ADSL

ADSL supports both voice and data at the same time, but it was created to allot more bandwidth downstream than upstream because it’s best for residential subscribers that usually need more downstream bandwidth for doing things like downloading video, movies, and music; online gaming; surfing; and getting emails—some that include sizeable attachments. ADSL will give you a downstream rate from 256Kbps to 8Mbps, but anything going upstream is only going to reach around 1Mbps.

POTS provides a channel for analog voice transmission and can transmit without a problem with ADSL over the same twisted-pair telephone line. Actually, depending on the type of ADSL, not just two, but three information channels commonly utilize the same wiring at the same time. This is why people can use a phone line and an ADSL connection at the same time and not affect either service.

ATM is the Data Link layer protocol typically used over the DSL layer 1 connection from the CPE that’s terminated at what’s known as the DSLAM—an ATM switch that contains DSL interface cards, or ATU-Cs. After ADSL connections meet their end at the DSLAM, it switches the data over an ATM network to something called an aggregation router—a layer 3 device where the subscriber’s IP connection then expires.

You know by now how important encapsulation is, so as you’ve probably guessed, any IP packets over an ATM and DSL connection must have this done. This happens in one of three ways (PPPoE,RFC1483 Routing,PPPoA), depending on your interface type and the service provider’s switch.

Digital Subscriber Line (DSL)

Coming in second in our subscriber-based popularity contest is DSL (digital subscriber line), a technology that uses your garden-variety copper phone wires to give you high-speed data transmission. DSL requires a phone line, a DSL modem (often included with service), either an Ethernet card or a router that has an Ethernet connection, and someone that can provide service wherever you happen to be located.

The acronym DSL originally meant digital subscriber loop, but now its meaning has morphed to digital subscriber line. DSL group types fall into two categories based upon the upstream or downstream speed connections:  

Symmetrical DSL The speed for both downstream and upstream connections are equal, or symmetrical.  

Asymmetrical DSL Different transmission speeds occur between two ends of a network— downstream speed is usually faster.  

Figure 1 shows an average home user with xDSL, which is a transmission technology that moves data over copper pairs.

xDSL connection from home user to central office

 All types of DSL are layer 1 technologies.
ATU-R = ADSL Transmission Unit - Remote
ATU-C = ADSL Transmission Unit - Central

The term xDSL covers a number of DSL variations, such as ADSL, high-bit-rate DSL(HDSL), Rate Adaptive DSL (RADSL), Synchronous DSL (SDSL), ISDN DSL (IDSL), and very-high-data-rate DSL (VDSL).

DSL flavors that don’t use the voice frequencies band, like ADSL and VDSL, allow DSL lines to carry both data and voice signals simultaneously. Others, like SDSL and IDSL, that occupy the complete frequency range, can only carry data. And by the way, the data service that the DSL connection gives you is always on.

The speed that DSL service can offer depends on how far you are from the CO—the closer the better. In fact, you can blaze at rates up to around 6.1Mbps if you’re physically close enough!

Differences between DSL and Cable Modem Networking

Okay, before I talk about the serial encapsulation connections used on Cisco routers (HDLC, PPP, and Frame Relay), I’m going to discuss cable modems and DSL (including ADSL and PPPoE) as solutions for connections to wide area networks because I think it will really help you understand the practical differences between DSL and cable modem networking.

DSL and cable Internet services truly do have a lot in common, but they still have some basic, essential differences that are important for you to understand:  

Speed Most would say that cable is faster than DSL Internet, but cable doesn’t always win the race in real-world use.  

Security DSL and cable are based on different network security models, and until recently, cable has been the reputed loser in this contest. But now, it’s pretty much a toss-up, and both offer adequate security that meets the needs of most users. And when I say adequate, I mean that there are still some very real security issues relating to both alternatives, no matter what your ISP says!

Popularity Cable Internet is definitely “best in show” in the U.S., but DSL is beginning to catch up.  

Customer Satisfaction Here, the reverse is true—in the U.S., DSL is top dog. But still, do you know anyone that’s really totally satisfied with their ISP?

Figure 1 shows how a connection can terminate from modems to either a PC directly or a router. Typically, your router would run DHCP on that interface, as well as PPPoE. Both DSL and cable high-speed Internet services are available to millions of residential and business consumers worldwide, but in some areas, only one (sometimes neither) service is available.

Figure 1 : Broadband access using cable or DSL

                                                     Always-on Voice, Video, and Data Services
Surprisingly, some of the differences between DSL and cable modem have nothing to do with the actual technologies—it comes down to the individual ISP. All other things being equal, issues like cost, reliability, and quality of customer support for both installation and maintenance issues vary significantly from one provider to the next.  

Cable Cable is a great cost-effective connection for a small office or home office, or SOHO—yes, there is an acronym for everything! And even in larger organizations, cable (or DSL for that matter) can be great to have as a backup link.  

Here are a few cable network terms:

Headend This is where all cable signals are received, processed, and formatted. The signals are then transmitted over the distribution network from the headend.

Distribution network These are relatively small service areas that usually range in size from 100 to 2,000 customers. They’re typically composed of a mixed, fiber-coaxial, or HFC architecture, with optical fiber substituting for the distribution network’s trunk portion. The fiber forms both the connection from the headend and an optical node that changes light to radio frequency (RF) signals that are then distributed through a coaxial cable throughout the specific area being serviced.

DOCSIS (data over cable service interface specification) All cable modems and like devices have to measure up to this standard.  

Figure 2 shows where you would find the various types of networks and how the terms I just listed would be used in a network diagram.

Figure 2: Cable network and terms

The problem is that ISPs often use a fiber-optic network that extends from the cable operator’s master headend, sometimes even to regional headends, out to a neighborhood’s hubsite that then arrives at a fiber-optic node, which serves anywhere from 25 to 2,000 or more homes. (Don’t get me wrong, all links have problems—I’m not picking on cable—really!)

And here’s another issue: If you have cable, open your PC’s command prompt, and type ipconfig check out your subnet mask. It’s probably a /20 or /21 class B address. Oh my. You already know that’s either 4,094 or 2,046 hosts per cable network connection. Not good!

When we say “cable,” we really mean using coax (coaxial) cable for transmission. And CATV, or community antenna television, is now used as a means to offer cost-effective broadcasting to subscribers. Cable is able to provide voice and data, plus analog and digital video, without requiring you to pony up your whole paycheck.

Your average cable connection gives you a maximum download speed of 2Mbps. And remember—you have to share that bandwidth with all the other subscribers. As if that weren’t enough, there are other things like overloaded web servers and plain old Net congestion that factor in as well. But your email-checking neighbors really aren’t making that much of a difference. So who or what is? Well, if you’re an online gamer, you would likely notice a bit more lag during peak periods (which could be a matter of virtual life and death!). And if somebody in your neighborhood is uploading a large amount of data—like, well, an entire collection of pirated Star Wars movies—that could definitely max out the entire connection and bring everyone’s browser to a crawl.

Cable modem access may or may not be faster or easier to install than DSL, and your mileage will vary, depending on where you live plus a variety of other factors. But it’s usually more available and a tad less pricey, making it a winner by a nose. But no worries, if cable access isn’t available in your neighborhood, DSL is okay—anything is better than dial-up!

WAN Protocols

Frame Relay A packet-switched technology that made its debut in the early 1990s, Frame Relay is a high-performance Data Link and Physical layer specification. It’s pretty much a successor to X.25, except that much of the technology in X.25 used to compensate for physical errors (noisy lines) has been eliminated. An upside to Frame Relay is that it can be more cost effective than point-to-point links, plus it typically runs at speeds of 64Kbps up to 45Mbps (T3). Another Frame Relay benefit is that it provides features for dynamic bandwidth allocation and congestion control.  

ISDN Integrated Services Digital Network (ISDN) is a set of digital services that transmit voice and data over existing phone lines. ISDN offers a cost-effective solution for remote users who need a higher-speed connection than analog dial-up links can give them, and it’s also a good choice to use as a backup link for other types of links like Frame Relay or T1 connections.

LAPB Link Access Procedure, Balanced (LAPB) was created to be a connection-oriented protocol at the Data Link layer for use with X.25, but it can also be used as a simple data link transport. A not-so-good characteristic of LAPB is that it tends to create a tremendous amount of overhead due to its strict time-out and windowing techniques.  

LAPD Link Access Procedure, D-Channel (LAPD) is used with ISDN at the Data Link layer (layer 2) as a protocol for the D (signaling) channel. LAPD was derived from the Link Access Procedure, Balanced (LAPB) protocol and is designed primarily to satisfy the signaling requirements of ISDN basic access.  

HDLC High-Level Data-Link Control (HDLC) was derived from Synchronous Data Link Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC works at the Data Link layer and creates very little overhead compared to LAPB. It wasn’t intended to encapsulate multiple Network layer protocols across the same link—the HDLC header doesn’t contain any identification about the type of protocol being carried inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own way of identifying the Network layer protocol, meaning each vendor’s HDLC is proprietary with regard to its specific equipment.

PPP Point-to-Point Protocol (PPP) is a pretty famous, industry-standard protocol. Because all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol and allows authentication and multilink connections to be run over asynchronous and synchronous links.  

PPPoE Point-to-Point Protocol over Ethernet encapsulates PPP frames in Ethernet frames and is usually used in conjunction with ADSL services. It gives you a lot of the familiar PPP features like authentication, encryption, and compression, but there’s a downside—it has a lower maximum transmission unit (MTU) than standard Ethernet does, and if your firewall isn’t solidly configured, this little attribute can really give you some grief! Still somewhat popular in the United States, PPPoE on Ethernet’s main feature is that it adds a direct connection to Ethernet interfaces while providing DSL support as well. It’s often used by many hosts on a shared Ethernet interface for opening PPP sessions to various destinations via at least one bridging modem.  

Cable In a modern HFC network, typically 500 to 2,000 active data subscribers are connected to a certain cable network segment, all sharing the upstream and downstream bandwidth. ( Hybrid fibre-coaxial , or HFC, is a telecommunications industry term for a network that incorporates both optical fiber and coaxial cable to create a broadband network.) The actual bandwidth for Internet service over a cable TV (CATV) line can be up to about 27Mbps on the download path to the subscriber, with about 2.5Mbps of bandwidth on the upload path. Typically users get an access speed from 256Kbps to 6Mbps. This data rate varies greatly throughout the U.S.  

DSL Digital subscriber line is a technology used by traditional telephone companies to deliver advanced services (high-speed data and sometimes video) over twisted-pair copper telephone wires. It typically has lower data carrying capacity than HFC networks, and data speeds can be range limited by line lengths and quality. Digital subscriber line is not a complete endto- end solution but rather a Physical layer transmission technology like dial-up, cable, or wireless. DSL connections are deployed in the last mile of a local telephone network—the local loop. The connection is set up between a pair of modems on either end of a copper wire that is between the customer premises equipment (CPE) and the Digital Subscriber Line Access Multiplexer (DSLAM). A DSLAM is the device located at the provider’s central office (CO) and concentrates connections from multiple DSL subscribers.

MPLS MultiProtocol Label Switching (MPLS) is a data-carrying mechanism that emulates some properties of a circuit-switched network over a packet-switched network. MPLS is a switching mechanism that imposes labels (numbers) to packets and then uses those labels to forward packets. The labels are assigned on the edge of the MPLS of the network, and forwarding inside the MPLS network is done solely based on labels. Labels usually correspond to a path to layer 3 destination addresses (equal to IP destination-based routing). MPLS was designed to support forwarding of protocols other than TCP/IP. Because of this, label switching within the network is performed the same regardless of the layer 3 protocol. In larger networks, the result of MPLS labeling is that only the edge routers perform a routing lookup. All the core routers forward packets based on the labels, which makes forwarding the packets through the service provider network faster. (Most companies are replacing their Frame Relay networks with MPLS today).  

ATM Asynchronous Transfer Mode (ATM) was created for time-sensitive traffic, providing simultaneous transmission of voice, video, and data. ATM uses cells that are a fixed 53 bytes long instead of packets. It also can use isochronous clocking (external clocking) to help the data move faster. Typically, if you are running Frame Relay today, you will be running Frame Relay over ATM.

WAN Support

Basically, Cisco just supports HDLC, PPP, and Frame Relay on its serial interfaces, and you can see this with the encapsulation? command from any serial interface (your output may vary depending on the IOS version you are running):

Corp# config t
Corp(config)# int s0/0/0
Corp(config-if)# encapsulation ?
atm-dxi           ATM-DXI encapsulation
frame-relay     Frame Relay networks
hdlc                 Serial HDLC synchronous
lapb                 LAPB (X.25 Level 2)
ppp                  Point-to-Point protocol
smds               Switched Megabit Data Service (SMDS)
x25                  X.25

Understand that if I had other types of interfaces on my router, I would have other encapsulation options, like ISDN or ADSL. And remember, you can’t configure Ethernet or Token Ring encapsulation on a serial interface.

Next, I’m going to define the most prominently known WAN protocols used today: Frame Relay, ISDN, LAPB, LAPD, HDLC, PPP, PPPoE, Cable, DSL, MPLS, and ATM. Just so you know, the only WAN protocols you’ll usually find configured on a serial interface are HDLC, PPP, and Frame Relay, but who said we’re stuck with using only serial interfaces for wide area connections?