Introduction to wide area networks

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Introduction-to-wide-area-networks

So what, exactly, is it makes something a wide area network (WAN) instead of a local area network (LAN)? well, there’s obviously the distance thing, but these days, wireless LANs can cover some serious turf. What about bandwidth? Well, here again, some really big pipes can be had for a price in many places, so that’s not it either. So what the heck is it then?

One of the main ways a WAN differs from a LAN is that while you generally own a LAN infrastructure, you usually lease WAN infrastructure from a service provider. To be honest, modem technologies oven blur this definition, but it still fits neatly into the context of Cisco’s exam objectives.

Anyway, we have already talked about the data link that you usually own (Ethernet), but now we’re going to find out about the kind you usually don’t own the type most often leased from a service provider.

The key to understanding WAN technologies is to be familiar with the different WAN terms and connection types commonly used by service providers to join your networks together.

Defining WAN terms

Before you run out and order a WAN service type from a provider, it would be a really good idea to understand the following terms that service providers typically use:

Customer Premises Equipment (CPE): Customer premises equipment (CPE) is equipment that’s owned by the subscriber and located on the subscriber’s premises.

Demarcation Point: The demarcation point is the precise spot where the service provider’s responsibility ends and the CPE begins. It’s generally a device in a telecommunications closet owned and installed by the telecommunications company (telco). It’s your responsibility to cable (extended demarc) from this box to the CPE, which is usually a connection to a CSU/DSU or ISDN interference.

Local Loop: The local loop connects the demark to the closest switching office, which is called a central office.

Central Office (CO): This point connects the customer’s network to the provider’s switching network. Good to know is that a central office (CO) is sometimes referred to as a point of presence (POP).

Toll Network: The toll network is a trunk line inside a WAN provider’s network. This network is a collection of switches and facilities owned by the ISP.

Definitely familiarize yourself with these terms because they’re crucial to understanding WAN technologies.

WAN connection Types

As you’re probably aware, a WAN can use a number of different connection types, and we are going to introduce you to each of the various types of WAN connections you’ll find on the market today. The following picture shows the different WAN connection types that can be used to connect your LANs together (DTE) over a DCE network.

Introduction-to-wide-area-networks
WAN connection types

Here’s a list explaining the different WAN connection types:

  • Leased Line: these are usually referred to as a point-to-point or dedicated connection. A leased-line is a pre-established WAN communications path that goes from the CPE through the DCE switch, then over to the CPE of the remote site. The CPE enables DTE networks to communicate at any time with no cumbersome setup procedures to muddle through before transmitting data. When you’ve got plenty of cash, this is really the way to go because it uses synchronous serial lines up to 45Mbps. HDLC and PPP encapsulations are frequently used on leased lines; we’ll go over them with you in detail in this article.
  • Circuit switching: When you hear the term circuit switching, think phone cell. The big advantage is a cost you only pay for the time you actually use. No data can transfer before an end-to-end connection is established. Circuit switching uses dial-up modems or ISDN and is used for low-bandwidth data transfers. Okay, we know what you’re thinking: “Modems? Did they say modem? Aren’t those only in museums by now?” after all, with all the wireless technologies available, who would use a modem these days? Well, some people do have ISDN and it still is viable (and we suppose someone does use a modem now and then), but circuit switching can be used in some of the newer WAN technologies as well.
  • Packet switching: This is a WAN switching method that allows you to share bandwidth with other companies to save money. Packet switching can be thought of as a network that’s designed to look like a leased line yet charges you more like circuit switching. But less cost isn’t always better there’s definitely downside: if you need to transfer data constantly, just forget about this option. Instead, get yourself a leased line. Packet switching will only work for you if your data transfers are the bursty type not continuous. Frame relay and X.25 are packet-switching technologies with speeds that can range from 56Kbps up to T3 (45Mbps).

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 we had other types of an interface on our router, we would have other encapsulation options, like ISDN or ADSL. And remember, you can’t configure Ethernet or Token Ring encapsulation or a serial interface.

Next, we’re 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 serial interfaces for wide-area connections?

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 a 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 the 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 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 regards to its specific equipment.

PPP: Point-to-Point (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 vendor’s equipment. It uses a Network control protocol field in the Data Link header to identify the Network layer protocol and allows authentication and multi-link connections to be run over asynchronous and synchronous links.

PPPoE: Point-to-Point over Ethernet encapsulation 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 US, PPPoE on Ethernet’s main feature is that it adds a direct connection to Ethernet interfaces while providing DSL supports 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 2000 active data subscribers are connected to a certain cable network segment, all sharing the upstream and downstream bandwidth. (Hybrid fiber-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 US.

DSL: Digital subscriber line is a technology used by traditional telephone companies to deliver advanced service (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. The digital subscriber line is not a complete end-to-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 edges 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 the 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.

Cable and DSL

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

DSL and cable internet service 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 the cable doesn’t always win the race in the 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 we say adequate, we 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 the show” in the US, but DSL is beginning to catch up.

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

The following picture 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.

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.

Introduction-to-wide-area-networks
Broadband access using Cable and DSL

Cable

The cable is a great cost-effective connection for a small office or home office or SOHOyes, 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 services areas that usually range in size from 100 to 2000 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 radiofrequency (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.

The following picture shows where you would find the various types of networks and how the term we just listed would be used in the network diagram.

Introduction-to-wide-area-networks
Cable networks and terms

The problem is that ISPs often use a fiber-optic network that extends from the cable operator’s master head end, sometimes even to regional headends, out to a neighborhood’s hub site that then arrives at a fiber-optic node, which serves anywhere from 25 to 2000 or more homes. (don’t get us wrong, all links have problems! We are not picking on cable really!)

And here’s another issue: If you have cable, open your PC’s command prompt, and type ipconfig and check out your subnet mask. It’s probably a /20 or /21 class B address. You already know that’s either 4094 or 2046 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. The 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 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!

Digital Subscriber Line (DSL)

Coming in seconds in our subscriber-based popularity contest in 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 a digital subscriber loop, but now its meaning has morphed to the 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 or a network downstream speed is usually faster.

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

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).

Introduction-to-wide-area-networks
xDSL connection from home user to central office

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 services 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!

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 videos, movies, and music; online gaming; surfing; and getting email 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 ASDL over the same twisted-pair telephone line. Actually, depending on the type of ADSL, not just two, but three information channels commonly utilized 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 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, depending on your interface type and the service provider’s switch:

PPPoE: We’ll see this in more detail below.

RFC1483 Routing: RFC1483 describes two different methods for carrying connectionless network traffic over an ATM network; routed protocols and bridged protocols.

PPPoA: Point-to-Point Protocol (PPP) over ATM is used to encapsulate PPP frames in ATM AAL5 (ATM Adaption layer 5). It is typically used with cable modems, DSL, and ADSL services and offers the usual PPP features of authentication, encryption, and compression and actually has less overhead in comparison to PPPoE.

PPPoE

Used with ADSL services, PPPoE (Point-to-Point Protocol over Ethernet) encapsulates PPP frames in Ethernet frames and uses common PPP features like authentication, encryption, and compression. But as we said earlier, it’s trouble if you’ve got a badly configured firewall. This is a tunneling protocol that layers IP and other protocols than run over PPP with the attributes of a PPP link so they can then be used to contact other Ethernet devices and initiate a point-to-point connection to transport IP packets.

The following picture displays the typical usage of PPPoE over ADSL. As you can see, a PPP session is connected from the PC of the end-user to the router and the subscriber PC IP address is assigned by the router via IPCP.

Introduction-to-wide-area-networks
PPPoE with ADSL

PPPoE is used to equip custom PPP-based software with the ability to deal with a connection that’s not using a serial line and to be at home in a packet-oriented network environment like Ethernet and to allow for a custom connection with login and password for internet connection accounting. Another factor is that the opposite side of the link’s IP address is only given to it and available for the specific period that the PPPoE connection is open, so reusing IP addresses dynamically is permitted.

PPPoE has a discovery stage and a PPP session stage that works like this: First, a host begins a PPPoE session, during which it has to execute a discovery process so it can determine the best server to meet the needs of the client machine’s request. After that, it has to discover the Ethernet MAC address of the peer device and create a PPPoE session ID. So even though PPP delimits a peer-to-peer relationship, the discovery part is innately a client-server relationship.

Okay, before getting into serial connections, there’s one last thing we want to cover Cisco LRE.

Cisco Long Range Ethernet (LRE)

The Cisco Long-range Ethernet solution employs something called VDSL (Very high data rate digital subscriber line) technology to significantly expand Ethernet service capacity. And LRE can achieve these impressive results: speeds from 5 to 15Mbps (full-duplex) at distances up to 5000 feet traveling over existing twisted-pair wiring!

So basically, Cisco LRE technology can give us broadband service on POTS, digital telephone, and ISDN traffic lines, and it also can operate in modes that are compatible with ADSL technologies. This flexibility is important because it makes it possible for service providers to make LRE available in structures and/or buildings that have broadband services already in place but need it enhanced very cool indeed.