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Establishing Serial Point-to-Point Connection

October 22, 2012


Organizations and home users connect to networks to share information and resources. Most organizations create an internal private network for the use of the employees. These types of networks enable employees to access the organizations resources, from within, and outside the organization. Employees can also share resource of different branches using a network. Most networks span over cities and states, which became a part of other networks. These networks together form the internet.

Wide Area Network

Wide Area Network (WAN) is network that spans a large geographical area. These networks are made up to two or more Local Area Networks (LAN). A LAN is a group computers connected to each other in a very small area. User on a LAN can connect to multiple networks, Networks in a WAN are usually connected using public or private service, such as, telephone lines.

The terms and components of WAN are:

  • Data Communications Equipment (DCE) : – Creates a physical connection between the source and destination computers over a network. It also is responsible for forwarding traffic and transferring data between computers. For example, modern is a DCE.
  • Data Terminal Equipment (DTE) : Connects computers to a WAN with the help of a DCE. DTE is end user equipment, such as, a router or computer. A device where a communication path begins or ends is called DTE.
  • Customer Premises Equipment (CPE) : Consist of network equipment, such as, DCE and DTE. CPE is the telecommunication equipment installed in the premises of an organization. Devices, such as, modems, telephones, and routers can be classified as CPE.
  • Demarcation Point : Refers to the point where ownership of communication is transferred from the sender to the receiver.
  • Toll Network : Consist of the sender’s interior infrastructure that enables the sender to transfer data.
  • Central Office (CO) switch : Connects subscriber’s telephone lines to other telephone networks using switching equipment in a toll network. The local lines terminate at a CO, and switching equipment switches signals over the telephone network.
  • Local Loop : Helps to transfer data from the sender’s switching equipment to the demarcation point.
  • Leased -Lines Connection : Provides a dedicated connection between two sites. A single cable connects two sites. It is preferred when the distance is no too large, and is a synchronous connections. the bandwidth can range from 2,400 kbps to 45 Mbps.
  • Circuit_Switched Connection : Used by PCs with a modem when connection to a ISP. These are dial-up connections, which include asynchronous and synchronous serial connections. The main disadvantage is that it is expensive when connections are made over a long distance. The bandwidth can vary between 300 kbps to 53 kbps.
  • Packet_Switched Connections : Uses logical circuits (virtual circuits) to build connections between two sites. The advantage is that logical circuits are bound to a physical path. This enables logical circuits to be built on physical paths.
  • Cell-Switched Connections : Transfer different kinds of traffic, such as, voice, video, and data, simultaneously. The advantage is that the distance can be large between nodes.

While connecting a LAN to a WAN you should be aware to the connection requirements. Some of the major factors that help to select a WAN are:

  • Frequency : Identify the usage frequency of the network. For example, if the network is used only during working hours, a higher transfer rate will be required during the day.
  • Transfer Rates : Identify the rate at which data needs to travel in a WAN connection. For example, there may be a need for transfer speeds, for bulk transactions of video conferencing.
  • Traffic Load : Identify the peak hours on the network. Verity if your WAN connection can handle heavy traffic load.
  • Cost : Identify the different schemes, such as, per-packet cost, and per-minute cost. Identify the best payment rate based on the WAN requirement. This will ensure that expenses are required without affecting the performance of the WAN.

Data-Link Protocols

Data-Link layer protocols are commonly called the Data Link Protocols. These protocols communicate between two devices, such as, Routers, and Bridges. Data packets are sent over the link created by the protocols.

Data_link Protocols consist of :

  • High Level Data Link Control  protocol(HDLC) :  HDLC is an ISO standard bit-oriented data link protocol that encapsulated data on synchronous serial data links.
  • Point-to-Point protocol (PPP) : Enables computers to connect to the internet using a dial-up telephone line and Integrated Services Digital Networks (ISDN).  PPP is more stable and has additional error-checking features.

HDLC Characteristics

Standard HDLC specifies a data encapsulation method on synchronous serial links using from characters and checksums. By default, synchronous serial lines use the HDLC serial encapsulation method. HDLC does not inherently support multiple protocols on a single link because it does not have a standard way to indicate which protocol it is carrying.

Cisco offers a proprietary version of HDLC. The Cisco HDLC frame uses a proprietary type field that acts as a protocol field, which makes it possible for multiple network layer protocols to share the same serial link. Cisco’s HDLC is a point-to-point protocol which can be used on leased lines between two Cisco devices. If communicating with a non-Cisco device, synchronous PPP is a more viable option.

Configuring HDLC

The encapsulation on a Cisco synchronous Serial interface is HDLC. HDLC needs to be configured f the data link protocol is change to some other protocol, and HDLC has to be set back to default.

To configure the HDLC protocol to be set as default:

  1. Open the router CLI.
  2. Switch to privileged mode.
  3. Enter the following commands in the command prompt, to set the HDLC protocol:              interface serial [module _0/1]port_1                  encapsulation hdlc
  4. Press Enter.
  5. Enter the following command in the command prompt, to view the data link layer encapsulation:   show interface serial 1
  6. Press enter. The output appears.


The PPP enables computers to connect to the internet using a dial-up telephone line and ISDN. PPP was originally designed to transport IP traffic over point-to-point links. PPP can transfer data over a synchronous and asynchronous links. It supports a transfer of data over different network protocols.

List the different fields of PPP frame:

Filed Name          Description

Flag                 : –          Contains a single byte which indicates the beginning or end of frame.

Address        : –         Contains the broadcast address which is a binary sequence.

Control      : –            Contains a single byte of binary sequence. This calls for transmission of user data in a non-sequence frame.

Protocol     : –            Contains two bytes of information about the encapsulation protocol.

Data               : –            Contains encapsulated data, which can range from 1 to 1,500 bytes.

FCS                 : –            Contains 2 to 4 bytes. Improves error detection.

Configuring PPP

Configuring the PPP enables to specify that the WAN uses the PPP protocol. The PPP parameter is specified using the encapsulation and interface command in the sub-configuration mode.

To specify the PPP protocol to be used on a WAN  : –

  1. Open router CLI.
  2. Switch to privileged mode.
  3. Switch to configuration mode.
  4. Enter the following commands in the command prompt, to specify the PPP.                     interface type [slot_2] port_1                 encapsulation PPP
  5. Press Enter.

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