Jaringan Komputer : Bab 4

DataLinkLayer
Pendahuluan

MerupakanLayer2padaModelOSI

Bertujuanmenyediakankoneksiantaraduakomputer/hostdenganmenggunakanpengalamatansecarafisik/HardwareAddressing

Komunikasihanyabisaterjadijikakeduahost/komputeryangberkomunikasitahualamatfisikmasing-masing

PengalamatansecarafisikbiasadisebutMACAddress

MACAddressDiambilkandariIDNICmasing-masingkomputer(cmdcommand->ipconfig/all).
MAC (Media Access Control) Address

MACAddressterdiridari48bittetapibiasanyaditulisdalam12bitHeksadesimaldenganketentuan6bitsebagaikodepabrikyangditentukanolehIEEEan6bitberikunyaadalahnomorserialperalatanyangdikeluarkanolehpabrik
Pengiriman Data

Untuk melakukan pengiriman data diperlukan kombinasi antara pengalamatan secara fisik dan pengalamatan secara logik

pengalamatan secara logik biasa disebut dengan IP Address (nomor IP), berada pada layer network

Nomor IP diperlukan oleh perangkat lunak untuk mengidentifikasi komputer pada jaringan

Namun nomor identitas yang sebenarnya diatur oleh NIC (Network Interface Card)atau kartu Jaringan yang juga mempunyai nomor unik.
Pengirimandata padaDataLinkLayer

Penentuanwaktupengirimandatayangtepatapabilasuatumediasedangterpakai,haliniperluuntukdeteksisinyalpembawa.

PadaEthernetmenggunakanmetodeCarrierSenseMultipleAccess/CollisionDetection(CSMA/CD).

Padajaringanyangdapatmelakukanaksessecarabersamaansimultan.MakabilaHostAmengirimkandatakeHostD,makaHostBdanCakanmelakukandeteksijalur,danapabilajalursedangdipakaimakaHostBdanCakanmenungguterlebihdahulu.
MetodeCSMA/CD

sebuahhostkomputeryangakanmengirimdatakejaringanpertama-tamamemastikanbahwajaringansedangtidakdipakaiuntuktransferdaridanolehhostkomputerlainnya(Listen).

Jikapadatahappengecekanditemukantransmisidatalaindanterjaditabrakan(collision),makahostkomputertersebutdiharuskanmengulangpermohonan(request)pengirimanpadaselangwaktuberikutnyayangdilakukansecaraacak(random).

Dengandemikianmakajaringanefektifbisadigunakansecarabergantian
Error Checking PengirimanData

Data-Link dapatmelakukandeteksierror danmemberikanperingatan(notification) kepadalapisandiatasnya,bahwaterjadikesalahantransmisi.

Teknikyang digunakanerror ddetectionadalahFrame Check Sequence (FCS) danCyclic Redundancy Check (CRC).

Data Link tidakmelakukanerror-correction
Generic Data Link Frame Format
Preamble or Start Field

When computers are connected to a physical medium, there must be a way they can grab the attention of other computers to broadcast the message, “Here comes a frame!”

Various technologies have different ways of doing this process, but all frames, regardless of technology, have a beginning signaling sequence of bytes.

Depending up frame format: Preamble = 7 bytes, Start or Start of Frame Delimiter (SFD) = 1 byte
Generic Data Link Frame Format
Address Field

We saw how IEEE 802.3 uses Destination and Source Addresses.

By the way: Any idea how a serial data link frame is addressed?

Unicastaddress –Single device

Broadcastaddress –All devices

Multicastaddress –Specific group of devices
Generic Data Link Frame Format
Type Field

Usually information indicating the layer 3 protocols in the data field, I.e. IP Packet.

Type field values of particular note for IEEE 802.3 frames include:

0×0600 XNS (Xerox)

0×0800 IP (the Internet protocol)

0×8137 Novell NetWare packet formatted for Ethernet II

0×6003 DECNET
“Ethernet” Frame Formats
802.2
802.3
802.3
Length Field

In some frame formats such as 802.3, there is a length field which specifies the exact length of a frame.

IEEE 802.3 specification limits the data portion to a maximum of 1500 bytes.

Designed to hold a Layer 3 IP packet.

When IEEE created 802.2, it saw the need for a protocol TYPE field that identified what was inside the “data” field.

IEEE called its 1 byte type field DSAP (Destination Service Access Point).

Turned out that 1 byte was not long enough to handle all the different number of protocols.
1.
To accommodate more protocols IEEE added the SNAP (SubnetworkAccess Protocol) header.

The fields of various Ethernet framing that are used for identifying the type of data contained in a frame:

Ethernet II or DIX (DEC, Intel, Xerox) –Most common

IEEE Ethernet (802.3)

IEEE 802.3 with SNAP header
Generic Data Link Frame Format
Data Field

Included along with this data, you must also send a few other bytes.

They are called padding bytes, and are sometimes added so that the frames have a minimum length for timing purposes.

LLC bytes are also included with the data field in the IEEE standard frames. (later)
Application Header + data
Data Encapsulation Example
010010100100100100111010010001101000…
Application Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2: Network Layer
Layer 1: Physical Layer
Generic Data Link Frame Format
FCS

Used to insure that the data has arrived without corruption.

More efficient than sending the data twice and comparing the results.

Necessary to prevent errors.
Three Kinds of FCS

Cyclic redundancy check (CRC)

performs polynomial calculations on the data

Two-dimensional parity

adds an 8thbit that makes an 8-bit sequence have an odd or even number of binary 1s

Internet checksum

adds the numbers to determine a number
Generic Data Link Frame Format
Stop Field (Other data link frame formats)

The computer that transmits data must get the attention of other devices, in order to start a frame, and then claim it again, to end the frame.

The length field implies the end, and the frame is considered ended after the FCS.

Sometimes there is a formal byte sequence referred to as an end-frame delimiter.

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