Data Link Layer - EthernetPertemuan 14

Matakuliah : H0484/Jaringan KomputerTahun : 2007

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa

akan mampu :

• Menggunakan Ethernet

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Outline Materi

• Standards• Frame format

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Frame Structure

• In synchronous transmission all transmissions are in frames

• Single frame format for all data and control exchanges

• Basic Structures:– Character Oriented Protocol (COP) - BSC

(Bisynchronous)– Bit Oriented Protocol (BOP) - HDLC, SDLC

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Generic Frame Structure (BOP)

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Data Link Protocols

• HDLC - High-Level Data Link Control• SDLC - Synchronous Data Link Control• PPP - Point to Point Protocol• Ethernet

HDLC can be considered as the base protocol format

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Ethernet Frame Format

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Ethernet Frame

The frame can be any number of bytes in length, depending on the specific

technology usedEthernet frame can be as long as 1522

bytes.

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Ethernet Layers

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Logical Link Control

• Interface to higher levels• Flow and error control• Transmission of link level PDUs between

two stations• Must support multiaccess, shared medium• Relieved of some link access details by MAC

layer• Addressing involves specifying source and

destination LLC users– Referred to as service access points (SAP)– Typically higher level protocol

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MAC Control Packet

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MAC Frame Format

• MAC control• Destination MAC address• Source MAC address• LLC

– MAC layer receives data from LLC layer• CRC

– MAC layer detects errors and discards frames

LLC optionally retransmits unsuccessful frames

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Media Access Control

• Assembly of data into frame with address and error detection fields

• Disassembly of frame– Address recognition– Error detection

• Govern access to transmission medium– Not found in traditional layer 2 data

link control• For the same LLC, several MAC options

may be available

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Media Access Control (MAC)

• Shared-media broadcast technology. • Ethernet’s MAC performs three functions:

Transmitting and receiving data packets Decoding data packets and checking them for valid

addresses before passing them to the upper layers of the OSI model

Detecting errors within data packets or on the network• Specified by the technology being used.• Dictates who can transmit and when (Only one station

may transmit at a time; otherwise, their signals would be scrambled)

• Two types: Deterministic (Token Ring) Opportunistic (First come, first served - Ethernet)

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Ethernet and IEEE 802.3

• Specify similar technologies.• Broadcast network.• Using CSMA/CD algorithm.• Hardware implementation.• Differences:

– Ethernet provides services corresponding to physical and datalink layer.

– IEEE 802.3 specifies the physical layer and the channel-access portion of the data link layer but does not define a LLC protocol.

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Protocol stack

Link Control Protocol (LCP)Authentication ProtocolsNetwork Control Protocol (NCP)Bit PaddingDigital Signals (DS) Services

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Encapsulated LCP packet

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An example