WAN Protocol Basics

In the past few months, I've written a number of articles on different protocols that you need to be familiar with if you're studying for your CCENT and CCNA certification exams. So far we've covered:

basic Networking Protocols, including TCP/IP and DNS and DHCP

Internet Control Protocols

both versions of the Routing Information Protocol, RIP version 1 and RIP version 2

Interior Gateway Routing Protocol

Today, we'll focus on Wide Area Networks. I'll briefly introduce you to the different connection types that are used on WANs and the main protocols that are used on these connections.

Wide Area Networks

Wide Area Networks can be seen as connection pipes that interconnect Local Area Networks. Usually WANs in contrast to LANs are not owned by the public; they are owned by service providers and their functionality-infrastructure is leased in order for LANs to be able to extend their expandability and make use of distant-remote services.

A number of different WAN connection types exist today. Choosing the right WAN connection type is up to you, but the information in this article will make your decision process much easier.

WAN Connection Types

Leased Line:

This is considered to be a dedicated point-to-point connection type where a permanent communication path exists between a Customer Premise Equipment (CPE) on one site and a CPE at the remote site communicating through a Data Communicating Equipment (DCE) within the providers' site. Synchronous serial lines are used for this connection and the most frequent protocols observed in these lines are HDLC (High-Level Data Link Control) and PPP (Point-to-Point Protocol). When cost in not an issue, you should use this type of connection.

Circuit Switching:

The concept of this WAN connection is based on the typical telephone switching network. A connection needs to be established prior to be able to transfer data. This type of connection is used for low bandwidth data transfers where charging is calculated based on actual connection time. ISDN (Integrated Services Digital Network) protocol is basically used on this connection type.

Packet Switching:

Always-on connection, where available bandwidth is shared between several users. No time-based charging. Charging is based on committed traffic rate. This type of connection is more appropriate for bursty data transfers. Special configuration is needed to support strict QoS requirements. Frame Relay is a packet switching connection type.

High Level Data Link Control (HDLC)

HDLC is a data-link layer protocol and because of the fact that there is no standard way of identifying the type of network protocol carried within the HDLC encapsulation, each vendor uses its own proprietary HDLC protocol.

Cisco uses its own HDLC implementation; therefore Cisco routers are not able to communicate with equipment running other vendors' HDLC implementation. Nevertheless, HDLC is the default encapsulation used by Cisco routers on synchronous serial links (leased line connections). When communicating with a non-Cisco device, synchronous Point-to-Point protocol (PPP) is the more feasible option to use.

On Cisco routers use the show interface command on serial interfaces to see the configured encapsulation method.

To see the physical connection type used, issue the show controllers command:

Point-to-Point Protocol (PPP)

PPP data link protocol is used on serial connections between dissimilar routers, for example a Cisco router and a non-Cisco router. PPP is designed to allow the simultaneous use of multiple network layer protocols and also supports two types of hostname authentications CHAP (Challenge Handshake Authentication Protocol) and PAP (Password Authentication Protocol).

PPP uses the services of the HDLC protocol for encapsulating datagrams over serial links. Moreover, it uses two additional control protocols to support its operation:

• Link Control Protocol (LCP) provides the means for configuring, establishing, maintaining and terminating the PPP connection. Among other things, LCP handles PPP authentication methods, error detection, compression techniques, support for multilink etc.

• Network Control Protocol (NCP) provides the means for encapsulating multiple network layer protocols across the PPP data link.

Use the show interface command to verify PPPs operation.

Notice from the output of the show interface serial 1/0 command the PPP encapsulation type. Also, notice that LCP is Open meaning is being running and maintaining the PPP connection. Finally, the last line is associated with the NCP. It shows that IP, CDP and AppleTalk are open.

Frame Relay

Frame Relay is a packet-switched technology. No connection setup phase takes place prior to data transmission. Moreover, the network infrastructure is shared among different users in contrast to leased line connections where the whole amount of bandwidth is always dedicated to the corresponding user. The main characteristics of Frame Relay technology are presented below:

• Contract terms are signed between the customer and service provider. Mainly the contract consists of a so-called Committed Information Rate (CIR) which is the amount of bandwidth the service provider has contractually guaranteed to provide to the customer at all times. The later may use more bandwidth if the network infrastructure is not congested, however this excess traffic is not guaranteed at all.
• Big money saved for both customer and service provider. The customer makes use of this packet switched technology at much lower price compared to the leased line option. From the other hand, the service provider does not have to install and maintain a huge number of leased line connections which always consume the whole bandwidth tube even if they are not really used.
• Frame Relay on Cisco routers is configured on serial interfaces. Unlike HDLC or PPP, configuring Frame Relay is achieved by specifying the appropriate encapsulation type among Cisco and IETF (Internet Engineering Task Force). The default encapsulation used for Frame Relay on Cisco routers is you guess correctly Cisco of course.
• Frame Relay uses what is called virtual circuits to route data across the service providers infrastructure towards the other communicating end. Service providers mainly use Permanent Virtual Circuits (PVCs) within their network to route packets forth and back. The PVCs once created remain operating as long as the customer pays the bill.
• PVCs are identified by the use of Data Link Connection Identifiers (DLCIs) which are typically assigned by the provider to end devices. These identifiers have only local significance in the sense that they are used to identify a specific data link and not the entire virtual circuit end-to-end. According to the DLCIs values assigned to the customers, the service provider is able to route packets appropriately.

Integrated Services Digital Network (ISDN)

ISDN is a Circuit Switched technology that is designed to run over existing telephone networks. It is a fully digital technology end-to-end. It consists of a number of protocols for transferring data, voice and video over the traditional telephone system. ISDN has the following major characteristics:

• Faster data transmission compared with analog modem connection.
• Perfect candidate for establishing a backup connection to a leased line connection.
• Comes with two flavors:
  • ISDN Basic Rate Interface (BRI) service also known as 2B+D consists of two data channels (B channels) that operate at 64 Kbps each and a single signaling channel (D channel) that operates at 16kbps.
  • ISDN Primary Rate Interface (PRI) also known as 23B+D in North America and Japan and 30B+D in Europe. In the case of 23B+D, it consists of 23 data channels operating at 64kbps each and one signaling channel operating at 64kbps as well.
• To be able to connect a Cisco router to the ISDN network you can either use a router with a built-in NT1 (U) interface (ISDNs two wire connection that runs into the home or office) or use an ISDN terminal adapter (TA) along with your routers serial interface

Summary: Important Key Points about WAN

1. Cisco routers use a proprietary HDLC encapsulation on all their serial interfaces by default.
2. PPP is used if you use different brands of routers or if you need extra features such as hostname authentication and connection management.
3. Two types of Frame Relay encapsulations are used on Cisco routers: Cisco and IETF. Default is the Cisco encapsulation. Use IETF when using different vendor routers.
4. Committed Information Rate (CIR) is the average rate in bits per second that the ISP guarantees to transfer. Charges for using this service are based on CIR value.
5. ISDN BRI provides two 64Kbps bearer channels and one 16kbps data channel for a total of 144kbps. ISDN PRI provides twenty three (in USA) 64Kbps bearer channels and one 64kbps data channel for a total of 1.544Mbps. In Europe thirty bearer channels are used for a total of 2.048Mbps.

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