Introduction to Wide Area Networks
So what, exactly, is it that 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, modern technologies even blur this definition, but it still fits neatly into the context of Cisco’s exam objectives.
Anyway, I’ve 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): 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 interface.
Local loop: The local loop connects the demarc 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 apoint 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 I’m going to introduce you to each of the various types of WAN connections you’ll find on the market today.Here’s a list explaining the different WAN connection types:
Leased lines:
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;
Circuit switching: When you hear the term circuit switching , think phone call. The big advantage is 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. Well, some people do have ISDN and it still is viable (and I do 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 a 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).
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.
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.
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.
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.
Digital Subscriber Line (DSL)
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.
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!
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.