Channel Allocation Methods in Computer Networks

 Channel Allocation kya hota hai? (What is Channel Allocation?)

• Jab multiple users ya devices ek hi common transmission medium (channel) ko share karte hain, toh yeh decide karna padta hai ki kaunsa user kab aur kitna channel bandwidth use karega. Isi process ko Channel Allocation kehte hain.

• Real-life example: Sochiye ek single-lane road hai aur uspe bohot saari gaadiyo ko nikalna hai. Agar koi rule ya traffic light nahi hogi, toh accident (collision) ho jayega. Traffic signals ya separate lanes banakar road ko manage karna hi channel allocation jaisa hai.

• Computer networks mein channel allocation ke do primary methods hote hain: Static Channel Allocation aur Dynamic Channel Allocation.

 1. Static Channel Allocation

Static channel allocation mein total channel capacity (bandwidth ya time) ko pehle se hi fixed portions mein divide karke har user ko permanently allocate kar diya jaata hai.

[ Total Channel Bandwidth ]
|--- User 1 (Fixed) ---|--- User 2 (Fixed) ---|--- User 3 (Fixed) ---|--- User N (Fixed) ---|

Static allocation ke do main techniques hote hain:

• FDM (Frequency Division Multiplexing): Total frequency spectrum ko multiple alag-alag frequency bands mein divide kiya jaata hai. Har user ko ek dedicated frequency band milti hai (jaise FM radio stations).

• TDM (Time Division Multiplexing): Pure channel bandwidth ko time slots mein divide kiya jaata hai. Har user ko round-robin fashion mein ek fixed periodic time slot milta hai.

Limitations of Static Allocation in Computer Networks

Static allocation traditional telephone systems ke liye accha tha, lekin computer data communication ke liye bohot inefficient hai kyu ki:

-> Bursty Traffic Problem: Computer users continuous data nahi bhejte, balki bursty traffic generate karte hain (e.g., ek web page load kiya, fir 2 minute padhte rahe). Jab user idle baitha hai, uska allocated slot/band waste hota hai.
-> Bandwidth Wastage: Agar 10 users mein se sirf 1 user active hai, tab bhi wo channel ka sirf 1/10th hissa hi use kar sakta hai. Baaki 9/10th capacity bekar padi rehti hai.
-> Scalability Issue: Agar users ki sankhya available slots se zyada ho jaye, toh naye users connect hi nahi ho sakte.

 2. Dynamic Channel Allocation

Dynamic channel allocation mein channel capacity pehle se permanently divide nahi hoti. Channel slots ya bandwidth on-demand (dynamically) tabhi assign hoti hain jab kisi user ke paas actual data bhejne ke liye hota hai.

Dynamic channel allocation ke paas 5 basic assumptions hote hain:

1. Station Model: Network mein N independent stations hain jo data frames generate karte hain.

2. Single Shared Channel: Sabhi stations communication ke liye ek hi shared channel use karte hain.

3. Collision Assumption: Agar do stations ek saath frame bhejte hain, toh frames aapas mein takra kar corrupt ho jaate hain (Collision). Corrupted frames ko discard karke dubara bhejna padta hai.

4. Time Model: Time Continuous ho sakta hai (station kabhi bhi transmit kar sakta hai) ya Slotted ho sakta hai (transmission sirf time slot ke shuruat mein hi start ho sakti hai).

5. Carrier Sensing: Stations transmission se pehle channel ko sun (sense) sakte hain ki channel busy hai ya idle (Carrier Sense vs No Carrier Sense).

 Classification of Dynamic Channel Allocation Protocols

Dynamic access protocols ko teen broad categories mein baanta jaata hai:

                       Dynamic Channel Allocation
                                   |
         +-------------------------+-------------------------+
         |                         |                         |
   Random Access           Controlled Access         Channelization Protocols
 (Contention-based)       (Collision-free)           (Spread Spectrum / Orthogonal)
         |                         |                         |
  - Pure ALOHA              - Reservation             - FDMA
  - Slotted ALOHA           - Polling                 - TDMA
  - CSMA                    - Token Passing           - CDMA
  - CSMA/CD, CSMA/CA

A. Random Access Protocols (Contention-based)

In protocols mein koi central coordinator nahi hota. Sabhi stations equal hote hain aur channel ke liye compete (contend) karte hain. Agar do stations ek saath transmit karein toh collision hota hai, jise handle karne ke liye backoff mechanisms use hote hain (e.g., ALOHA, CSMA/CD).

B. Controlled Access Protocols

In methods mein stations ek doosre ke saath coordinate karte hain taaki collision kabhi na ho:

-> Reservation: Transmission se pehle stations ek small reservation frame bhej kar time slot book karte hain.
-> Polling: Ek Primary device sabhi Secondary devices se baari-baari puchta hai ki kya unke paas data hai.
-> Token Passing: Ek special bit pattern (Token) network mein ghoomta rehta hai; sirf wahi station transmit kar sakta hai jiske paas token ho.

C. Channelization Protocols

Frequency, time, ya special codes (CDMA) ke through shared bandwidth ko simultaneously share kiya jaata hai without interference.

 Static vs Dynamic Channel Allocation Comparison

 Summary Table