Kendali Data-Link

Fungsi Lapisan Data Link

• Frame Sync. – membuat abstraksi dari channel "frame-at-a-time" untuk layer yang lebih tinggi
• Addressing - bila banyak node berbagi link transmisi
• Flow Control – kendali laju transmisi untuk mencegah over-run
• Error Control – membetulkan kesalahan transmisi (dengan pengiriman ulang)
• Sequence Control -
• Link Management - inisiasi, perawatan, & penghentian koneksi

1 - Flow Control

Teknik untuk mengontrol laju data sehingga pengirim tidak over-run penerima

1. Stop-and-wait
2. Sliding-window

Stop-and-wait

1. Pengirim mengirim frame
2. Penerima menerima frame & mengakuinya
3. Pengirim menunggu untuk menerima "ack" sebelum mengirim frame berikutnya (Jika penerima belum siap untuk menerima frame lain dengan menahan ack)

Utilisasi (Efisiensi) dari Stop & Wait

1. Waktu propagasi: waktu yang dibutuhkan untuk sinyal untuk melakukan perjalanan dari S ke R. Jadi bit pertama yang ditransmisikan pada t = 0 tiba pada R pada t = Tp = d / V.
2. Waktu transmisi: waktu yang dibutuhkan untuk memancarkan semua bit
3. frame pada pengirim = Tt = L / B.

2 - Deteksi Error

3 – Kendali Error

4 - High-Level Data Link Control (HDLC)

A synchronous data link protocol
Widely used, and basis for many other data link control protocols
Connections can be multipoint or point-to-point
Can be used in half-duplex or full-duplex
Three types of stations

• Primary station - (Mainframe) Controls the operation of the link, issues commands, and receive responses
• Secondary station - (Terminal) Usually only communicates (response) to a primary station
• Combined station - Can be both a primary and a secondary

Two link configurations

• Unbalanced configuration - One primary and one or more secondary stations
• Balanced configurations - Two combined stations

Three data transfer modes

• Normal response mode (NRM) - Unbalanced config. Primary station always dictates who sends and receives
• Asynchronous balanced mode (ABM) - Two combined stations
• Asynchronous response mode - Unbalanced config. Secondary station can send at any given time, but only one secondary can be active at a time

¢Two common techniques
¢• Parity checks
¢• Cyclic redundancy checks (CRC)
¢– Parity Check
¢• One extra “parity” bit is added to each word
¢– Odd parity: bit added so as to make # of 1’s odd
¢– Even parity: makes total # of 1’s even
¢• Single parity is very effective with white noise, but not very robust with noise bursts (which may extend over whole word duration.)
¢• Additional block check characters (longitudinal check characters) are used: parity bit for first bits of all chars, 2nd bits of all chars and so on
¢• This decreases residual error rates by 2 to 4 orders of magnitude.