---

Connecting LANs: Network Interconnection Devices

---

1. Connecting Devices and the OSI Model

When Local Area Networks (LANs) grow or need to interconnect, specific hardware devices are employed. These devices operate at different layers of the OSI model:

---

2. Detailed Breakdown of Interconnecting Devices

A. Repeater (Layer 1 - Physical)

• Function: Signals traveling along a cable attenuate (weaken) over distance. A repeater receives an attenuated electrical/optical signal, regenerates (cleans and restores) its original amplitude and timing, and retransmits it.
• Key Characteristics:
  • Operates purely at the bit level without inspecting frames or headers.
  • Does NOT filter traffic: Forwarding is unconditional.
  • Does NOT separate collision domains or broadcast domains.
  • Extends physical network cable length limits.

B. Hub (Layer 1 - Multi-port Repeater)

• Function: Acts as a central connection point in a physical star topology.
• Operation: Any signal received on one incoming port is regenerated and broadcast to all other connected ports.
• Limitation:
  • All connected stations share one single collision domain and one broadcast domain.
  • Half-duplex operation; collisions occur if two stations transmit simultaneously.

C. Bridge (Layer 2 - Data Link)

• Function: Divides a large LAN into two or more smaller segments, filtering traffic based on hardware MAC addresses.
• Operation:
  • Maintains a Forwarding Table (MAC Table) learned dynamically via backward learning (inspecting the source MAC of incoming frames).
  • If destination MAC is on the same segment, the frame is filtered (dropped).
  • If destination MAC is on another segment, the frame is forwarded to that port only.
  • If destination MAC is unknown or broadcast (FF:FF:FF:FF:FF:FF), it floods to all other ports.
• Advantage: Separates collision domains, reducing contention and improving throughput.

D. Layer 2 Switch (Multi-port Bridge)

• Function: Modern high-speed evolution of a transparent bridge.
• Operation:
  • Employs dedicated hardware ASICs (Application-Specific Integrated Circuits) for wire-speed frame switching.
  • Microsegmentation: Every switch port is its own dedicated collision domain.
  • Supports Full-Duplex transmission (stations can transmit and receive at full wire speed simultaneously with zero collisions).
  • Uses forwarding techniques:
    1. Store-and-Forward: Receives entire frame, checks CRC checksum for errors; discards corrupt frames before forwarding.
    2. Cut-Through: Forwards frame immediately after reading the 6-byte destination MAC address; lowest latency, but propagates corrupt frames.
    3. Fragment-Free: Reads the first 64 bytes (detecting collisions) before forwarding.

E. Router (Layer 3 - Network)

• Function: Interconnects completely different logical networks (e.g., LAN to WAN, Subnet A to Subnet B).
• Operation:
  • Forwards packets based on Logical IP Addresses rather than MAC addresses.
  • Runs routing protocols (OSPF, RIP, BGP) to calculate optimal packet paths across complex internetworks.
  • Separates Broadcast Domains: Routers do not forward Layer 2 broadcasts (FF:FF:FF:FF:FF:FF) across interfaces by default, preventing broadcast storms.

F. Gateway (Application / Multi-layer)

• Function: Connects two completely incompatible network architectures by translating protocols across all 7 layers (e.g., connecting an IBM SNA mainframe network to a TCP/IP network).

---

3. Collision vs. Broadcast Domain Summary

Device	OSI Layer	Forwards Based On	Divides Collision Domain?	Divides Broadcast Domain?
Repeater / Hub	Layer 1	Electrical signal / bits	No (1 shared domain)	No (1 shared domain)
Bridge / Switch	Layer 2	48-bit MAC Address	Yes (1 per port)	No (1 shared domain)
Router	Layer 3	32/128-bit IP Address	Yes (1 per port)	Yes (1 per interface)
Gateway	Layers 4–7	Application protocols	Yes	Yes

---