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OSI model Layers & Devices

September 19, 2020 , , , ,

OSI model Layers & Devices

OSI model Layers & Devices

Physical (Layer 1)

Purpose: Transmits bits over a physical channel—defines electrical/optical/radio signaling, connectors, cabling, and timing.

Typical devices:

  • Repeaters and Hubs: Regenerate/amplify signals without frame awareness. Evidence: “A repeater operates on the physical layer… regenerates the signal” [Cisco Press, “Network Fundamentals”].
  • Media Converters: Convert one physical medium to another (e.g., fiber-to-copper). Evidence: “Media converters… enable the connection of dissimilar media types such as twisted pair with fiber optic cabling” [FS.com Documentation].
  • Transceivers/PHYs (SFP, QSFP, GBIC): Interface NICs/switch ports to fiber/copper. Evidence: “SFP… a compact transceiver used for both telecommunication and data communications applications” [SFF Committee INF-8074i].

Physical media:

  • Copper: UTP/STP (Cat5e/6/6A/7/8), coaxial. Evidence: “Category 5e/6… standardized twisted pair cabling for Ethernet” [TIA-568].
  • Fiber: Single-mode, multimode, with 850/1310/1550 nm optics. Evidence: “Single-mode fiber… long-distance; multimode… short distance” [ITU-T G.652/G.651].
  • Wireless RF: Wi‑Fi, Bluetooth, microwave links. Evidence: “The physical layer… includes radio transmission” [ISO/IEC 7498-1].

Data Link (Layer 2)

Purpose: Framing, MAC addressing, error detection, and link control over a single link or LAN segment.

Typical devices:

  • Bridges and Switches: Forward frames based on MAC addresses. Evidence: “A bridge… operates at the data link layer” [IEEE 802.1D]; “Switches are multiport bridges” 
  • Wireless Access Points: Bridge 802.11 wireless to 802.3 wired. Evidence: “An AP connects wireless stations to a distribution system” [IEEE 802.11-2020].
  • NICs: Implement MAC and PHY, handle frame transmit/receive. Evidence: “Network interface controllers implement link-layer protocols” 
  • Layer 2 Tunneling/Access: Ethernet bridges, PPP, Frame Relay, ATM switches. Evidence: “Bridging is a link-layer function” [IEEE 802.1Q]; “PPP… data link layer protocol” [RFC 1661].

Physical media (as used by L2 technologies):

Ethernet over copper/fiber; Wi‑Fi over RF; PPP over serial; MPLS over various L2 carriers (note MPLS labels sit between L2/L3). Evidence: “Ethernet MAC is defined by IEEE 802.3” [IEEE 802.3]; “PPP… encapsulates network layer protocols over point-to-point links” [RFC 1661].

Key functions:

Framing and FCS, MAC addressing, VLAN tagging (802.1Q), link aggregation (802.1AX), loop avoidance (STP/RSTP/MSTP). Evidence: “Bridges use the Spanning Tree Protocol” [IEEE 802.1D]; “VLAN tagging defined in 802.1Q”.

Network (Layer 3)

Purpose: Logical addressing and routing across multiple links; provides internetworking.

Typical devices:

  • Routers: Forward packets based on IP or other network-layer addresses. Evidence: “Internet Protocol (IP) is the network layer protocol” [RFC 791]; routers “forward IP packets” [RFC 1812].
  • Layer 3 Switches: Hardware-accelerated routing within switches. Evidence: “Multilayer switching integrates L3 forwarding” [Cisco CVDs].
  • Firewalls (routing mode): Filter/route based on L3/L4 info. Evidence: “A firewall enforces an access control policy” [RFC 2979, applicability; vendor docs clarify L3/4 filtering].

Physical media:

  • Any underlying L2/L1 carrying IP: Ethernet, fiber, Wi‑Fi, serial, cellular. Evidence: “IP is independent of the underlying transmission facilities” [RFC 791].
  • Key functions:
  • Packet forwarding, TTL/hop-limit, fragmentation (IPv4), routing protocols (RIP/OSPF/IS-IS/EIGRP/BGP) running on routers. Evidence: “Routers decrement TTL” [RFC 791]; “OSPF is an interior gateway protocol” [RFC 2328].

Transport (Layer 4)

Purpose: End-to-end transport services like reliability, flow control, multiplexing.

Typical devices:

  • Hosts/Servers/Clients: Implement TCP/UDP/SCTP stacks. Evidence: “Transport layer… provides reliable, transparent data transfer” [RFC 1122].
  • Load Balancers (transport-aware): Distribute flows at L4. Evidence: “Load balancing at transport layer uses TCP/UDP ports” [RFC 793 context; vendor docs].

Physical media:

  • Not specific; rides atop L3/L2/L1. Evidence: Layering abstraction per [ISO/IEC 7498-1].
  • Key functions:
  • TCP segmentation/reassembly, ports, congestion control; UDP multiplexing. Evidence: “TCP… reliable, connection-oriented” [RFC 793]; “UDP… connectionless” [RFC 768].

Session (Layer 5)

Purpose: Establish, manage, and terminate application sessions; synchronization and dialog control.

Typical devices:

  • Primarily host-based functions; some middleboxes help (session border controllers in VoIP). Evidence: “The session layer establishes, manages and terminates connections” [ISO/IEC 7498-1]; “SBCs… control signaling and media” [RFC 5853].

Physical media:

  • Inherits from lower layers; no specific medium.

Key functions:

  • Session negotiation, checkpoints, dialog control (e.g., RTP/RTCP control aspects, NetBIOS sessions). Evidence: “Session services… dialog control” [ISO/IEC 7498-1].

Presentation (Layer 6)

Purpose: Data representation, translation, encryption, compression.

Typical devices:

  • Hosts implement codecs/encoders and TLS libraries; some appliances offload TLS/SSL. Evidence: “Presentation layer provides syntax and semantics… including encryption” [ISO/IEC 7498-1]; “SSL accelerators offload cryptographic operations” [F5/Netscaler docs].

Physical media:

  • None specific; operates on data streams above transport.

Key functions:

  • Format translation (ASCII/EBCDIC, XDR), compression, TLS/SSL encryption. Evidence: “XDR… external data representation” [RFC 4506]; “TLS provides privacy and data integrity” [RFC 5246/8446].

Application (Layer 7)

Purpose: Network services to end-users/applications (HTTP, DNS, SMTP, etc.).

Typical devices:

  • Application servers, proxies, Layer 7 firewalls/WAFs, DNS resolvers, email gateways. Evidence: “The application layer provides services for network applications” [ISO/IEC 7498-1]; “HTTP is an application-layer protocol” [RFC 9110].

Physical media:

  • Not tied to a medium; relies on lower layers to carry application data.

Key functions:

  • Protocol-specific logic, resource requests, name resolution, file transfer. Evidence: “DNS is an application-layer protocol” [RFC 1035]; “SMTP transfers mail” [RFC 5321].