Point to Point connections. Serial Communications WAN technologies are based on serial transmission at the physical layer. This means that the bits of.

Point to Point connections

Serial Communications

• WAN technologies are based on serial transmission at the physical layer.

• This means that the bits of a frame are transmitted one at a time over the physical medium.

Serial Communications

• In long-haul communications and most computer networks the serial connections are preferred because – Serial cables can be longer than parallel cables,

because there is much less interaction (crosstalk) among the conductors in the cable

– The cost of cable and synchronization difficulties make parallel connections impractical.

– The most significant advantage is simpler wiring.

Time Division Multiplexing

• TDM divides the bandwidth of a single link into separate channels or time slots

• TDM transmits several sources of information over the same link by allocating a different time interval (time slot) for the transmission of each channel.

WAN Connection Types

WAN Encapsolation Protocols

HDLC Protocol

• HDLC is an ISO standard Data Link layer protocol. (standard HDLC)

• HDLC uses serial transmission to provide error-free communication between two points.

• Cisco HDLC is a point-to-point protocol that can be used on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices.

• Cisco HDLC is the default Layer 2 protocol for Cisco router serial interfaces.

HDLC Encapsulation

• HDLC defines a Layer 2 framing structure

HDLC Encapsulation

• Standard HDLC – does not support multiple protocols on a single

link, as it does not have a way to indicate which protocol is being carried.

• Cisco HDLC – The frame uses a ’protocol' field that enables multiple

network layer protocols to share the same serial link.

Point-to-Point Protocol (PPP)

• Point-to-Point Protocol (PPP) is also data link protocol used on serial links just like HDLC. However PPP is has more advanced features when compared with HDLC which is quite primitive.

• Can be used over asynchronous serial or synchronous serial media.

PPP Components

PPP Components

Three main components:– HDLC:

• HDLC protocol for encapsulating datagrams over point-to-point links.

– LCP:• Extensible Link Control Protocol (LCP) to establish, configure,

maintain and terminate the data link connection.

– NCPs:• Family of Network Control Protocols (NCPs) for establishing

and configuring different network layer protocols. • Allows simultaneous use of multiple Network layer protocols

PPP Layered Architecture

• PPP can be configured on multiple types of interfaces:– Asynchronous serial – Synchronous serial – High-Speed Serial Interface (HSSI) – Integrated Services Digital Network (ISDN)

Network Control Protocols (NCPs)

Data Link Layer

Network LayerIP IPX Many Others

Link Control Protocol (LCP)

Synchronous or AsynchronousPhysical Medium Physical Layer

PPP Layered Architecture

– PPP uses the Link Control Protocol (LCP) to negotiate and setup control options on the WAN link.

– Authentication, Compression, Error Detection, Multilink for load balancing, PPP Callback and link monitoring functions.

Synchronous or AsynchronousPhysical Medium Physical Layer

Network Control Protocols (NCPs)

Data Link Layer

Network LayerIP IPX Many Others

Link Control Protocol (LCP)

Link Control Protocol (LCP)

Network LayerIP IPX Many Others

PPP Layered Architecture

• PPP uses the Network Control Protocols (NCPs) to permit multiple network layer protocols to operate on the same communications link.

Synchronous or AsynchronousPhysical Medium Physical Layer

Network Control Protocols (NCPs)

Data Link Layer

Link Control Protocol (LCP)

Network LayerIP IPX Many Others

PPP Layered Architecture

• For every network layer protocol used, a separate Network Control Protocol (NCP) is provided.

Synchronous or AsynchronousPhysical Medium Physical Layer

Data Link Layer

IPCP IPXCP Many Others

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PPP Session Establishment

• PPP session establishment progresses through three phases:– link establishment phase– Authentication phase– network layer protocol phase

Link Establishment Phase

• Before PPP exchanges any network layer datagrams (for example, IP), the LCP must first open the connection and negotiate configuration options.

• This phase is complete when the receiving router sends a configuration-acknowledgment frame back to the router initiating the connection.

Link Establishment Phase

• As part of this phase, LCP also allows for an optional link-quality determination test. – The link is tested to determine whether the link quality is

good enough to bring up network layer protocols.

Authentication Phase

• After the link has been established and the authentication protocol decided on, the peer may be authenticated.

• Authentication, if used, takes place before the network layer protocol phase is entered.

• PPP uses PAP and CHAP as authentication protocols

Network Layer Protocol Phase• In this phase the PPP devices send NCP packets to

choose and configure one or more network layer protocols, such as IP.

• Once each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link.

• The PPP link remains configured for communications until either of the following: – LCP (change in authen) or NCP frames close the link – A user intervenes – If the LCP closes the link, it informs the network layer protocols

so that they can take appropriate action