Cable and DSL Broadband Access Technologies

Broadband access technologies provide high-speed, always-on communication between homes and businesses via the Internet. Broadband access technologies enable data, voice, video, and other multimedia communication practical for homes and businesses. There are several broadband access technologies available today. These access technologies include cable, Digital Subscriber Line (DSL), WiMax, Direct Broadcast Satellite (DBS), Integrated Services Digital Network (ISDN), and fixed wireless technologies, such as Local Multipoint Distribution System (LMDS), Multichannel Multipoint Distribution System (MMDS), and Digital Electronic Messaging Service (DEMS). Although there are several broadband access technologies, the only two technologies that fall within the scope of the current CCNP certification exam are cable and DSL. These technologies are described in additional detail in the following sections.

Cable Access Technologies

Cable modems are network bridge devices that operate at Layer 1 and Layer 2 of the OSI model and connect home or branch office networks to the Internet through the cable television system. On the network side, cable modems support Ethernet, and on the cable side, they support Data-over-Cable Service Interface Specifications (DOCSIS). However, it should be noted that although cable modems generally follow the DOCSIS standard, implementations commonly can and do vary by provider. DOCSIS is beyond the scope of the current TSHOOT certification exam and will not be described in any further detail in this chapter.

Cable companies use Hybrid Fiber Coaxial (HFC) cable networks to provide fiber and coaxial connections to the customer. HFC is used to describe a standard telecommunications industry term for a broadband network that combines both optical fiber and coaxial cable. The HFC network is comprised of the following components:

• Cable Modems
• Cable Modem Termination Systems
• Local Headend

Cable modems (CMs) communicate with routers called Cable Modem Termination Systems (CMTSs) over the HFC plant using the DOCSIS standard. Cable modems are considered Customer Premise Equipment (CPE). The function of the cable modem is twofold, as follows:

• It modulates the digital data that is received from the computer via the Ethernet port or connection into an RF signal that is sent over the coaxial connection.

• It demodulates the RF signal that is received via the coaxial connection back into digital data, which it then sends to the computer via the Ethernet connection.

The CMTS is a piece of equipment commonly located in a cable company’s Local Headend (LE). The CMTS provides high-speed data services, such as cable Internet or Voice over IP, to cable subscribers. The CMTS performs the following two functions:

• It is responsible for the modulation of digital data signals received from the Internet into an RF signal, which is eventually sent to the user.

• It is responsible for the demodulation of RF signals back into digital data and then forwards this data to the Internet.

The LE is the cable operator facility where the received signals are processed, formatted, and distributed over the cable network. The CMTS resides at the LE. Like the cable modem and CMTS, the LE is responsible for the following two actions:

• It receives television signals from sources such as satellite dishes and antennas.
• It distributes these received television signals to subscribers.

DSL Access Technologies

Digital Subscriber Line (DSL) is an increasingly popular method used for branch office connectivity. A family of DSL types has been developed to provide high-speed Internet services over the existing telephone line infrastructure. The idea behind DSL technologies is to use a wider frequency band for communicating data over the existing twisted-pair lines (telephone lines) at the same time as voiceband. This means that a second frequency band, above the voiceband, must be defined to perform data modulation.

DSL types can be divided into two broad categories: symmetrical DSL (SDSL) and asymmetrical DSL (ADSL). Although both are part of the DSL family of technologies, these types of DSL have different characteristics. It is therefore important to understand the differences between the two. These two DSL variants, or “flavors,” and their respective characteristics will be described in the following sections.

SDSL provides equal capacity for data traveling both upstream and downstream. Symmetrical transmission is most suited to users who both upload and download significant amounts of data. The following section describes SDSL variants.

Symmetrical DSL is a DSL variant that runs over one pair of copper wires and supports data only. SDSL does not support analog calls because it takes over the entire bandwidth of the line. SDSL is a proprietary technology that was never standardized. SDSL provides up to T1 and E1 downstream and upstream throughput. SDSL has a distance limit of 10,000 feet. SDSL is also typically offered in the speeds shown in Table 13-1 below:

Table 13-1. Symmetrical DSL Variant Speeds

Additional SDSL variants include SHDSL, HDSL, HDSL2, and IDSL. These different flavors are described as follows:

• Symmetrical High data-rate DSL (SHDSL) uses frequencies that overlap with those used by traditional telephone services to provide symmetrical data rates, which means it does not support analog calls. SHDSL provides symmetrical throughput from 192 Kbps to 2.3 Mbps; however, an optional extended SHDSL mode makes it possible to allow even greater symmetrical throughput speeds. SHDSL has a distance limitation of 26,000 feet; it is also referred to as G.SHDL because it is defined in ITU-T recommendation G.991.2.

• High bit-rate DSL (HDSL) is the most mature of the xDSL approaches. HDSL provides either T1 or E1 speeds using two copper pairs. HDSL is commonly used in place of the traditional T1 circuits. HDSL does not allow for standard telephone service over the copper pairs. HDSL has a distance limitation of 12,000 feet.

• HDSL2 is the second generation of HDSL. HDLS2 provides symmetrical service at T1 speeds using a single-wire pair rather than the two pairs used by HDSL. HDSL2 was also developed as a standard, allowing for interoperability between different vendors’ equipment. HDSL2 has a distance limitation of 22,000 feet.

• ISDN DSL (IDSL) uses ISDN-type technology to provide a data communication channel across existing copper telephone lines at a rate of 1.44 Kbps. IDSL uses a single-wire pair for symmetrical speeds across existing ISDN lines. This means that data is transmitted digitally rather than in analog form. While IDSL has a distance limitation of 18,000 feet, local telephone providers can increase this limitation to up to 45,000 feet using repeaters.

ADSL is the most common type of DSL service. Unlike SDSL, which provides symmetrical (same) upstream and downstream speeds, ADSL provides different downstream and upstream speeds. In asymmetrical connections, the downstream throughput is higher than the upstream throughput. ADSL is defined in ANSI T1.413-1998 Issue 2. Standard ADSL has an 8 Mbps downstream rate and a 1 Mbps upstream rate.

Like SDSL, there are several ADSL variants. However, unlike SDSL variants, ADSL variants allow for voice and data to be sent simultaneously over the existing telephone line. This is because ADSL operates at higher frequencies than PSTN/POTS, so they can coexist on the same media. ADSL variants, which include G.Lite ADSL, ADSL2, RADSL, VDSL, and VDSL2, are described as follows:

• G.Lite ADSL is also referred to as splitterless ADSL because it allows voice and data to coexist on the existing telephone line without using a splitter. G.Lite ADSL has a distance limitation of up to 25,000 feet. G.Lite ADSL has a downstream rate of 1.5 Mbps and an upstream rate of 0.5 Mbps.

• ADSL2 extends the capability of basic ADSL in data rates to 12 Mbps downstream and 3.5 Mbps upstream, with a mandatory capability of ADSL2 transceivers of 8 Mbps downstream and 800 Kbps upstream. However, actual speeds may be reduced depending on line quality and the distance from the subscriber to the CO. ADSL2, also referred to as G.DMT.bis, is standardized in ITU G.992.3. Splitterless ADSL2, on the other hand, is standardized in ITU G.992.4, and its data rate mandatory capability is reduced to 1.536 Mbps downstream and 512 Kbps upstream. It is also referred to as G.lite.bis. ADSL2 has a distance limitation of about 20,000 feet.

• ADSL2+, or ADSL2Plus, is standardized in ITU G.992.5. ADSL2+ extends the capability of basic ADSL by doubling the number of downstream bits. The data rates can be as high as 24 Mbps downstream and 1.4 Mbps upstream, depending on the distance from the CO to the home of the subscriber. ADSL2+ also allows port bonding, where multiple ports are physically provisioned to the subscriber and the total bandwidth is equal to the sum of all provisioned ports. ADSL2+ has a distance limitation of about 20,000 feet.

• Rate-adaptive DSL (RADSL) has the same transmission limits as ADSL, but it has the ability to adjust transmission speed automatically according to the length and quality of the local line. While this is considered the defining characteristic of RADSL, it should be noted that standard ADSL also allows the DSL modem to adapt speeds of data transfer. With RADSL, the connection speed is established when the line syncs up, and it varies between 600 Kbps and 7 Mbps downstream, and between 128 Kbps and 1 Mbps upstream. RADSL has a distance limitation of 18,000 feet.

• Very High bit-rate DSL (VDSL or VHDSL) is the fastest DSL technology. Downstream rates range between 13 Mbps and 52 Mbps and upstream rates range between 1.5 Mbps and 2.3 Mbps over a single wire pair. VDSL can also operate in symmetrical mode at 26 Mbps. VDSL is standardized in ITU G.993.1 and has a distance limitation of only 4,500 feet, which is considered a very short local loop.

• Very High bit-rate DSL2 (VDSL2 or VHDSL2) is an enhancement of the VDSL standard. It is standardized in ITU-T G.993.2. VDSL2 also allows for the transmission of asymmetrical and symmetrical aggregate data rates up to 200 Mbps on twisted pairs, using a bandwidth of up to 30 MHz. ADSL-like long reach performance is one of the key advantages of VDSL2. Long Reach VDSL2 (LR-VDSL2)-enabled systems are capable of supporting speeds of around 1 Mbps to 4 Mbps downstream over distances of 16,000 feet, increasing the bit rate up to symmetrical 100 Mbps as the loop is shortened.

NOTE: While the sections above provide a summary of the different DSL flavors, keep in mind that additional detailed information can be found in the current ROUTE guide, which is available online.

Configuring Cisco IOS Routers for Cable Access Technologies

The configuration of Cisco IOS routers for cable access is a straightforward process. Given that routers connect to the Ethernet port of the cable modem, the router is simply configured as a Dynamic Host Configuration Protocol (DHCP) client, which allows the router to receive addressing information from the Internet Service Provider (ISP). Following this, any additional configuration that needs to be implemented can include the Cisco IOS DHCP server, which will allow hosts connected to the LAN to receive their IP addressing configuration parameters from the router. In most cases, the DHCP pool is configured with the import all DHCP configuration command, which allows addressing parameters received from the ISP (e.g., WINS and DNS server information) to be imported into the LAN DHCP pool and provided to the hosts that reside on the LAN.

Additionally, Port Address Transmission (PAT) is also implemented on the router, allowing the hosts residing on the LAN to access the Internet. In some cases, some ISPs provide business-class cable and allocate the customer a public IP address range. This allows hosts that use those public addresses (e.g., any servers) to be accessed from the Internet. Any other internal hosts using private addressing require either Network Address Transmission (NAT) or PAT to be implemented on the router so that they can access the Internet.

Detailed configuration information can be found in the current ROUTE guide, which is available online. This will not be included in this chapter.

Configuring Cisco IOS Routers for DSL Access Technologies

Unlike cable access connectivity configuration, DSL access connectivity configuration is a little more detailed because Cisco IOS software supports multiple options for subscriber-side configuration when connecting to a DSL network. These methods include the following:

• Cisco Long Range Ethernet (LRE) Solution
• Routed Bridged Encapsulation
• Multiprotocol Encapsulation over Asynchronous Transfer Mode (ATM)
• Service Selection Gateway
• PPP over ATM (PPPoA)
• PPP over Ethernet (PPPoE)

The Cisco LRE solution leverages VDSL technology to extend Ethernet services over existing Category 1, 2, or 3 twisted-pair cabling at speeds from 5 Mbps to 15 Mbps and distances up to 5,000 feet. Cisco LRE technology delivers broadband service on the same lines as POTS, digital telephone, and ISDN traffic. In addition, Cisco LRE supports modes compatible with ADSL technologies, allowing providers to provision LRE to buildings where broadband services already exist. LRE is not a widely implemented technology due to numerous issues that are beyond the scope of the TSHOOT certification exam. You are not required to demonstrate any familiarity with LRE configuration in the TSHOOT exam.

Routed Bridged Encapsulation (RBE) is a Cisco implementation of bridged Ethernet over ATM wherein a stub-bridged segment is terminated on a point-to-point routed interface. The router routes on an Ethernet header carried over a Point-to-Point Protocol (PPP), such as RFC 1483 ATM or RFC 1490 Frame Relay. RBE was developed to address the known RFC1483 bridging issues, including broadcast storms and security. RBE operates exclusively over ATM. Scalability, performance, and security can be achieved by using the unique characteristics of xDSL subscribers. While you are not expected to configure RBE, an example is included simply as a reference to highlight the differences between RBE and PPPoA, which you are required to know for the TSHOOT certification exam. The following example illustrates a basic RBE configuration:

NOTE: Because AAL5 SNAP is the default encapsulation, the encapsulation aal5snapcommand issued under the Permanent Virtual Circuit (PVC) can be omitted.

Multiprotocol Encapsulation over ATM was originally standardized in RFC 1483, which was then rendered obsolete by RFC 2684. Multiprotocol Encapsulation over ATM describes two different methods for carrying connectionless network interconnect traffic over an ATM network, which are routed PDUs and bridged PDUs. Routing allows multiplexing of multiple protocols over a single ATM virtual circuit (VC). The protocol of a carried protocol data unit (PDU) is identified by prefixing the PDU with an IEEE 802.2 Logical Link Control header. Bridging performs higher-layer protocol multiplexing implicitly by ATM VCs. The configuration of Multiprotocol Encapsulation over ATM is similar to the configuration of RBE, with the exception being the omission of the atm route-bridged ip command under the interface or subinterface. Multiprotocol Encapsulation over ATM supports both ATM Adaptation Layer 5 (AAL5) Subnetwork Access Protocol (SNAP) and AAL5 Multiplexed Encapsulation (MUX). While you are not required to implement Multiprotocol Encapsulation over ATM, the following example illustrates a basic configuration of the method:

The Cisco Service Selection Gateway (SSG) is a switching solution for service providers who offer Intranet, Extranet, and Internet connections to subscribers, using broadband access technology such as DSL, cable modems, or wireless LAN. You are not required to go into any detail on SSG, as this is a service provider solution. SSG will not be described in any additional detail in this chapter.

PPP over Ethernet (PPPoE) provides the ability to connect a network of hosts over a simple bridging access device to a remote access concentrator or aggregation concentrator. Each host uses its own PPP stack, thus presenting the user with a familiar user interface. Access control, billing, and type of service can be done on a per-user, rather than a per-site, basis. By default, PPPoE runs on top of ATM AAL5 SNAP; however, PPPoE can also be configured to use ATM AAL5 MUX.

PPPoE can be configured over both Ethernet and ATM interfaces. This guide will not be delving into detailed configuration instructions; however, you can find this information in the current ROUTE guide, which is available online. However, to recap what you should already know, assuming that you have completed the ROUTE exam, it is important both to remember and to keep the following points in mind when characterizing and configuring PPPoE:

• You must install PPPoE client software on all hosts that connect to the Ethernet segment.
• Based on RFC 2516, the Maximum Transmission Unit (MTU) of PPPoE interfaces must be set to 1492 bytes.
• PPPoE requires the configuration of a username and password pair for authentication.
• PPPoE allows you to have multiple PPPoE sessions per PVC.
• PPPoE is susceptible to broadcast storms and possible denial-of-service (DoS) attacks.
• PPPoE runs on top of AAL5 SNAP; however, it supports AAL5 MUX.

The following example illustrates a basic PPPoE configuration over Ethernet. Additional features, such as NAT and DHCP, are not included in this configuration example:

The following example illustrates a basic PPPoE configuration over ATM. Additional features, such as NAT and DHCP, are not included in this configuration example:

PPPoA is specified in RFC 2364. Though PPPoE is used in most countries, the U.K. and some parts of the U.S. use PPPoA to provide ADSL service. PPPoA is a network protocol for encapsulating PPP frames in AAL5. While PPPoA uses AAL5 SNAP for encapsulation, by default, the most common encapsulation type used is AAL5 MUX.

The network architecture of PPPoA is similar to that of PPPoE. However, the process for the establishment of connectivity is slightly different. Additional detailed information on the differences is included in the current ROUTE guide, which is available online. Unlike PPPoE, PPPoA is supported using only ATM interfaces. Some important PPPoA points that you should be familiar with are as follows:

• PPPoA requires the configuration of a username and password pair for authentication.
• PPPoA allows for IP address conservation at the CPE using NAT or PAT.
• The username and password are provisioned on the CPE.
• Only one IP address per CPE on one VC can be used due to the single username and password pair.
• The CPE username and password means only the provider can make changes.

The following example illustrates a basic PPPoA configuration using dialer profiles. Additional features, such as NAT and DHCP, are not included in this configuration example:

R1(config)#interface atm0/0

R1(config-if)#pvc 2/200

R1(config-if-atm-vc)#encapsulation aal5mux ppp dialer

R1(config-if-atm-vc)#dialer pool-member 55

R1config-if-atm-vc)#no shut

R1config-if-atm-vc)#exit

R1(config-if)#exit

R1(config)#interface Dialer55

R1(config-if)#ip address negotiated

R1(config-if)#dialer pool 55

R1(config-if)#encapsulation ppp

R1(config-if)#dialer-group 1

R1(config-if)#exit

The following example illustrates a basic PPPoA configuration using virtual templates. Other additional features, such as NAT and DHCP, are not included in this configuration example:

R1(config)#interface ATM0/0

R1(config-if)#pvc 2/200

R1(config-if-atm-vc)#encapsulation aal5mux ppp virtual-template 55

R1(config-if-atm-vc)#exit

NOTE: Additional detailed configuration instructions and examples for PPPoE and PPPoA can be found in the current ROUTE guide, which is available online. If you have forgotten how to configure these technologies, it is strongly recommended that you revisit this guide for a quick refresher, as you can expect to be tested on these technologies in the current TSHOOT exam.