Wide area network (WAN)


Defining WAN Terms

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

  • Customer premises equipment (CPE)

    Customer premises equipment (CPE)is equipmentthat’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 acentral 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

    A WAN can use a number of different connection types. The different WAN connection types that can be used to connect your LANs together (DTE) over a DCE network. 

    Here’s a list explaining the different WAN connection types:
    1. 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; I’ll go over them with you in detail in a bit
    2. 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
    3. 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).
  • WAN Support

    Basically, Cisco just supports HDLC, PPP, and Frame Relay on its serial interfaces.

    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.

    WAN protocols used today: FrameRelay, ISDN, LAPB, LAPD, HDLC, PPP, PPPoE, Cable, DSL, MPLS, and ATM. Just so youknow, the only WAN protocols you’ll usually find configured on a serial interface are HDLC,PPP, and Frame Relay.
  • Frame Relay

    FrameRelay is a high-performance Data Link and Physical layer specification. 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. 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 connections.
  • 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.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 multi protocol 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 multi link 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.
  • 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. 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

    Multi-Protocol 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. Allthe 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).
  • Data Terminal Equipment and Data Communication Equipment

    By default, router interfaces aredata terminal equipment (DTE), and they connect into data communication equipment (DCE)like a channel service unit/data service unit (CSU/DSU).

Link Control Protocol (LCP) Configuration Options

Link Control Protocol (LCP)offers different PPP encapsulation options, including the following:

  • Authentication

    This option tells the calling side of the link to send information that can identify the user. The two methods are PAP and CHAP.
  • Compression

    This is used to increase the throughput of PPP connections by compressing the data or payload prior to transmission. PPP decompresses the data frame on the receiving end.
  • Error detection

    PPP uses Quality and Magic Number options to ensure a reliable, loop-free data link.
  • Multi-link

    Starting with IOS version 11.1, multi link is supported on PPP links with Cisco routers. This option makes several separate physical paths appear to be one logical path at layer 3. For example, two T1s running multi-link PPP would show up as a single 3Mbps path to a layer 3 routing protocol.
  • PPP callback

    PPP can be configured to call back after successful authentication. PPP callback can be a good thing for you because you can keep track of usage based upon access charges, for accounting records, and a bunch of other reasons. With callback enabled, a calling router (client)will contact a remote router (server) and authenticate as I described earlier. (Know that both routers have to be configured for the callback feature for this to work.) Once authentication is completed, the remote router will terminate the connection and then re-initiate a connection to the calling router from the remote router.

PPP Authentication Methods
There are two methods of authentication that can be used with PPP links:

  • Password Authentication Protocol (PAP)

    ThePassword Authentication Protocol (PAP)is theless secure of the two methods. Passwords are sent in clear text, and PAP is only performed uponthe initial link establishment. When the PPP link is first established, the remote node sends theusername and password back to the originating router until authentication is acknowledged.
  • Challenge Handshake Authentication Protocol (CHAP)

    TheChallenge Handshake Authentication Protocol (CHAP)is used at the initial startup of a link and at periodic checkups onthe link to make sure the router is still communicating with the same host. After PPP finishes its initial link-establishment phase, the local router sends a challenge requestto the remote device. The remote device sends a value calculated using a one-way hash function called MD5. The local router checks this hash value to make sure it matches. If the valuesdon’t match, the link is immediately terminated.

Frame Relay

Frame Relay is still one of the most popular WAN services deployed over the past decade, and there’s a good reason for this—cost!

By default, Frame Relay is classified as a non-broadcast multi-access (NBMA) network,meaning it doesn’t send any broadcasts. 

  • Committed Information Rate (CIR)

    Frame Relay provides a packet-switched network to many different customers at the same time. This is a really good thing because it spreads the cost of the switches among many customers. But remember, Frame Relay is based on the assumption that all customers won’t ever need to transmit data constantly, and all at the same time. Frame Relay works by providing a portion of dedicated bandwidth to each user, and it also allows the user to exceed their guaranteed bandwidth if resources on the telco network happen to be available. So basically, Frame Relay providers allow customers to buy a lower amount of bandwidth than what they really use. There are two separate bandwidth specifications with Frame Relay:

    Access rate

    The maximum speed at which the Frame Relay interface can transmit.

    CIR

    The maximum bandwidth of data guaranteed to be delivered

    Frame Relay Encapsulation Types

    When configuring Frame Relay on Cisco routers, you need to specify it as an encapsulation on serial interfaces. you can’t use HDLC or PPP with Frame Relay. When you configure Frame Relay, you specify an encapsulation of Frame Relay (as shown in the following output).But unlike HDLC or PPP, with Frame Relay, there are two encapsulation types:Cisco and IETF (Internet Engineering Task Force).
  • Data Link Connection Identifiers (DLCIs)

    Frame Relay PVCs are identified to DTE end devices byData Link Connection Identifiers (DLCIs). A Frame Relay service provider typically assigns DLCI values, which are used on Frame Relay interfaces to distinguish between different virtual circuits. Because many virtual circuits can be terminated on one multi-point Frame Relay interface, many DLCIs are often affiliated with it.

    DLCIs are local to your routerDLCI 100 DLCI 200RouterARouterB

    DLCI numbers that are used to identify a PVC are typically assigned by the provider andstart at 16.

    You configure a DLCI number to be applied to an interface like this:

    RouterA(config-if)#frame-relay interface-dlci ?
    < 16-1007 >Define a DLCI as part of the current subinterface
    RouterA(config-if)#frame-relay interface-dlci 16
    DLCIs identify the logical circuit between the local router and a Frame Relay switch.
  • Local Management Interface (LMI)

    Local Management Interface (LMI)is a signaling standard used between your router and the first Frame Relay switch it’s connected to. It allows for passing information about the operation and status of the virtual circuit between the provider’s network and the DTE (your router). It communicates information about the following:

    Keepalives
    These verify that data is flowing.

    Multicasting

    This is an optional extension of the LMI specification that allows, for example,the efficient distribution of routing information and ARP requests over a Frame Relay network. Multicasting uses the reserved DLCIs from 1019 through 1022.

Global addressing

This provides global significance to DLCIs, allowing the Frame Relaycloud to work exactly like a LAN.

  • Troubleshooting Using Frame Relay Congestion Control

    verify the Frame Relay congestion control information with the show frame-relay pvc command and get this:
    RouterA#sh frame-relay pvc
    PVC Statistics for interface Serial0/0 
    (Frame Relay DTE)
    Active     Inactive      Deleted       Static
    Local          1            0            0            0
    Switched       0            0            0            0
    Unused         0            0            0            0
    DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
    inputpkts 1300          output pkts 1270       in bytes 21212000
    out bytes 21802000       dropped pkts 4         in pkts dropped 147
    outpkts dropped 0       out bytes dropped 0      in FECN pkts 147
    in BECN pkts 192        out FECN pkts 147
    out BECN pkts 259        in DE pkts 0             out DE pkts 214
    outbcastpkts 0         out bcast bytes 0
    pvc create time 00:00:06, last time pvc status changed 00:00:06
    Pod1R1#
    


    What you want to look for is the in BECN pkts 192 output because this is what’s telling the local router that traffic sent to the corporate site is experiencing congestion. BECN means that the path that a frame took to “return” to you is congested.

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