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This chapter describes the Cisco uBR910 series cable data service unit (DSU), which connects an existing local area network to the IP network provided by a cable service provider. The Cisco uBR910 series DSU allows subscribers to replace their network's telco connection to a remote network such as the Internet with network connectivity provided by a Multiple System Operator (MSO), which is typically a cable operator that provides data and voice IP services in addition to cable TV services.
The chapter provides the following information about the Cisco uBR910 series DSU:
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Note The Cisco uBR910 series includes the Cisco uBR914 DSU. |
For information on configuring the Cisco uBR910 series DSU and on configuring the attached network equipment, see the Cisco uBR910 Series Cable DSU Software Configuration Guide.
This section provides an overview of the Cisco uBR910 series DSU, its hardware, and its basic operation:
The Cisco uBR910 series DSU connects to the cable broadband network and to the access router that is serving as the gateway for the existing LAN. The access router continues to act as the gateway for the LAN, while the Cisco uBR910 series DSU sends the traffic between the access router and the cable network, performing the overhead required to maintain the data cable connection. This simplifies network administration and allows the access router to continue its current function with minimal configuration changes.
The Cisco uBR910 series DSU is based on the Data-Over-Cable Service Interface Specification (DOCSIS) and interoperates with any bidirectional, DOCSIS-qualified CMTS. In its connection with the CMTS, the Cisco uBR910 series DSU functions as a cable modem---a modulator/demodulator that conveys data communications over the cable television network.
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Note The Cisco uBR910 series DSU functions as a DOCSIS 1.0 compliant cable modem in terms of its interoperation with the CMTS and cable broadband network. However, it uses a serial interface to connect to a local area network, as opposed to most cable modems, which use an Ethernet interface to connect to one or more individual PCs or other customer premises equipment (CPE) devices. |
The Cisco uBR910 series DSU connects to the access router through a serial interface compatible with the serial interfaces most commonly used in the Cisco 1600 series, Cisco 1700 series, Cisco 2500 series, Cisco 2600 series, and Cisco 3600 series routers. This allows the Cisco uBR910 series DSU to be easily integrated into existing networks.
Figure 1-1 shows a typical Cisco uBR910 series DSU configuration.
Depending on the version of Cisco IOS software used, different software images might be available for the Cisco uBR910 series DSU, providing a wide range of functionality. At its most basic level of functionality, the Cisco uBR910 series DSU functions as a cable modem that routes LAN traffic to and from a DOCSIS cable network. Other images can provide additional functionality, such as a DHCP server or Network Address Translation (NAT).
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Note The Cisco uBR910 series DSU uses Cisco IOS Release 12.1(1)XD or 12.1(3)T and higher images. All Cisco IOS releases that support the Cisco uBR910 series DSU include at least one image that provides basic DOCSIS connectivity and high-speed Internet data access. |
The Cisco uBR910 series includes a 19-inch rack-mount model (Cisco uBR914) with a field-replaceable router interface module.
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Note The serial interface on the Cisco uBR910 series DSU can be connected only to a compatible serial interface on an access router that is acting as a LAN gateway. Cisco does not support connecting the Cisco uBR910 series DSU to a telco device such as a PBX or to a public switched network. |
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Note The serial interface's maximum data rate specifies only the maximum possible bandwidth on the serial link itself. The overall network performance of the Cisco uBR910 series DSU depends on a great many factors, including the access router being used, the levels of service purchased from the cable service provider, and the overall performance of the broadband cable data network. |
The next section, "Physical Description", describes the physical features of all Cisco uBR910 series DSUs, including the features that are specific to each model.
The Cisco uBR910 series includes a rack-mount model with field-replaceable interface configurations (Cisco uBR914). The following sections describe the physical specifications and features for each model.
The Cisco uBR910 series DSUs have the following physical features:
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Note The Cisco uBR910 series DSU does not support wall mounting. |
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Note The Cisco uBR910 series DSU ships from the Cisco factory with the console port enabled. By default, downloading a Cisco IOS configuration file disables the console port and erases the previously saved configurations. If desired, the service provider can leave the console port enabled to allow the local system administrator to configure and troubleshoot the DSU as needed. |
The Cisco uBR914 DSU is a 19-inch rack-mount model with a slot for one field-replaceable router interface module. Figure 1-2 shows the rear panel.
The following sections describe the Cisco uBR914 and its features in more detail:
See "Connector and Cable Specifications," for the pinouts and cabling information for each of these connectors. For information on using the console port, see the Cisco uBR910 series Software Configuration Guide.
The Cisco uBR914 DSU contains three LEDs on the front panel and five LEDs on the back panel that provide information about the current status and network connections. Figure 1-3 illustrates the LEDs on the front panel. Figure 1-4 illustrates the LEDs on the back panel (not including the LEDs on the router interface module). Table 1-1 lists the LEDs on the Cisco uBR914 and the router interface modules, as well as their meanings.
| Label | Description | Color | Function | ||
|---|---|---|---|---|---|
| Front Panel | |||||
Power | System Status | Green | On = Cisco uBR910 series power-on and self-test diagnostics have completed successfully, the system image has been booted, and the system is operational. Off = No power. If the OK LED turns off and remains off during the boot process, it indicates the Cisco uBR910 series has failed its self-test diagnostics. | ||
RPS | Redundant Power Supply | Red | On = RPS connected and active. Off = RPS not present (the Cisco uBR910 series is using its onboard AC-input power supply). | ||
Activity | Cable Activity | Green | On = Normal operation. | ||
| Back Panel | |||||
CBL LNK | Cable RF Link | Green | On = Cable connection up; RF link up. | ||
CBL ACT | Cable Activity | Green | On = Cable activity. | ||
DS 1 | Downstream Signal Lock Status | Green | On = Cisco uBR910 series is locked to a downstream frequency (channel). | ||
US 1 | Upstream Signal Quality | Green | On = Cisco uBR910 series has established upstream communications with the CMTS, has completed ranging state 2 (as per DOCSIS), has entered provisioning state, and is communicating within 6 dB of desired final power level (generally within 3 dB). Off = Cisco uBR910 series has not completed secondary ranging; the CMTS has heard from the DSU, however, and there is some upstream continuity. | ||
DSNR 1 | Downstream Signal-to-Noise Ratio | Green | On = Receiving quality, downstream signal; SNR is greater than 5 dB above the downstream lock threshold. Off = Receiving low or marginal downstream signal strength or quality.
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The Cisco uBR914 contains the following data connectors (see Figure 1-2):
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Note By default, console port access is automatically disabled when the DOCSIS configuration file specifies that a Cisco IOS configuration file should also be downloaded. The service provider can leave the console port enabled to allow access for a local system administrator to configure and troubleshoot the Cisco uBR914. |
The Cisco uBR914 DSU uses either an onboard AC-input power supply or the redundant power supply (RPS) option. See Table A-1 in "Technical Specifications," for the input power supply power specifications, including input voltage and operating frequency ranges.
The Cisco uBR914 DSU provides a power switch. This switch is intended to remain in the ON position after the cable system technician installs, connects, powers on, and initializes the unit. The unit is turned off only when removing the unit from service or when replacing the router interface module.
The same power supply supports both domestic (U.S.) and international operation. Different power cords are required, however, depending on the country of operation.
The Cisco uBR910 series DSU also supports the Redundant Power Supply (RPS) option. The RPS can provide redundant power to up to four external devices, such as routers, switches, and hubs. To use the RPS, the Cisco uBR910 series AC power supply must be replaced with a power adapter plate that connects to the RPS. See the documentation supplied with the Cisco uBR910 series RPS Adapter Plate for more details.
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Warning The device is designed to work with TN power systems. |
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Warning Never defeat the ground conductor or operate the equipment in the absence of a suitably installed ground conductor. Contact the appropriate electrical inspection authority or an electrician if you are uncertain that suitable grounding is available. |
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Caution Use only a Cisco-provided power supply and cord. Using any other vendors's power supply and cord can cause loss of data or permanent damage. |
This section provides a functional overview of the operation of the Cisco uBR910 series DSU, which can be divided into the following categories:
The Cisco uBR910 series DSU ships from the Cisco factory ready to bridge traffic between its serial and cable interfaces. However, before the Cisco uBR910 series can transmit any traffic, the CMTS at the headend must properly provision the unit as follows:
1. The appropriate service must be purchased from the cable system. A license for the appropriate Cisco IOS software image must also be purchased, depending on the features being used.
2. When the Cisco uBR910 series is first brought online, the CMTS at the headend downloads a DOCSIS configuration file to the unit. This file---which is a binary file that must be in the format required by the DOCSIS 1.0 specification---configures the Cisco uBR910 series for the appropriate level of services and sets other parameters as needed.
3. At this point, the Cisco uBR910 series is completely configured for basic DOCSIS operations, but if additional features beyond those in the default Cisco IOS software image are desired, the DOCSIS configuration file can also specify that the Cisco uBR910 series should download another Cisco IOS image from the CMTS.
4. Finally, the Cisco uBR910 series must also be configured to route traffic between its cable and serial interfaces. This can be done in the following ways:
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Note The CMTS typically downloads the DOCSIS configuration file, Cisco IOS image (if needed), and Cisco IOS configuration file (if needed) only once when the Cisco uBR910 series is initially brought online. However, a new configuration file or image can be downloaded whenever necessary, such as when the cable service offers new services or when subscribers upgrade their services. |
To ensure that subscribers obtain the exact services they have ordered, the Cisco uBR910 series DSU arrives from the Cisco factory with a unique identifier (UID) that consists of a serial number as well as the media access control (MAC) address that is assigned to the cable interface. These factory-assigned values are on a label at the bottom of the chassis; for convenience, these values are also in a barcode label that can be easily scanned in for easy entry into the service provider's provisioning and billing system.
Using the MAC address of the Cisco uBR910 series as the key, the CMTS downloads the specific DOCSIS configuration file and Cisco IOS image that will provide the services this particular subscriber has purchased. Service technicians at the headend typically create a number of standard configuration files to match the range of services offered by the provider; these configuration files can be created manually or with tools provided by Cisco Systems for this purpose.
The following sections describe the initial power-on and provisioning sequence in more detail, and define the requirements that must be met by both the Cisco uBR910 series and the CMTS before provisioning can be successful.
When connected and first powered on, the Cisco uBR910 series DSU performs the following DOCSIS-mandated procedure for automatic installation and configuration:
1. The Cisco uBR910 series boots the Read Only Memory (ROM) from the ROMMON partition of its Flash memory.
2. The Cisco uBR910 series performs a self-test, initializes processor hardware, and boots the main operating system software---the Cisco IOS release image stored in Flash memory.
3. The Cisco uBR910 series acquires the downstream channel by matching the clock sync signal that is regularly sent out by the CMTS on the downstream channel.
4. The Cisco uBR910 series then waits for an Upstream Channel Descriptor (UCD) message from the CMTS and configures itself for the upstream frequency specified in that message.
5. The Cisco uBR910 series waits for the next upstream bandwidth allocation map message (MAP). These messages are regularly sent from the CMTS to find the next shared request timeslot. The Cisco uBR910 series then sends a ranging request message on the next available shared request timeslot, communicating its user ID (UID, which is its unique MAC address), using a temporary service identifier (SID) of 0 (zero) to indicate it has not yet been allocated an upstream channel.
6. In reply to the ranging request, the CMTS sends a ranging response containing a temporary SID to be used for the initial configuration and bandwidth allocation. As needed, the Cisco uBR910 series adjusts its transmit power levels using the power increment value given by the CMTS in its ranging response message.
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Note At this point, the Cisco uBR910 series has established connectivity with the CMTS but is not yet online. The next steps allocate permanent upstream and downstream frequencies, as well as the configuration required for IP network connectivity. |
7. After the next MAP message broadcast, the Cisco uBR910 series uses a shared request timeslot to invoke the Dynamic Host Configuration Protocol (DHCP) to establish IP connectivity with the TCP/IP network at the headend.
8. The DHCP server---typically a dedicated server at the headend, but it could also be a CMTS such as a Cisco uBR7200 series universal broadband router---sends a response containing the IP address for the Cisco uBR910 series, as well as the IP addresses for the default gateway and time of day (ToD) server. It also sends the IP address for the authorized Trivial File Transfer Protocol (TFTP) server from which the Cisco uBR910 series should download its DOCSIS configuration file. Depending on the particular network configuration, other information could be provided, such as the IP addresses for a syslog server or security server.
9. The Cisco uBR910 series configures itself for the specified IP address and gets the current date and time from the specified ToD server.
10. Using the TFTP protocol, the Cisco uBR910 series downloads the specified DOCSIS configuration file and configures itself for the appropriate parameters. The DOCSIS configuration file defines the Cisco uBR910 series' operating mode such as the provisioned downstream and upstream service assignments, assigned frequencies, data rates, modulation schemes, Class of Service (CoS), type of services to support, and other parameters. Cisco provides tools to help automate the creation of configuration files.
11. The Cisco uBR910 series sends another registration request to the CMTS containing the CoS parameters given in the DOCSIS configuration file.
12. The CMTS verifies that the Cisco uBR910 series is using the appropriate CoS profile and converts the temporary SID into a data SID with a service class index that points to the applicable CoS profile.
13. The Cisco uBR910 series completes its secondary ranging and is then online, passing data between the cable network and the PCs and other customer premises devices connected to the Cisco uBR910 series DSU.
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Note At this point, the Cisco uBR910 series is online and operational for DOCSIS traffic. However, to enable routing between the cable and serial interfaces, the following steps are also usually done. |
14. If the DOCSIS configuration file specifies that the Cisco uBR910 series must download a secondary Cisco IOS image and a Cisco IOS configuration file, the Cisco uBR910 series uses TFTP to download the image and configuration files to its local memory. It then installs the new Cisco IOS image and runs the configuration file.
15. If baseline privacy is configured and enabled on both the Cisco uBR910 series and CMTS, the Cisco uBR910 series and CMTS negotiate the appropriate encryption/decryption parameters. After encryption is enabled, all information sent within Ethernet packets is encrypted to prevent interception or modification by an unauthorized party.
Figure 1-5 illustrates the traffic flow during this process.
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Note For more detail on the provisioning process, see the DOCSIS 1.0 Radio Frequency Interface (RFI) specification (SP-RFII01-990731 or later revision). |
After the Cisco uBR910 series DSU goes online, it begins sending the traffic between the cable interface and the access router connected to its serial interface.
The following requirements must be met before a DOCSIS cable modem can be provisioned:
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Note The CMTS uses a cable modem's unique MAC address to ensure that each cable modem downloads only the files appropriate for the levels of service that have been purchased. |
When Cisco IOS images are updated to new releases, the service provider can download them as needed to Cisco uBR910 series DSUs installed in the field (based on the software licenses purchased). See the release notes for a particular release for a complete list of features and Cisco IOS images that are currently supported.
Service providers can use the MAC address assigned to the Cisco uBR910 series DSU's cable interface to uniquely identify each particular Cisco uBR910 series DSU in the field to ensure that the downloaded image matches the services that have been purchased for that unit. The MAC address can also be used to determine which DOCSIS and Cisco IOS configuration files should be downloaded to properly configure the Cisco uBR910 series. These downloads can take several minutes to complete, during which time network connectivity is not available; however, the image and configuration files need to be downloaded only once, until the subscriber requests an upgraded image or updated configuration.
See the Cisco uBR910 Series Software Configuration Guide for a description on how to download a software image and configuration file to a Cisco uBR910 series DSU installed in the field.
This section provides an overview of how data is transmitted to and from the Cisco uBR910 series DSU across the cable system's HFC network. Figure 1-6 illustrates a typical broadband data cable system, showing the network path between the Cisco uBR910 series DSU and the CMTS headend equipment (Cisco uBR7200 series universal broadband router or other DOCSIS-compliant CMTS).
The Cisco uBR910 series DSU provides the connection between the customer's LAN and the cable system, modulating the data transmitted to and from the devices on the local network so that it can be carried over the coaxial cable installed by the service provider. To avoid interfering with the cable video signals that are also transmitted over this same coaxial cable, the DOCSIS specification allows only certain frequencies to be used for data transmissions. Separate frequencies are used for the data sent from the CMTS to the cable modem (the downstream direction) and for the data sent from the cable modem to the CMTS (the upstream direction).
The CMTS divides the cable plant into downstream channels and upstream segments or clusters of nodes. Each Cisco uBR910 series DSU on the network is configured to receive data on a particular downstream channel. A downstream channel contains one or more upstream segments; partitioning the upstream plant into smaller segments significantly reduces the number of potential ingress sources and failure points.
Because 90% of the data transmitted on the Internet is, on average, sent from the network to the user, the cable system allocates the majority of bandwidth for downstream data (data sent from the CMTS to the Cisco uBR910 series). Downstream transmissions use a 6 MHz data channel in the 88 to 860 MHz range, providing an approximate maximum bandwidth of 27 or 26 Mbps. This bandwidth is shared among all subscribers who have been assigned to this particular downstream channel.
The CMTS allocates a particular downstream frequency to the Cisco uBR910 series DSU when it first registers on the system. If that frequency becomes unusable because of noisy conditions, the CMTS instructs the Cisco uBR910 series (and all other DOCSIS-compliant cable modems on that downstream) to transfer operations to a new downstream frequency.
The CMTS receives the downstream data from its own connection to the Internet or from WAN connections with other networks. It addresses the data to the appropriate Cisco uBR910 series DSU and modulates it for transmission on the cable network. When the data arrives at the subscriber's site, the Cisco uBR910 series modulates it for transmission over the serial connection to the access router, which then forwards it to the appropriate customer premises device.
The data transmitted in the upstream direction (from the user to the network) is typically much less than that on the downstream direction, so a smaller bandwidth is allocated to it. The upstream transmissions share a 200 kHz-wide to 3.2 MHz-wide channel in the 5 to 42 MHz range, providing a bandwidth of up to 10 Mbps.
Typically, however, service providers allocate different upstream bandwidths depending on the services purchased by a subscriber. For example, a business purchasing basic service might be allocated a 128 kbps upstream, while businesses purchasing premium services might be allocated a 384 kbps upstream. The CMTS enforces this maximum bandwidth on a per cable modem basis, so that each user receives the services purchased.
The CMTS allocates an upstream frequency to the Cisco uBR910 series DSU when it first registers on the system. The CMTS can then move the Cisco uBR910 series to a new upstream or a new upstream frequency as needed to respond to current network conditions and noise levels.
Depending on the bandwidths allocated to each user, the quality of the physical plant, and the CMTS used at the headend, users on a single downstream can be allocated across several upstreams. This ensures a responsive network for all users and simplifies bandwidth management when new users are added or when a user wants to upgrade to a higher bandwidth service.
The Cisco uBR910 series DSU receives the upstream data from the access router, which receives it from devices on the local area network. The Cisco uBR910 series DSU modulates this data for transmission on the coaxial cable system to the CMTS. The CMTS then routes the data to the appropriate destination (local server, Internet, and so forth) through its WAN interfaces.
The Cisco uBR910 series typically uses a request/grant mechanism to obtain upstream bandwidth, where each user on the upstream must make a specific request before being allowed to transmit on the upstream. DOCSIS rules and the maximum bandwidth cap prevent any single user from monopolizing the upstream bandwidth. If supported by the CMTS, the Cisco uBR910 series supports DOCSIS 1.1 concatenation, whereby multiple transmissions can be combined into one bandwidth request, reducing overhead and maximizing bandwidth efficiency.
The Cisco uBR910 series also supports unsolicited grants, in which part of the upstream bandwidth is reserved for a particular use. This allows service providers to sell services with both a maximum bandwidth and a guaranteed bandwidth. (The CMTS must also support this feature and be configured on a per-user basis.)
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Note End-to-end throughput varies based on the design and loading of network components, the mix of traffic, the processing speed and interface of the host server(s), the processing speed and local Ethernet performance of the subscriber's LAN, and other parameters. Because the network can be configured to support multiple levels of service to meet differing market price/performance requirements, the subscriber's service level agreement also affects throughput. DOCSIS also specifies fundamental performance limitations to give a larger number of customers good performance, rather than permitting a few users to consume the entire capacity. |
Security over the cable interface is provided by the optional Baseline Privacy Interface (BPI) specified by the DOCSIS 1.0 specification. Both the CMTS and Cisco uBR910 series must support BPI security and enable its use before this option can be used.
When using BPI security, the CMTS and Cisco uBR910 series DSU encrypt all data before transmitting it on the cable interface. Data is encrypted using a 40-bit or 56-bit data encryption algorithm, which prevents unauthorized parties from intercepting and reading the data as it travels across the cable network.
When using the BPI option, the Cisco uBR910 series uses a uniquely assigned key encryption key (kek) to connect to the CMTS. The kek authorizes the Cisco uBR910 series to negotiate a traffic encryption key (tek), which the Cisco uBR910 series and CMTS use to encrypt and decrypt the data sent on the cable interface. The keys have a limited lifespan and the Cisco uBR910 series must request a new key before the current one expires.
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Note This feature is not available in all Cisco IOS software images; in addition, this is an export-controlled feature. Check to make sure that the Cisco IOS software image you are using supports baseline privacy. |
This section describes the serial interface that the Cisco uBR910 series DSU uses to connect to the access router---this connection is between the router interface module on the Cisco uBR910 series DSU and the WAN interface on the access router. This connection is characterized by the following parameters:
| Cisco Access Router | Maximum Supported WIC Interface Speed |
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Cisco 805 | 512 Kbps |
Cisco 1600 series | 2 Mbps |
Cisco 1700 series | 2 Mbps |
Cisco 2500 series | 2 Mbps |
Cisco 2600 series | 2 Mbps to 8 Mbps (depending on slot used)1 |
Cisco 3600 series | 2 Mbps to 8 Mbps (depending on slot used)1 |
| 1See the product literature for the Cisco 2600 series and Cisco 3600 series , available on CCO, for more details. |
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Note See the documentation for the access router for a complete description of its interface and the connector that it requires. If the access router is a Cisco router using a WAN interface module that is compatible with one of the router interface modules listed above, see "Connector and Cable Specifications," for a list of Cisco cables to connect the serial interfaces. |
In WAN applications, the DCE device typically serves as the gateway to the wide area network, while the DTE device serves as the entry point into the local area network (such as a router). Because the Cisco uBR910 series DSU is serving as the WAN side of the serial interface, it functions as the DCE device. The access router that serves as the gateway to the LAN typically functions as the DTE device.
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Note The serial interfaces on most Cisco routers can be configured as either DTE or DCE devices. However, Cisco requires that the router interface module on the Cisco uBR910 series DSU be configured as the DCE, while the WAN interface module on the connected access router be configured as the DTE. |
The Cisco uBR910 series DSU supports the following physical layer signaling standards, depending on the router interface module being used:
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Note The serial interface's maximum data rate specifies only the maximum possible bandwidth on the serial link itself. The overall network performance of the Cisco uBR910 series DSU depends on a great many factors, including the access router being used, the type of service purchased from the cable service provider, and the overall performance of the broadband cable data network. |
The Cisco uBR910 series DSU supports the following datalink layer (Layer 2) signaling standards:
Serial signals can travel a limited distance at any given bit rate; generally, the slower the data rate, the greater the distance. All serial signals are subject to distance limits, beyond which a signal significantly degrades or is completely lost. However, in a typical installation of the Cisco uBR910 series DSU, the serial cable distance limitations are not a factor.
This is because the Cisco uBR910 series DSU will be installed close to the access router that is serving as the gateway for the local area network at the subscriber's site. For this reason, Cisco's serial cables for the Cisco uBR910 series are 10 feet in length. If the coaxial cable drop site is not close enough to the access router to allow this, Cisco recommends extending the coaxial cable length, as opposed to extending the serial cable length.
The Cisco uBR910 series DSU operates as an IP bridge between its cable and serial interfaces:
The Cisco uBR910 series DSU can also function as an IP router, using static routes and the RIPv2 routing protocol.
In addition to this basic function, the Cisco uBR910 series DSU can support a number of additional features, depending on the Cisco IOS software image being used and the features supported by the CMTS. See the Cisco uBR910 Series Software Configuration Guide and the release notes that accompanied the Cisco uBR910 series DSU for information on the specific features that are available.
Posted: Thu Jun 15 10:56:47 PDT 2000
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