Example Data Link Protocols

Quick review Reference Models? Layers? Flow Control? Bit stuffing? Connection Oriented? Synchronous transmission? Full-duplex communication? Data Link protocol?

Example Data Link Protocols High-Level Data Link Control -HDLC The Data Link Layer in the Internet

SDLC protocol (Synchronous Data Link Control) High-Level Data Link Control -HDLC

(HDLC) History The original ISO standards for HDLC are: –ISO 3309 – Frame Structure –ISO 4335 – Elements of Procedure –ISO 6159 – Unbalanced Classes of Procedure –ISO 6256 – Balanced Classes of Procedure The current standard for HDLC is ISO 13239, which replaces all of those standards. (ADCCP ) Advanced Data Communication Control Procedure (HDLC) High-Level Data Link Control (LAP ) Link Access procedure (LAPB)

(HDLC) Features One of the most common data link layer protocols (CISCO routers). Bit-oriented protocol. Use bits to stuff flags occurring in data. Provides both connection-oriented and connectionless service. Synchronous Data link control protocol. Implements error and flow control (adjust window size based on receiver capability)). Used for point-to-point and multipoint links access. Reliable protocol: –Use Selective repeat and go-back-N,with a 3-bit sequence number Up to seven unacknowledged frames may be outstanding at any instant. Full-duplex communication (receive and transmit at the same time)

Data Transfer Mode There are three modes of data transfer operations: –Normal Response Mode (NRM) The secondary station initiate a transmission after permission from the primary station (usually half-duplex). Typically use SDLC. Mainly used in terminal-mainframe networks. –Asynchronous Response Mode (ARM) The secondary station can initiate a transmission without receiving permission from the primary station. Used mostly in point-to-point configurations. –Asynchronous Balanced Mode (ABM) Both sides can send and receive. Mainly used in full duplex point-to-point links, for communication between combined stations. Used in the protocol set known as X

HDLC – Frame Format  Two 8-bit flag fields to identify the start and end of frame (The value of the flag is always ( )).  Address field:  Multipoint links: contains the address of the destination.  Point-to-point links: used to distinguish command from response (Why?).  This field is set to ( ) in broadcasting (to all-stations).  Control field: (Identify the frame type).  Data or information field contains user data from the network layer.  The Frame Check Sequence (FCS)  Computed over the Address, Control, and Information fields.  Error detection field. FlagFCSDataControlAddressFlag 816 Grater than or =

HDLC frame types  Information frame (I-frame):  Carries data from the upper network layer it can also carry flow and error control information (piggybacking)  Supervisory Frame (S-Frame):  Carries only flow and error control Information (ACK, NAK)  Used when piggybacking can’t be used or no data is sent with acknowledgement or sending commands…

HDLC frame types  Unnumbered Frames (U-frame) carries link management:  Set up the logical link between the primary station and a secondary station.  Inform the secondary station about the mode of operation which is used)

I_Frame Control Field  First bit is 0 to indicate I-frame.  Seq: holds the frame sequence number (3-bits).  P/F:  P/F = 1: means Poll (If the frame is sent by the primary )  P/F = 0: means final (If the frame is sent by the secondary).  Next: holds the ACK number (piggybacking) to be sent to the sender (3-bits). Seq Next

S-frame control field There are four different supervisory frames: TypeNext NameType ACK( Indicate the next frame expected)Receiver Ready00 Negative ACK (Retransmit all outstanding frames starting at Next (Go Back N)) Reject01 ACK(Indicate the next frame expected + stop sending) Receiver Not Ready10 Negative ACK (Retransmission oonly the frame specified in Next(Selective Repeat)) Selective Reject11 Type 3 frames are undefined in SDLC and LAPB

U-frame control field

Protocol operation

The Data Link Layer in the Internet

Point-to-Point communication Point-to-point communication is primarily used in the internet in two situations: Router to router

Point-to-Point communication Point-to-point communication is primarily used in the internet in two situations: Dial-up host-router PPP—The Point-to-Point Protocol

PPP-The Point-to-Point Protocol  A data link layer protocol. Character(Byte) -oriented protocol Use byte stuffing to stuff flags occurring in data.  Used for Point-to-Point Connections only.  Used as data link to connect :  Home users (PC) to ISP using a telephone line and a modem.  Two routers

PPP Features Defines the format of the frame to be exchanged between devices. Defines the link establishment and data transfer between devices. Supports multiple protocols. Defines how two devices authenticate each other Error detection – (not correction)

PPP Support protocols  Link management:The link control protocol (LCP) is responsible for :  Establishing, configuring, and negotiating a data-link connection.  Monitors the link quality.  Terminate the data-link connection.  Authentication: Authentication is optional (negotiated between the two devices). PPP supports two authentication protocols:  Password Authentication Protocol (PAP).  Challenge Handshake Authentication Protocol (CHAP).  Network protocols configuration: (NCPs) responsible for:  Establishing and configuring different network layer protocols (such as IP, AppleTalk, Decnet )  For IP network protocol, NCP handles dynamic IP address assignment

Frame format (Byte). Identify the frame boundaries. Flag No need for an address for prodcasting. Can be removed – negotiated. Address Can be removed – negotiated Control

Frame format It tells what kind of packet is in the Payload field. Length is 2 or 1byte(negotiated) Protocol Contains user’s data or other information Default maximum length 1500 bytes. length negotiated ( at link configuration ) Data field (payload) length negotiated Checksum (FCS)

Multiplexing in PPP

PPP Transition states

PPP Transition states

PPP Transition states

PPP Transition states

PPP Transition states

LCP Frame Types

An example

An example (continued) IPCP Networ k DataLi nk

References

Questions ?

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