Suppose ten computers are connected to one shared communication channel. All ten devices need the channel, but they do not transmit continuously.
The important question is: “Should we permanently divide the channel among all users, or should we give capacity only to the users that actually need it?”
This question creates the Channel Allocation Problem and leads to two important approaches: Static Channel Allocation and Dynamic Channel Allocation.
Channel allocation is the process of deciding how the available communication capacity of a shared network medium is distributed among multiple users or stations.
In Static Channel Allocation, each user receives a fixed portion of the communication resource in advance.
In Dynamic Channel Allocation, channel access is assigned according to current traffic demand instead of permanently reserving resources for every user.
What Is Channel Allocation in Computer Networks?
A communication channel has limited capacity.
If several devices share the same channel, the network needs a method for deciding how that capacity will be distributed.
This process is called channel allocation.
What Is the Channel Allocation Problem?
Consider five devices connected to one common communication medium.
All five users may need the communication channel, but their requirements can be different.
For example:
- User A may have no data to send.
- User B may send only a small message.
- User C may suddenly need to send a large file.
- User D may remain idle.
- User E may generate frequent traffic.
Therefore, the network must answer:
Why Is Channel Allocation Required?
Limited Capacity
A communication channel cannot provide unlimited bandwidth to every user.
Many Users
Multiple devices may need access to the same network medium.
Changing Traffic
Computer traffic changes rapidly rather than remaining constant.
Fairness
Active stations should receive a reasonable opportunity to transmit.
Efficiency
Available bandwidth should not remain unnecessarily unused.
Delay Control
Users should not wait unnecessarily long for network access.
Traffic Coordination
Simultaneous access must be coordinated where transmissions can interfere.
Scalability
The network should remain usable as the number of devices grows.
Objectives of Channel Allocation
A good channel-allocation method tries to balance several goals.
High Utilization
Use as much available communication capacity as possible.
High Throughput
Successfully transfer a large amount of useful data.
Low Delay
Reduce the time stations wait before successful transmission.
Fairness
Avoid giving one station permanent control of the channel.
Low Overhead
Do not spend too much capacity on control information.
Traffic Adaptation
Respond effectively when network demand changes.
Types of Channel Allocation
Channel resources are divided in advance.
Resources are assigned according to current traffic demand.
Static Channel Allocation
Static channel allocation divides the communication channel into fixed parts and permanently assigns those parts to users.
The allocation normally remains fixed even when a user has no data to send.
How Static Channel Allocation Works
Suppose a 100 Mbps communication channel is shared by four users.
A simple equal static allocation might give:
- User A = 25 Mbps
- User B = 25 Mbps
- User C = 25 Mbps
- User D = 25 Mbps
The important point is that User A cannot automatically use User B's reserved 25 Mbps simply because User B is idle.
Main Characteristics
- Resource allocation is predetermined.
- Every user receives a fixed share.
- Resource availability is predictable.
- Idle user capacity can remain unused.
- Best suited to relatively predictable or continuous traffic.
Static Channel Allocation Using FDM
FDM stands for Frequency Division Multiplexing.
The available frequency range is divided into smaller frequency bands.
Different users are assigned different bands.
Guard Bands
Small unused frequency ranges may be placed between adjacent channels to reduce interference.
Main Advantage
Different users can transmit at the same time because they use different frequency bands.
Main Limitation
If one user is idle, its allocated frequency band may remain unused.
Static Channel Allocation Using TDM
TDM stands for Time Division Multiplexing.
Instead of dividing frequency, TDM divides transmission time into slots.
Each user receives one or more fixed time slots.
If User B has nothing to send during its fixed slot, that slot may remain unused in basic fixed TDM.
Static Channel Allocation Formula
Suppose:
- Total channel rate = R
- Number of equally allocated users = N
Then each user's nominal fixed share is:
For example:
Solved Static Allocation Numerical
A communication link has a capacity of 120 Mbps and is statically divided equally among six stations.
Find the capacity assigned to each station.
Therefore, each station receives 20 Mbps.
Now Suppose Only Two Stations Are Active
In a strict equal static allocation, each active station still owns only its fixed 20 Mbps share.
The remaining four allocations can remain unused.
Why Static Allocation Can Waste Bandwidth
ACTIVE
UNUSED
UNUSED
UNUSED
Computer communication is often unpredictable.
A user may remain silent for many seconds and then suddenly send a large amount of data.
If the channel is permanently divided, inactive users can hold capacity that active users need.
Advantages and Limitations of Static Allocation
Advantages
- Simple allocation
- Predictable capacity
- Easy to understand
- Users have predetermined resources
- Useful when demand is stable
- No random competition for separately assigned capacity
Limitations
- Idle bandwidth can be wasted
- Poor adaptation to bursty traffic
- Heavy users cannot automatically borrow unused capacity
- Fixed allocation may become inefficient when demand changes
- May require guard bands or synchronization overhead
Dynamic Channel Allocation
Dynamic channel allocation does not permanently reserve a fixed portion of the channel for every station.
Instead, channel access is assigned according to current communication demand.
How Dynamic Channel Allocation Works
A Station Gets Data
One or more devices become ready to transmit.
Access Method Is Applied
The network follows an access rule such as contention, polling, reservation or scheduling.
A Station Gets an Opportunity
One or more stations are allowed to use communication resources.
Transmission Occurs
The selected station transmits according to the protocol.
Channel Becomes Available Again
Future users can receive access according to new traffic conditions.
Dynamic Channel Allocation Assumptions
To understand multiple-access algorithms, textbooks commonly use a simplified model of the shared channel.
Station Model
The network contains several independent stations that generate frames for transmission.
Single Channel
All stations share one common communication channel.
Collision Assumption
If incompatible frames overlap on the shared medium, they can collide and the transmission becomes unsuccessful.
Time Model
Time may be treated as continuous or divided into defined slots depending on the protocol.
Carrier-Sense Model
Some protocols allow a station to determine whether the channel appears busy before transmission, while other protocols do not.
Independent Decisions
Stations may make their own transmission decisions unless a centralized controller coordinates access.
Continuous Time
A station can begin transmission at many possible moments rather than waiting for a globally defined slot boundary.
Slotted Time
Time is divided into fixed slots, and transmissions are required to begin at permitted slot boundaries.
| Continuous Time | Slotted Time |
|---|---|
| Transmission start can occur at arbitrary times | Transmission starts at defined slot boundaries |
| No global slot synchronization required | Slot synchronization is required |
| Pure ALOHA follows this basic idea | Slotted ALOHA follows this idea |
Carrier Sense
A station listens to or senses the medium before transmitting.
CSMA protocols use this idea.
No Carrier Sense
A station does not first determine whether another transmission is already active.
Pure ALOHA is a classic example of a protocol without carrier sensing.
Types of Dynamic Channel Allocation
Stations compete for access.
Example concepts: ALOHA, CSMA.
Access is coordinated.
Example concepts: polling, token, reservation.
A controller or scheduler assigns communication opportunities.
Centralized Dynamic Channel Allocation
In centralized allocation, one controller or coordinating system makes channel-access decisions.
The controller may decide:
- Which station can transmit
- When it can transmit
- How much resource it receives
- Which waiting station gets priority
Advantages
- Easy to enforce priorities
- Can provide organized access
- Can reduce uncontrolled collisions
Limitations
- Controller introduces management overhead
- Central control can become a bottleneck
- Controller failure can affect allocation
Distributed Dynamic Channel Allocation
In distributed allocation, stations cooperate or compete without one permanent central controller making every decision.
Each station follows a common protocol.
Advantages
- No single central allocation point required
- Can scale naturally in shared-medium systems
- Flexible access
Limitations
- Stations may compete
- Collisions may occur in contention-based systems
- Fairness can be harder to guarantee
Contention-Based Dynamic Allocation
Contention means stations compete for access to the shared channel.
There may be no advance guarantee that a particular station will be the next successful transmitter.
Examples
- ALOHA
- Slotted ALOHA
- CSMA
- CSMA/CD
- CSMA/CA
Controlled Dynamic Allocation
Controlled-access methods coordinate transmission opportunities more explicitly.
Examples
- Reservation
- Polling
- Token Passing
These methods can reduce collisions, although the control mechanism itself creates overhead.
Channel Allocation Under Light and Heavy Traffic
Light Load
Only a few stations need to transmit.
Simple contention methods may provide fast access because competition is low.
Medium Load
Several stations are active.
Access coordination and backoff become increasingly important.
Heavy Load
Many stations continuously request access.
Uncontrolled contention can produce high delay and many collisions.
Why Computer Traffic Is Called Bursty
Computer users normally do not transmit data continuously at a constant rate.
For example, while reading a webpage your computer may temporarily send almost nothing.
When you click a download link, it may suddenly require a large amount of capacity.
Idle → Small Request → Idle → Large Transfer → Idle → Another Request
This irregular traffic pattern is called bursty traffic.
Dynamic allocation usually handles bursty traffic more efficiently because unused channel resources can be made available to other active users.
Static vs Dynamic Channel Allocation
| Feature | Static Allocation | Dynamic Allocation |
|---|---|---|
| Allocation | Fixed in advance | Changes with demand |
| Resource Sharing | Each user has predetermined share | Active users share available capacity dynamically |
| Idle User Capacity | Can remain unused | Can potentially be used by other active users |
| Complexity | Lower | Higher |
| Traffic Adaptation | Poor | Good |
| Bursty Traffic | Less efficient | Better suited |
| Predictability | High fixed allocation | Access can vary |
| Collision Possibility | Separate fixed resources can avoid contention | Possible in contention-based methods |
| Control Overhead | Usually low after allocation | Can require access-control overhead |
| Bandwidth Utilization | Can be poor when users are idle | Usually better for changing traffic |
| Examples | Fixed FDM, fixed TDM | ALOHA, CSMA, polling, reservation, token/scheduling concepts |
Static Allocation Does Not Mean FDM Only
That is incomplete.
The real idea is that resources are permanently or predictably allocated in advance.
FDM can provide fixed frequency shares, while fixed TDM can provide fixed recurring time slots.
Performance Factors in Channel Allocation
Channel Utilization
How much of the available capacity is actually used?
Throughput
How much useful information is successfully transmitted?
Access Delay
How long does a station wait before it gets permission or successfully transmits?
Fairness
Do all stations receive reasonable access opportunities?
Throughput and Channel Utilization
Throughput represents the rate of successful useful data delivery.
Channel utilization indicates how effectively available communication capacity is being used.
Fairness in Channel Allocation
A high-throughput allocation system can still be poor if one station receives most of the communication opportunities while other stations wait indefinitely.
Therefore a MAC allocation method should consider fairness.
Unfair Situation
Station A repeatedly gains access while Station B waits for a very long time.
Fair Situation
All active stations receive reasonable access opportunities according to the network policy.
Access Delay
Access delay is the time a station waits before it gets an opportunity to transmit successfully.
Access delay can increase because of:
- Many competing stations
- Collisions
- Backoff periods
- Polling cycles
- Waiting for a token
- Reservation periods
- Priority rules
Channel Allocation and Queueing
If frames arrive faster than they can obtain access to the shared channel, frames begin waiting in buffers.
This creates queueing delay.
As offered traffic approaches or exceeds available capacity:
- Queues can become longer
- Delay can increase rapidly
- Buffers can become full
- Frames may eventually be dropped
Static vs Dynamic Capacity Example
Consider a 100 Mbps channel shared by four users.
Static Case
Each user receives:
If only User A is active, its basic fixed allocation remains 25 Mbps.
Idealized Dynamic Idea
If User A is the only active user and the dynamic access mechanism permits it, User A may potentially use most of the available communication capacity.
Centralized vs Distributed Dynamic Allocation
| Feature | Centralized | Distributed |
|---|---|---|
| Decision Maker | Central controller or scheduler | Stations follow distributed protocol rules |
| Coordination | Direct | Distributed |
| Collision Control | Can explicitly schedule access | Depends on the protocol |
| Failure | Controller may be critical | No single allocation controller required |
| Examples | Polling/scheduling concepts | ALOHA and CSMA-type contention concepts |
Real-Life Examples of Channel Allocation
Frequency-Based Systems
Different users or services may be allocated different frequency ranges.
Time-Slot Systems
Users can be assigned repeating transmission time slots.
Shared Ethernet History
Traditional shared Ethernet dynamically handled competing stations using CSMA/CD.
Wi-Fi
Wireless devices dynamically compete for a shared radio channel using CSMA/CA-based access.
Modern Wireless Networks
Modern cellular systems dynamically schedule radio resources using mechanisms much more advanced than basic textbook allocation models.
Token-Based Systems
Transmission opportunity can be dynamically given to the station currently holding a control token.
Which Channel Allocation Method Should Be Used?
Static Allocation
Useful when communication demand is predictable and reserved capacity is valuable.
Dynamic Allocation
Better when active users and traffic demand change frequently.
Controlled Dynamic Access
Useful where predictable access or central coordination is important.
Questions a Network Designer Should Ask
- How many users share the channel?
- Is traffic continuous or bursty?
- How important is predictable capacity?
- Can collisions occur?
- How important is low delay?
- Is fairness required?
- Can a central controller be used?
- How much control overhead is acceptable?
Best Real-Life Analogy: Static vs Dynamic Allocation
Reserved Classroom Seats
Four seats are permanently reserved for four students.
If three students are absent, those seats still remain empty.
Open Library Seats
Seats are used by whichever students currently need them.
When one student leaves, another student can use the available seat.
Static Allocation vs Dynamic Allocation in One Line
Dynamic Allocation = Resource opportunity follows current communication demand.
Common Student Mistakes
The physical communication resource can still be shared, but each user receives a predetermined portion such as a frequency band or time slot.
Fixed TDM can also allocate recurring resources to users.
Some dynamic allocation methods use contention and may collide, while controlled or scheduled methods can coordinate access without random collisions.
The total physical channel capacity is still limited. Dynamic allocation only changes how that limited capacity is shared.
Static allocation can be useful when traffic is stable and predictable resource availability is required.
Dynamic systems can introduce contention, queueing, polling, scheduling and other control delays.
Channel allocation decides access to communication resources. Routing decides paths through networks.
Multiplexing is a technique for combining multiple signals on a communication medium. Channel allocation is the broader decision of how communication resources are assigned among users.
Dynamic allocation can be centralized or distributed.
Bursty means traffic appears in irregular bursts separated by quieter periods.
Easy Memory Tricks
Channel Allocation Quick Revision Notes
- Channel allocation distributes communication capacity among users.
- The main allocation approaches are static and dynamic.
- Static allocation divides resources in advance.
- Dynamic allocation responds to current traffic demand.
- Static allocation can use fixed frequency bands.
- Static allocation can also use fixed repeating time slots.
- FDM divides the available frequency spectrum.
- TDM divides transmission time into slots.
- If total capacity R is divided equally among N users, each gets R/N.
- Static allocation can waste capacity when assigned users are idle.
- Computer traffic is often bursty.
- Bursty traffic contains periods of low activity followed by sudden transmissions.
- Dynamic allocation generally handles bursty traffic better.
- Dynamic allocation can be centralized or distributed.
- Centralized allocation uses a controller or scheduler.
- Distributed allocation allows stations to follow common access rules.
- Contention methods allow stations to compete for access.
- Controlled methods coordinate transmission opportunities.
- Dynamic channel models often assume several independent stations.
- A shared-channel model commonly assumes one common channel.
- Simultaneous transmissions may collide in contention networks.
- Time can be continuous or slotted.
- Some protocols use carrier sensing.
- ALOHA does not perform carrier sensing.
- CSMA uses carrier sensing.
- Channel utilization measures effective use of capacity.
- Throughput measures successful useful data delivery.
- Fairness measures how reasonably access opportunities are distributed.
- Access delay is the waiting time before successful channel use.
- No single allocation method is ideal for every network.
Important Channel Allocation Exam Questions
Very Short Answer Questions
Short Answer Questions
Long Answer Questions
Channel Allocation MCQs With Answers
A. Assigning communication capacity among users
B. Assigning domain names
C. Creating files
D. Encrypting passwords
A. Allocated in advance
B. Allocated only after collisions
C. Never allocated
D. Assigned by DNS
A. Current traffic demand
B. Fixed permanent share only
C. File size only
D. IP version
A. Frequency
B. Time only
C. Passwords
D. Routing tables
A. Time slots
B. DNS names
C. Only frequency
D. IP addresses
A. Idle capacity can be wasted
B. It has infinite bandwidth
C. It has no channels
D. It cannot transmit data
A. Dynamic allocation
B. Fixed static allocation only
C. No allocation
D. Physical addressing
A. Traffic occurs in irregular bursts
B. Every user sends continuously
C. All frames are corrupted
D. No data is transmitted
A. A controller or scheduler
B. No decision mechanism
C. Only DNS
D. A printer
A. Stations follow distributed access rules
B. One controller always assigns every slot
C. No channel exists
D. Only routers can transmit
A. ALOHA
B. Fixed FDM
C. Fixed TDM only
D. Static frequency reservation only
A. CSMA
B. Pure ALOHA
C. Fixed FDM
D. Fixed TDM
A. Do not perform carrier sensing before every transmission
B. Always use fixed frequency channels
C. Always use polling
D. Never transmit
A. Transmission begins at defined slot boundaries
B. Time is ignored
C. Frequency is infinite
D. Every packet uses a different cable
A. Successfully delivered useful data rate
B. Number of passwords
C. Screen resolution
D. Number of cables
A. Effective use of available capacity
B. Number of monitors
C. Processor temperature
D. File names
A. Stations receive reasonable access opportunities
B. Only one station always transmits
C. No station is allowed to transmit
D. All users need identical IP addresses
A. 25 Mbps
B. 40 Mbps
C. 50 Mbps
D. 100 Mbps
A. Dynamic allocation can adapt to traffic demand
B. Static allocation always gives all capacity to one active user
C. FDM divides time
D. TDM divides only frequency
A. Computer traffic is often bursty
B. Computers always send continuously
C. Networks have unlimited capacity
D. All users always need equal bandwidth
Frequently Asked Questions About Channel Allocation
What is channel allocation in computer networks?
Channel allocation is the process of distributing available communication capacity among multiple stations or users sharing a network medium.
What are the two main types of channel allocation?
The two main approaches are static channel allocation and dynamic channel allocation.
What is static channel allocation?
Static channel allocation divides communication resources among users in advance, such as fixed frequency bands or recurring time slots.
What is dynamic channel allocation?
Dynamic channel allocation assigns access according to current communication demand instead of permanently reserving a fixed portion for every station.
Why is static channel allocation inefficient for computer traffic?
Computer traffic is often bursty. When a statically allocated user is idle, its reserved capacity can remain unused even if another user needs more bandwidth.
What is bursty traffic?
Bursty traffic contains periods of little or no transmission followed by sudden periods of higher data transmission.
What is the difference between FDM and TDM?
FDM divides a communication resource into different frequency bands, while TDM divides transmission time into recurring slots.
What are the assumptions of dynamic channel allocation?
Common textbook assumptions include multiple independent stations, one shared channel, possible collisions, continuous or slotted time and carrier-sense or no-carrier-sense operation.
What is centralized channel allocation?
Centralized allocation uses a controller or scheduler to decide which user or station receives communication resources.
What is distributed channel allocation?
Distributed allocation allows stations to make transmission decisions according to common protocol rules rather than relying on one central controller for every decision.
Which allocation is better for bursty computer traffic?
Dynamic allocation is generally better suited to bursty traffic because idle capacity can be made available to active stations. The exact performance still depends on the protocol and network conditions.
Is dynamic channel allocation always better than static allocation?
No. Static allocation can be useful when traffic is predictable and guaranteed resources are important, while dynamic allocation is usually more flexible for changing traffic demand.
Conclusion
Channel allocation answers a basic networking question: How should limited communication capacity be shared among multiple users?
There are two main approaches.
Static Channel Allocation divides resources in advance. Examples include fixed frequency allocation and fixed time-slot allocation.
Its major advantage is simplicity and predictability, but its biggest problem is wasted capacity when assigned users are idle.
Dynamic Channel Allocation gives communication opportunities according to current traffic demand.
This makes dynamic allocation especially useful for computer networks because computer traffic is normally bursty.
Dynamic allocation may be:
Centralized → a controller decides
Distributed → stations follow shared rules
Contention Based → users compete
Controlled → transmission opportunities are coordinated
The easiest way to remember the complete chapter is:
Static = Fixed Resource
Dynamic = Demand-Based Resource
And the biggest conceptual difference is:
Static asks “What was allocated to you?”
Dynamic asks “Who needs the channel now?”
Next Chapter: Chapter 12 – ALOHA Protocols in Computer Networks: Pure ALOHA, Slotted ALOHA, Vulnerable Time & Throughput Explained
