TCP/IP Protocol Suite: Layer 1 - Physical Layer

Layer 1 - Physical Layer

• The physical layer is the bottom layer of the TCP/IP protocol suite, and it deals with sending information on the network

• At layer 1, we need a device that is not quite as smart as a switch, and options include a hub or a repeater

  A hub and a repeater serve different purposes in a network:

  1. Hub:

  • Function: A hub is a multi-port device that connects multiple computers or network devices in a local area network (LAN).

  • Operation: It operates at the physical layer (Layer 1) of the OSI model and broadcasts incoming data packets to all ports, regardless of the destination.

  • Intelligence: Hubs are not intelligent devices; they do not filter or route the data.

  • Collision Domain: All devices connected to a hub share the same collision domain, which can lead to data collisions and network inefficiency.

  2. Repeater:

  • Function: A repeater is a device that regenerates and amplifies signals to extend the distance over which data can travel in a network.

  • Operation: It also operates at the physical layer (Layer 1) of the OSI model but has only two ports (one incoming and one outgoing). It strengthens the signal without altering the data content.

  • Intelligence: Repeaters are simpler than hubs; their sole purpose is to enable longer transmissions by counteracting signal degradation over distance.

  • Collision Domain: Repeaters do not impact the collision domain; they simply regenerate the signal within the existing network segment.

  In summary, while both devices operate at the physical layer, a hub connects multiple devices within a network and broadcasts data to all ports, affecting network performance. A repeater, on the other hand, is used to extend the range of the network by regenerating signals without contributing to network congestion.

Hubs

• A layer 1 hub is a multi-port repeater that takes signals that come in on one port and forwards them out all other ports
• Hubs have no concept of addressing, framing, or packets; they only forward signals
• In a hub environment, all devices connected to the hub share the same bandwidth and are on the same network
• A hub acts as a logical bus, but physically it's a star topology with the hub in the center
• With a hub, only one device can talk at a time, and if two devices try to send signals at the same time, there's a collision
• A hub is a single collision domain, and devices connected to it use half-duplex communication (send or receive, but not both at the same time)
• Devices connected to a hub use CSMA/CD (carrier sensing multiple access with collision detection) to avoid collisions

CSMA/CD

• Carrier sensing: devices check to see if the network is clear before sending data
• Multiple access: multiple devices can be connected to the hub and share the collision domain
• Collision detection: devices can detect when a collision occurs and respond accordingly

Broadcast Domain

• A broadcast is like an all-points bulletin that is sent to every device on the network
• In a layer 1 environment, a broadcast is sent to every device on the network segment
• A broadcast domain represents the limits of how many devices will hear the broadcast

Other Layer 1 Devices

• Repeaters: take signals that come in on one side and send them out the other
• Media converters: convert electrical signals to light signals or vice versa

Limitations of Hubs

• Hubs are not very smart and only forward every bit they hear
• They don't care about layer 2 or layer 3 addressing or anything higher in the protocol stack
• In modern networks, hubs are not commonly used and are being replaced by layer 2 switches