Actually, you may already have heard of demarcs. If you’ve ever had the glorious experience of reporting a problem to your service provider, they’ll usually tell you everything tests out fine up to the demarc, so the problem must be the CPE, or customer premises equipment. In other words, it’s your problem not theirs.
The idea behind a WAN is to be able to connect two DTE networks through a DCE network. The DCE network includes the CSU/DSU, through the provider’s wiring and switches, all the way to the CSU/DSU at the other end. The network’s DCE device (CSU/DSU) provides clocking to the DTE-connected interface (the router’s serial interface).
As mentioned, the DCE network provides clocking to the router; this is the CSU/DSU. If you have a nonproduction network and you’re using a WAN crossover type of cable and do not have a CSU/DSU, then you need to provide clocking on the DCE end of the cable by using the clock rate command.
High-Level Data-Link Control (HDLC) Protocol
The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented, Data Link layer protocol. It specifies an encapsulation method for data on synchronous serial data links using frame characters and checksums. HDLC is a point-to-point protocol used on leased lines. No authentication can be used with HDLC.In byte-oriented protocols, control information is encoded using entire bytes. On the other hand, bit-oriented protocols use single bits to represent the control information. Some common bit-oriented protocols include SDLC, LLC, HDLC, TCP, and IP. HDLC is the default encapsulation used by Cisco routers over synchronous serial links.
And Cisco’s HDLC is proprietary—it won’t communicate with any other vendor’s HDLC implementation.
Point-to-Point Protocol (PPP)
Remember that it’s a Data Link layer protocol that can be used over either asynchronous serial (dial-up) or synchronous serial (ISDN) media. It uses Link Control Protocol (LCP) to build and maintain data-link connections.Network Control Protocol (NCP) is used to allow multiple Network layer protocols (routed protocols) to be used on a point-to-point connection.
Since HDLC is the default serial encapsulation on Cisco serial links and it works great, why and when would you choose to use PPP? Well, the basic purpose of PPP is to transport layer 3 packets across a Data Link layer point-to-point link, and it’s nonproprietary. So unless you have all Cisco routers, you need PPP on your serial interfaces—the HDLC encapsulation is Cisco proprietary, remember? Plus, since PPP can encapsulate several layer 3 routed protocols and provide authentication, dynamic addressing, and callback, PPP could be the best encapsulation solution for you instead of HDLC.
PPP contains four main components:
EIA/TIA-232-C, V.24, V.35, and ISDN A Physical layer international standard for serial communication.
HDLC A method for encapsulating datagrams over serial links.
LCP A method of establishing, configuring, maintaining, and terminating the point-to-point connection.
NCP A method of establishing and configuring different Network layer protocols. NCP is
designed to allow the simultaneous use of multiple Network layer protocols. Some examples of protocols here are IPCP (Internet Protocol Control Protocol) and IPXCP (Internetwork Packet Exchange Control Protocol).
Burn it into your mind that the PPP protocol stack is specified at the Physical and Data Link layers only. NCP is used to allow communication of multiple Network layer protocols by encapsulating the protocols across a PPP data link.
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.
Multilink: Starting with IOS version 11.1, multilink 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 multilink 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 Session Establishment
When PPP connections are started, the links go through three phases of session establishment;Link-establishment phase: LCP packets are sent by each PPP device to configure and test the link. These packets contain a field called the Configuration Option that allows each device to see the size of the data, compression, and authentication. If no Configuration Option field is present, then the default configurations will be used.
Authentication phase: If required, either CHAP or PAP can be used to authenticate a link.
Authentication takes place before Network layer protocol information is read. And it’s possible that link-quality determination will occur simultaneously.
Network layer protocol phase: PPP uses the Network Control Protocol (NCP) to allow multiple Network layer protocols to be encapsulated and sent over a PPP data link. Each Network layer protocol (e.g., IP, IPX, AppleTalk, which are routed protocols) establishes a service with NCP.
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. And it’s a rare network design or designer that has the privilege to ignore that all-important cost factor!By default, Frame Relay is classified as a non-broadcast multi-access (NBMA) network, meaning it doesn’t send any broadcasts like RIP updates across the network.
Frame Relay has at its roots a technology called X.25, and it essentially incorporates the components of X.25 that are still relevant to today’s reliable and relatively “clean” telecommunications networks while leaving out the no-longer-needed error-correction components.
Below are some of the Frame Relay technology information you need to know about;
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. In reality, it’s the average amount that the service provider will allow you to transmit.
Virtual Circuits: Frame Relay operates using virtual circuits as opposed to the actual circuits that leased lines use. These virtual circuits are what link together the thousands of devices connected to the provider’s
“cloud.” Frame Relay provides a virtual circuit between your two DTE devices, making them appear to be connected via a circuit when in reality, they’re dumping their frames into a large, shared infrastructure. You never see the complexity of what’s actually happening inside the cloud because you only have a virtual circuit.
Data Link Connection Identifiers (DLCIs): Frame Relay PVCs are identified to DTE end devices by Data 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 multipoint Frame Relay interface, many DLCIs are often affiliated with it.
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 Relay cloud to work exactly like a LAN.
- Status of virtual circuits: This provides DLCI status. The status inquiries and messages are used as keepalives when there is no regular LMI traffic to send.