McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Lapisan Data Link BAGIAN III

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Position lapisan data-link

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Tugas-tugas Lapisan Data Link

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Sub lapisan LLC dan MAC

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Standar-standar IEEE untuk LAN

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bab-Bab Bab 10 Error Detection and Correction Bab 11 Data Link Control and Protocols Bab 12 Point-To-Point Access Bab 13Multiple Access Bab 14Local Area Networks Bab 15 Wireless LANs Bab 16 Connecting LANs Bab 17 Cellular Telephone and Satellite Networks Bab 18 Virtual Circuit Switching

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bab 10 Deteksi galat dan koreksi (Error Detection and Correction)

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Data dapat terkorupsi selama transmisi, reliable communication, error harus dapat dideteksi dan dikoreksi. Catatan: Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., Tipe-tipe Error Error bit-tunggal (Single-Bit Error) Error deburan (Burst Error)

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Dalam error bit tunggal (single-bit error), berarti hanya ada satu bit yang berubah dalam data unit. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Gambar 10.1 Error bit-tunggal

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Error deburan (burst error) jika terdapat lebih dari 2 bit yang berubah pada data unit. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Gambar 10.2 Burst error of length 5

McGraw-Hill©The McGraw-Hill Companies, Inc., Deteksi Error Redundancy Parity Check Cyclic Redundancy Check (CRC) Checksum

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Deteksi error menggunakan konsep menambahkan bit-bit (redundancy), berarti menambahkan bit ekstra untuk mendeteksi error pada tujuan (penerima). Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Gambar 10.3 Redundancy

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Gambar 10.4 Metode deteksi

McGraw-Hill©The McGraw-Hill Companies, Inc., Konsep paritas genap (even-parity)

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Pada parity check, suatu bit paritas ditambahkan pada setian satuan (unit) data sehingga jumlah bitnya ditambah 1, dapat diterapkan dua jenis paritas yaitu paritas ganjil (even- parity atau odd parity). Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Catatan 1 Suppose the sender wants to send the word world. In ASCII the five characters are coded as The following shows the actual bits sent

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 2 Now suppose the word world in Contoh 1 is received by the receiver without being corrupted in transmission The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 3 Now suppose the word world in Contoh 1 is corrupted during transmission The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Gambar 10.6 Sistem paritas dua-dimensi

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 4 Suppose the following block is sent: However, it is hit by a burst noise of length 8, and some bits are corrupted When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., CRC generator and checker

McGraw-Hill©The McGraw-Hill Companies, Inc., Binary division in a CRC generator

McGraw-Hill©The McGraw-Hill Companies, Inc., Pembagian biner pada metode CRC

McGraw-Hill©The McGraw-Hill Companies, Inc., Polinomial

McGraw-Hill©The McGraw-Hill Companies, Inc., Polinomial sebagai bilangan pembagi

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Tabel 10.1 Polinomial Standar NamePolynomialApplication CRC-8x 8 + x 2 + x + 1ATM header CRC-10x 10 + x 9 + x 5 + x 4 + x 2 + 1ATM AAL ITU-16x 16 + x 12 + x 5 + 1HDLC ITU-32 x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1 LANs

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 5 It is obvious that we cannot choose x (binary 10) or x 2 + x (binary 110) as the polynomial because both are divisible by x. However, we can choose x + 1 (binary 11) because it is not divisible by x, but is divisible by x + 1. We can also choose x (binary 101) because it is divisible by x + 1 (binary division).

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 6 The CRC-12 x 12 + x 11 + x 3 + x + 1 which has a degree of 12, will detect all burst errors affecting an odd number of bits, will detect all burst errors with a length less than or equal to 12, and will detect, percent of the time, burst errors with a length of 12 or more.

McGraw-Hill©The McGraw-Hill Companies, Inc., Checksum

McGraw-Hill©The McGraw-Hill Companies, Inc., Data unit and checksum

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 The sender follows these steps: The unit is divided into k sections, each of n bits.The unit is divided into k sections, each of n bits. All sections are added using one’s complement to get the sum.All sections are added using one’s complement to get the sum. The sum is complemented and becomes the checksum.The sum is complemented and becomes the checksum. The checksum is sent with the data.The checksum is sent with the data. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 The receiver follows these steps: The unit is divided into k sections, each of n bits.The unit is divided into k sections, each of n bits. All sections are added using one’s complement to get the sum.All sections are added using one’s complement to get the sum. The sum is complemented.The sum is complemented. If the result is zero, the data are accepted: otherwise, rejected.If the result is zero, the data are accepted: otherwise, rejected. Catatan:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 7 Anggap pada block berikut yang terdiri atas 16 bit, dikirimkan menggunakan suatu checksum 8 bit Kedua bilangan dijumlahkan menggunakan penjumlahan one’s complement Sum Checksum Pola bit yang dikirimkan

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 8 Sekarang anggap suatu penerima menerima pola bit yang dikirim oleh pengirim seperti Contoh 7 dan anggap tanpa error pada transmisi Jika ketiga bilangan tersebut dijumlahkan,maka akan diperoleh bilangan yang bernilai 1 semuanya, jika dilakukan operasi komplemen (complementing), menghasilkan bilangan yang bit- bitnya bernilai 0 semua, hal ini menunjukan tidak terjadi error Sum Complement berarti polanya benar.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Contoh 9 Anggap pada transmisi terjadi suatu error deburan (burst error) yang panjangnya 5 bir dan ber pengaruh pada 4 bit Jika pada penerima ketiga bilangan tersebut dijumlahkan, akan diperoleh Partial Sum Carry 1 Sum Complement Terjadi korupsi pada pola bit yang diterima.

McGraw-Hill©The McGraw-Hill Companies, Inc., Koreksi Retransmission Forward Error Correction Burst Error Correction

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Table 10.2 Data and redundancy bits Number of data bits m Number of redundancy bits r Total bits m + r

McGraw-Hill©The McGraw-Hill Companies, Inc., Positions of redundancy bits in Hamming code

McGraw-Hill©The McGraw-Hill Companies, Inc., Redundancy bits calculation

McGraw-Hill©The McGraw-Hill Companies, Inc., Contoh kalkulasi bit redundancy

McGraw-Hill©The McGraw-Hill Companies, Inc., Error detection using Hamming code

McGraw-Hill©The McGraw-Hill Companies, Inc., Contoh koreksi error deburan (burst error)