Error Control in Data Link Layer

Error Control kya hota hai?

Error Control ek technique hai jiska use hota hai ye ensure karne ke liye ki sender se bheja gaya data receiver tak sahi aur accurate pahunch raha hai ya nahi.

Socho aapne WhatsApp pe ek message bheja, lekin network problem ke wajah se kuch shabdon ka meaning badal gaya.
Error Control aise hi problems ko detect karta hai aur ya to message dobara bhejne ko kehta hai ya khud hi correct kar leta hai.

Errors aate kyu hain?

Network mein data electric signals ke form mein travel karta hai. Transmission ke dauraan kuch problems ho sakti hain jaise:

Noise (electric disturbance)
Signal weak hona ya distort ho jaana
Loose cable ya faulty hardware
Lightning ya electromagnetic waves

In sab wajahon se bits flip ho jaati hain (0 → 1 ya 1 → 0)

Error Control ka Objective

Data mein error detect karna
Galti hui to correct karna
Data kharab ho to dobara bhejne ko kehna
Overall communication ko reliable banana

Types of Errors

Type	Example
Single-bit error	1001 → 1101 (sirf 1 bit flip)
Burst error	10010011 → 11011111 (multiple bits flip)

Error Control Techniques (2 Main Parts)

1. Error Detection

(Errors ko pakadne ke liye)

a) Parity Check

Data ke saath ek extra bit (parity bit) add karte hain
Wo check karta hai ki 1s ka count even hai ya odd

Example (Even Parity):
Data: 1010101 → 4 ones → even → Parity bit = 0
Final data: 10101010
Agar receiver par 1s ka count odd aaya → error hua

Simple
Sab errors detect nahi kar paata

b) Checksum

Data ke total ka ek summary value (checksum) banta hai
Receiver bhi checksum calculate karta hai
Agar same aaya → data sahi hai

Example:
Data chunks ka addition → Invert karke checksum
Receiver fir se add karke match karta hai

Better than parity
Burst errors pakadna mushkil hota hai

c) CRC (Cyclic Redundancy Check)

Polynomial division se ek unique CRC code generate hota hai
Receiver bhi same calculation karta hai

Example:
Data: 110100
Divisor: 1011
Jo remainder bachega, wahi CRC code hoga

Powerful & reliable
Bas error detect karta hai, correct nahi

2. Error Correction

(Errors ko sahi karne ke liye)

a) Hamming Code

Extra bits (redundant bits) data mein add karte hain
In bits se receiver khud pata laga sakta hai ki kaunsa bit galat hai
Us bit ko flip karke correct kar leta hai

No need to resend
Extra space lagta hai

b) ARQ (Automatic Repeat Request)

Sender data bhejta hai
Receiver agar error detect kare to NACK (Not ACK) bhejta hai
Sender dobara data bhejta hai

Types of ARQ:

Type	Explanation
Stop-and-Wait ARQ	1 frame bhejo, wait for ACK. Error aaya to resend
Go-Back-N ARQ	Multiple frames bhej do, agar ek frame me error aaya to uske baad ke sab dobara bhejo
Selective Repeat	Sirf wahi frame dobara bhejo jisme error tha

ARQ Flow Diagram:

Quick Summary Table

Technique	Use	Detects Error	Corrects Error
Parity Bit	Simple check	Yes	No
Checksum	Group check	Yes	No
CRC	Reliable check	Yes	No
Hamming Code	Auto correction	Yes	Yes
ARQ	Retransmission	Yes	(by resending)

Real-Life Analogy

Courier Example:

Aapne parcel bheja
Receiver parcel check karta hai (error detection)
Agar damage hai to return karta hai (ARQ)
Aap dubara sahi parcel bhejte ho
Ya agar chhoti damage hai to receiver khud theek kar leta hai (Hamming Code)

Conclusion

Error control ka kaam hai galtiyon ko pakadna aur sahi karna
Detection techniques: Parity, Checksum, CRC
Correction techniques: Hamming Code, ARQ
Ye sab methods milke ensure karte hain ki data accurately aur reliably receiver tak pahuche

The Hingineer