt. The system asks for a confirmation and then switches sides. The standby NI-2 resumes and completes the boot sequence, and the previously online NI-2 reboots to standby state.
The green STATUS LED shows the online NI-2. Only the online NI-2 lights this LED.
Redundant data is transmitted and received simultaneously on both the primary and secondary NI-2 OC-3 trunk 0/1 interfaces. If both interfaces are working correctly, OC-3 switchover occurs instantly, as the same data is transmitted simultaneously on two links. The far end also
transmits simultaneously on two links. Either of the two incoming data streams may be selected as the active received traffic source, regardless of which NI-2 is online.
If the standby NI-2 receive path is defective, a manual switch from active to protect status is not allowed.
Physical trunk connections are located on the system I/O card. The common backplane I/O board serves as a DS-3 Y-cable, connecting the online NI-2 trunk ports to a common DS-3 coaxial cable.
There is no DS-3 link redundancy—only a single physical link.
The active and standby DS-3 NI-2 cards share a common physical interface for ports 0/1, 0/2, and 0/3. The coaxial ports can be accessed by either the primary or secondary NI-2 card, but only by the online card.
Line Cards
As you read earlier, individual ports on DSL modem cards are the termination of the DSL connection itself. The modem cards may be asymmetric or symmetric types and are frequently called line cards. The line cards convert xDSL modulation into digital data streams to and from the NI-2 card and negotiate the line rate with the CPE. All Cisco IP DSL switches have three prevailing types of line cards. Not all line cards are available on all DSLAM models due to chassis differences. The three main types of line cards are as follows:
•
The four-port Flexi line card, also called an Asymmetric Transceiver Unit-Central office (ATU-C), not only can provide full-rate and half-rate DMT, but also can be programmed to support the legacy Carrierless Amplitude Phase modulation (CAP) technology. Although it is increasingly being displaced by the worldwide standards of DMT, CAP was widely deployed around the world several years ago. Therefore, the Flexi offers a migration for service providers to the newer, standardized technologies on the same IP DSL switch platform.•
ADSL line cards (ATU-Cs) come in both four- and eight-port versions. These cards automatically recognize and adapt to either the ANSI DMT2 standard (T1.413.2) or the ITU standards for both full-rate (256 tones) and half-rate (128 tones) DMT.•
The G.SHDSL Symmetric Transceiver Unit-Central office (STU-C) is an eight-port line card providing the ITU symmetric service standard.All line cards except the Flexi are automatically discovered by the NI-2. The NI-2 first queries the card to determine its validity for the particular chassis type. Then the NI-2 analyzes the line card’s software image. If the image on the new line card is not the same version as the image for the card type contained on the NI-2, the NI-2 automatically downloads its own copy of the software image directly to the line card. This takes about 2 or 3 minutes, depending on the particular line card.
As shown in Figure 4-9, the Flexi card’s LEDs indicate what type of DSL service is in place.
Figure 4-9 Flexi Line Card LEDs
The Flexi is autodiscovered by the NI-2, but the software image upgrade requires manual intervention by the carbon-based unit to configure that Flexi for either DMT or CAP. In other
The Flexi is autodiscovered by the NI-2, but the software image upgrade requires manual intervention by the carbon-based unit to configure that Flexi for either DMT or CAP. In other