Data Link Protocols and Ethernet WAN description

HDLC Basics

Every data Link Protocol has one basic job to perform which is to control the correct delivery of every data that been sent over a physical medium.When a packet is encapsulated in a Data Link Frame, it also gave the packet a header and a trailer. The trailer consist of a FCS(Frame Check Sequence) Field which make sure that the frame which has been delivered has no errors and has arrived correctly. HDLC (High-Level Data Link Control) as a Data link Protocol also provides this function.

When we send a HDLC frame through a router, it can only go to one place which is the the other end of the link. The HDLC contains the following fields which are :

• Flag
• Address
• Control 
• Type 
• FCS

PPP (Point-To-Point) Data Link Protocol

The PPP is an another most important used Data link protocol. It is mostly used for the connection of synchronus and asynchronus circuits. It is also used as a DSL(Digital Subscriber Line) internet service for customers.

There are two types of PPP derivatives which are as follows:

1. Point-To-Point over Ethernet(PPPoE)
2. Point-To-Point over ATM (PPPoA)

Ethernet as a WAN

During the beginning of the Internet age, The Ethernet was only appropriate for deployment in LAN's.The restrictions on cable lengths was one of the limit as the ethernet can only go as far as 100m. But as the passage of time, The IEEE improved Ethernet standards in every way so that the Ethernet can be use as a WAN technology. 

There are two types of Ethernet WAN's

1. Ethernet Emulation
2. EoMPLS (Ethernet over Multi Protocol Label Switching)

The Ethernet emulation is used as a general term to tell that the service acts like one Ethernet link. The EoMPLS is a technology that is used on a SP (Service Provider) Cloud. The EoMPLS service provides a point-to-point connection between two customer devices and shows the type of connectivity as if a fiber Ethernet link exists between the two customer devices.

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WAN Technology Description

The LAN (Local Area Network) typically include Nearby devices but when we talk about WAN (Wide Area Network), we are literally talking about devices that can be Far apart. The main priority for WAN is to move data between LAN's. The device which is used to connect the WAN with LAN is called a Router.
To connect our LAN with WAN, we order a Leased Line which is provided to us by an ISP (Internet Service Provider). Leased Line is available in variety of speed. The Leased Line is known by many names like Leased Circuit, Serial Line/Link, P2P Line/Link, T1, WAN Line/Link and Private Line. For connecting a Leased Line on a site, The site requires a CPE (Customer Premises Equipment) which consist of :

• A Router
• Serial Interface Card
• CSU/DSU (Channel Service Unit/Data Service Unit)                                       

The Serial cables that are used between a router and an external CSU/DSU are DTE (Data Terminal Equipment) and DCE (Data Communication Equipment). DTE has a male connector while DCE has a female connector which helps to connect both cables directly. The DCE cable works like a crossover cable because it swaps the transmitting and recieving pairs.
The DCE cables help the CSU/DSU to perform one primary Function which is called CLOCKING.
The Clocking Function tells the router when to exactly send the bits through signaling over the cable.
When we connect the DCE cable in the serial port, the router automatically senses it and supplies Clocking.
Leased Line provides a Layer 1 service which is the reason Leased Line doesn't provide standard Data Link Protocol which is a Layer 2 service.
On the next post we will explain to you the two most popular Data lank protocol which are

HDLC (High-Level Data Link Protocol)
PPP (Point-To-Point Protocol)