Share this page : facebooktwitterlinkedinmailfacebooktwitterlinkedinmail
PPP
Definition:

Point-to-Point connections are dedicated communication lines between two locations, such as T1/T3 or E1/E3 lines using PPP as the Layer 2 protocol.  It is a connection-oriented protocol.

Connection-oriented protocol:

Connection-Oriented Protocols require that two devices establish a formal connection that ensures they are ready to communicate. Connection-oriented Layer 2 protocols include PPP, ATM, and Frame Relay.

Connectionless protocol:

Connectionless Protocols do not require a formal connection before two devices can communicate. One device just send the frame, has no idea if the other device can receive it or not. Ethernet is an example of a connectionless Layer 2 protocol.

Establish a PPP connection
  1. Establish connection and configure link using the Link Control Protocol (LCP).  Link Control Protocol (LCP) packets are the packets exchanged between two devices during the PPP connection establishment process. To bring up the connection, both ends of the link must first send PPP LCP packets to configure and test the p2p line.
  2. Use authentication to verify identity(optional); For dedicated p2p links it is not needed.
  3. Configure Layer 3 protocols using the Network Control Protocol (NCP). Network Control Protocol (NCP) is a Layer 3-specific protocol used during the PPP connection establishment process to establish and configure the different Layer 3 protocols—such as IP, IPX, or Appletalk—running over the connection.   IP Control Protocol (IPCP) is the PPP Network Control Protocol used to configure, enable, and disable the IP protocol on both ends of the point-to-point link.
PPP Frame

PPP_Frame

  • Flag: 8 bits or 1 byte that indicate the beginning or end of a frame so that PPP can determine where each frame ends and the next begins. The flag consists of the binary sequence 01111110 or Hexadecimal 7E.
  • Address: 8 bits or 1 byte. In HDLC, it is the address of the destination of the frame. in PPP, it has no real meaning. It is always set to the binary 11111111, which equivalent hexadecimal FF, the broadcast address.
  • Control: the 8 bits or 1 byte field is used in HDLC for various control purposes, but in PPP it is always set to binary 00000011 or hexadecimal 03.
  • Protocol: This 16 bit or 2 byte field identifies the protocol of the packet encapsulated in the data field of the frame. For IP traffic, this field is set to binary 00000000 00100001 or hexadecimal 0021.
  • Data: this variable length field contains either data or control information, depending on the frame type. For regular PPP data frames, the IP packet is encapsulated here; for control frames, control information, such as the LCP and NCP used to configure the link, or keepalives to see if the other side is till there, are placed here instead.
  • Frame Check Sequence ( FCS): A 2( default) or 4 byte checksum computed over the frame to provide basic protection against errors in transmission. This is a CRC code similar to the one used for other Layer 2 protocol error protection schemes such as the one used in Ethernet. The FCS is calculated over the address, control, protocol, information and padding fields.

 

Frame relay and ATM
Virtual Circuuits

Virtual Circuits are the logical connections running over a single physical communications line and are used to connect two devices or locations. A virtual circuit, also known as a virtual connection, acts like a dedicated connection through the provider’s network even though the physical lines might be shared by multiple customers. Frame Relay and ATM networks make use of virtual circuits. Virtual circuits come in two types. A permanent virtual circuit is like a leased line in that the service provider defines a path to each customer location. Permanent virtual circuits are always on and ready to use. A switched virtual circuit is dynamically establishes only when data needs sending and terminates when the transmission is complete.

VPN: A Virtual Private Network (VPN) is a private network built across a public network such as the service provider’s network or the Internet. It can’t solve the network congestion, but can reduce the latency.

ATM

Asynchronous transfer mode (ATM) is a Layer 2 WAN protocol that can transport voice, video, and data on the same network with guaranteed performance or quality of service for each type of traffic. Customers can build a Virtual Private Network using ATM.

Characteristic:

  • Cell-switching technology
  • Fixed length 53-byte cells : switches don’t have to spend time determining the start and end of a variable length frame.
  • Ensure that delay-sensitive data such as voice or video is not adversely affected by long data frames.
  • Label different kinds of data, guarantee performance or quality of service for each type of traffic.
  • Allows many customers to share a service provider’s network resources and create private VPNs.
  • Complicated and expensive to deploy, operate and manage.
  • Not the fastest WAN option by today’s standards.

Leave a Reply

Your email address will not be published.