A collision in networking occurs when two or more devices on the same network segment attempt to transmit data simultaneously over a shared communication medium. This concept is fundamental to understanding how traditional Ethernet networks operate, particularly in half-duplex environments.
In ear…A collision in networking occurs when two or more devices on the same network segment attempt to transmit data simultaneously over a shared communication medium. This concept is fundamental to understanding how traditional Ethernet networks operate, particularly in half-duplex environments.
In early Ethernet implementations using hubs and coaxial cables, all devices shared the same bandwidth and collision domain. When multiple devices transmitted at the same time, their electrical signals would interfere with each other, corrupting the data and making it unusable. This event is called a collision.
To handle collisions, Ethernet networks use a protocol called CSMA/CD (Carrier Sense Multiple Access with Collision Detection). Here is how it works: Before transmitting, a device listens to the network to check if it is clear. If the medium is free, the device sends its data. If a collision is detected during transmission, the device stops sending, broadcasts a jam signal to notify other devices, and then waits a random period before attempting to retransmit.
A collision domain is defined as the network segment where collisions can occur. Devices connected to a hub share one collision domain, meaning all connected devices compete for the same bandwidth. As more devices are added, collision probability increases, reducing network efficiency.
Modern networks largely eliminate collisions through the use of switches and full-duplex communication. Switches create separate collision domains for each port, allowing devices to send and receive data simultaneously on dedicated pathways. Full-duplex mode enables simultaneous two-way communication, effectively eliminating the conditions that cause collisions.
Understanding collisions remains important for network professionals because legacy equipment still exists, and the concepts help explain why switches are preferred over hubs. Additionally, collision-related issues can still occur in misconfigured networks or when duplex mismatches exist between connected devices.
Collisions in Network Fundamentals - CCNA Study Guide
What Are Collisions?
A collision occurs when two or more devices on a shared network medium attempt to transmit data at the same time. When these signals overlap, the data becomes corrupted and unusable, requiring retransmission. Collisions are a fundamental concept in networking, particularly in legacy Ethernet environments that use a shared communication channel.
Why Are Collisions Important?
Understanding collisions is crucial for several reasons:
• Network Performance: Excessive collisions degrade network throughput and increase latency • Network Design: Knowledge of collisions helps you design efficient network topologies • Troubleshooting: Identifying collision-related issues is essential for network maintenance • Technology Evolution: Understanding why we moved from hubs to switches requires collision knowledge
How Collisions Work
In a shared medium environment (like a hub-based network), all devices share the same bandwidth. The mechanism that handles collisions is called CSMA/CD (Carrier Sense Multiple Access with Collision Detection):
1. Carrier Sense: A device listens to the network to check if it is idle 2. Multiple Access: Multiple devices have equal access to the medium 3. Collision Detection: If two devices transmit simultaneously, a collision is detected 4. Jam Signal: A jam signal is sent to notify all devices of the collision 5. Backoff Algorithm: Devices wait a random period before attempting retransmission
Collision Domains
A collision domain is a network segment where collisions can occur. Key points include:
• Hubs create one large collision domain - all ports share the same domain • Switches break up collision domains - each port is its own collision domain • Routers also separate collision domains at each interface • Bridges reduce collision domains by segmenting networks
Half-Duplex vs Full-Duplex
• Half-Duplex: Devices can send OR receive, but not both at once. Collisions are possible. • Full-Duplex: Devices can send AND receive at the same time. Collisions cannot occur because separate channels are used for each direction.
Modern switched networks operating in full-duplex mode have effectively eliminated collisions.
Exam Tips: Answering Questions on Collisions
1. Remember device behavior: Hubs forward traffic to all ports and create one collision domain. Switches create separate collision domains per port.
2. Count collision domains: When asked to count collision domains in a topology, remember: - Each switch port = 1 collision domain - Each router interface = 1 collision domain - All devices connected to a hub = 1 shared collision domain
3. Know CSMA/CD steps: Be prepared to identify the correct sequence of carrier sense, transmission, collision detection, jam signal, and backoff.
4. Understand the solution: Switches and full-duplex connections are the solutions to collision problems.
5. Watch for keywords: Questions mentioning hubs, shared media, or half-duplex are likely testing collision concepts.
6. Late collisions: These occur after the first 64 bytes and typically indicate cable length issues or duplex mismatches.
7. Differentiate from broadcast domains: Collision domains and broadcast domains are different concepts. Switches break up collision domains but not broadcast domains. Routers break up both.
Common Exam Scenarios
• Calculating the number of collision domains in a network diagram • Identifying which device reduces collisions • Explaining why switches are preferred over hubs • Troubleshooting performance issues related to excessive collisions • Understanding duplex mismatch symptoms