Part I: Networking Fundamentals

Chapter 1. Introduction to Computer Networking

Chapter 2. The TCP/IP and OSI Networking Models

TCP/IP Networking Model

A networking model (networking architecture or networking blueprint), refers to a comprehensive set of documents that define everything that should happen for a computer network to work.

The TCP/IP model both defines and references a large collection of protocols that allow computers to communicate. TCP/IP uses documents called Requests for Comments (RFC).

Data Encapsulation Terminology

Figure 2-11 Five Steps of Data Encapsulation: TCP/IP

  1. Create and encapsulate the application data with any required application layer headers.
  2. Encapsulate the data supplied by the application layer inside a transport layer header.
  3. Encapsulate the data supplied by the transport layer inside an Internet layer (IP) header.
  4. Encapsulate the data supplied by the Internet layer inside a data link layer header and trailer. This is the only layer that uses both a header and a trailer.
  5. Transmit the bits.

OSI Networking Model

Figure 2-13 OSI Model Compared to the Two TCP/IP Models

Describing Protocols by Referencing the OSI Layers

Networking documents often describe TCP/IP protocols and standards by referencing OSI layers, both by layer number and layer name. For instance, a common description of a LAN switch is “layer 2 switch,” with “layer 2” referring to OSI layer 2.

Layer Name Protocols and Specifications Devices
Application, presentation, session (Layers 5–7) Telnet, HTTP, FTP, SMTP, POP3, VoIP, SNMP Firewall, intrusion detection systems, hosts
Transport (Layer 4) TCP, UDP Hosts, firewalls
Network (Layer 3) IP Router
Data link (Layer 2) Ethernet (IEEE 802.3), HDLC, Frame Relay, PPP LAN switch, wireless access point, cable modem, DSL modem
Physical (Layer 1) RJ-45, EIA/TIA-232, V.35, Ethernet (IEEE 802.3) LAN hub, LAN repeater, cables

OSI Layering Concepts and Benefits


OSI Encapsulation Terminology

Figure 2-14 OSI Encapsulation and Protocol Data Units

The TCP/IP model uses terms such as segment, packet, and frame to refer to various layers and their respective encapsulated data. OSI uses a more generic term: protocol data unit (PDU).

Chapter 3. Fundamentals of LANs

An Overview of Modern Ethernet LANs

Types of cabling:

Most IEEE standards define a different variation of Ethernet at the physical layer. For the data link layer:


Common Name Speed Alternative Name Name of IEEE Standard Cable Type, Maximum Length
Ethernet 10 Mbps 10BASE-T IEEE 802.3 Copper, 100 m
Fast Ethernet 100 Mbps 100BASE-TX IEEE 802.3u Copper, 100 m
Gigabit Ethernet 1000 Mbps 1000BASE-LX, 1000BASE-SX IEEE 802.3z Fiber, 550 m (SX) 5 km (LX)
Gigabit Ethernet 1000 Mbps 1000BASE-T IEEE 802.3ab 100 m

The term Ethernet is often used to mean "all types of Ethernet", but in some cases it is used to mean "10BASE-T Ethernet"

A Brief History of Ethernet


Repeaters extended the length of LANs by cleaning up the electrical signal and repeating it (a Layer 1 function) but without interpreting the meaning of the electrical signal. [p56]

Building 10BASE-T Networks with Hubs

Hubs are essentially repeaters with multiple physical ports. It simply regenerates the electrical signal that comes in one port and sends the same signal out every other port.

Ethernet UTP Cabling

Transmitting Data Using Twisted Pairs

UTP cabling consists of matched pairs of wires that are indeed twisted together, with current on the two wires in opposite directions.

UTP Cabling Pinouts for 10BASE-T and 100BASE-TX

10BASE-T and 100BASE-TX Ethernet define that one pair should be used to send data in one direction, with the other pair used to send data in the other direction.

The wires in the UTP cable must be connected to the correct pin positions in the RJ-45 connectors in order for communication to work correctly.


The following applies to 10BASE-T and 100BASE-TX only:

Devices That Transmit on 1,2 and Receive on 3,6 Devices That Transmit on 3,6 and Receive on 1,2
PC NICs Hubs
Routers Switches
Wireless Access Point (Ethernet interface)
Networked printers (printers that connect directly to the LAN)

1000BASE-T Cabling

1000BASE-T differs from 10BASE-T and 100BASE-TX as far as the cabling and pinouts:

Improving Performance by Using Switches Instead of Hubs

CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur:

  1. A device with a frame to send listens until the Ethernet is not busy.
  2. When the Ethernet is not busy, the sender(s) begin(s) sending the frame.
  3. The sender(s) listen(s) to make sure that no collision occurred.
  4. If a collision occurs, the devices that had been sending a frame each send a jamming signal to ensure that all stations recognize the collision.
  5. After the jamming is complete, each sender randomizes a timer and waits that long before trying to resend the collided frame. When each random timer expires, the process starts over with Step 1.

Increasing Available Bandwidth Using Switches

The term collision domain defines the set of devices whose frames could collide. For example, all devices connected to the hub are in the same collision domain. To avoid collisions, and to recover when they occur, devices in the same collision domain use CSMA/CD.

Switches significantly reduce, or even eliminate, the number of collisions on a LAN:

The switch’s logic requires that the switch look at the Ethernet header, which is considered a Layer 2 feature. As a result, switches are considered to operate as a Layer 2 device, whereas hubs are Layer 1 devices.

Buffering (temporarily holds the frame in memory) also helps prevent collisions.

Switch features provide significant performance improvements:

Shared Ethernet vs. Switched Ethernet

Doubling Performance by Using Full-Duplex Ethernet

In an Ethernet network using hubs, CSMA/CD imposes half-duplex logic on each device, meaning that only one device can send at a time. LAN switches with only one device cabled to each port of the switch allow the use of full-duplex operation; Ethernet card can send and receive concurrently.

Ethernet Addressing

LAN Addressing Term or Feature Description
MAC Media Access Control. 802.3 (Ethernet) defines the MAC sublayer of IEEE Ethernet.
Ethernet address, NIC address, LAN address Other names often used instead of MAC address. These terms describe the 6-byte address of the LAN interface card.
Burned-in address The 6-byte address assigned by the vendor making the card.
Unicast address A term for a MAC that represents a single LAN interface.
Broadcast address An address that means “all devices that reside on this LAN right now.” (FFFF.FFFF.FFFF)
Multicast address On Ethernet, a multicast address implies some subset of all devices currently on the Ethernet LAN. (0100.5exx.xxxx, any value can be used in the last half)

Ethernet Framing

Framing defines the meaning of the bits transmitted and received over a network.

Figure 3-14 LAN Header Formats

Identifying the Data Inside an Ethernet Frame

Type/Length filed:

Error Detection

Errors (bit changes) occur due to electrical interference. Trailer containing a Frame Check Sequence (FCS) field used for error detection.

Chapter 4. Fundamentals of WANs

The WAN topics in this chapter describe mainly how enterprise networks use WANs to connect remote sites.

OSI Layer 1 for Point-to-Point WANs

Routers provide the capability to connect many different types of OSI Layer 1 and Layer 2 technologies. A router is connected to each end of a WAN connection.

Figure 4-3 Point-to-Point Leased Line: Components and Terminology

WAN Cabling Standards


Clock Rates, Synchronization, DCE, and DTE


Name(s) of Line Bit Rate
DS0 64 kbps
DS1 (T1) 1.544 Mbps (24 DS0s, plus 8 kbps overhead)
DS3 (T3) 44.736 Mbps (28 DS1s, plus management overhead)
E1 2.048 Mbps (32 DS0s)
E3 34.368 Mbps (16 E1s, plus management overhead)
J1 (Y1) 2.048 Mbps (32 DS0s; Japanese standard)

OSI Layer 2 for Point-to-Point WANs


Figure 4-6 HDLC Framing

High-Level Data Link Control (HDLC) defines framing to:

Point-to-Point Protocol

Point-to-Point Protocol (PPP) behaves much like HDLC. The framing looks identical to the Cisco proprietary HDLC framing. PPP has become the most popular and feature-rich of WAN data link layer protocols. [p91]

Point-to-Point WAN Summary

WAN Terminology [p91-92]

All the following terms may be used to refer to a point-to-point leased line:

Frame Relay and Packet-Switching Services

Packet-switching service: a company can connect a large number of routers using a single serial link from each router into the packet-switching service. Once connected, each router can send packets to all the other routers—much like all the devices connected to an Ethernet hub or switch can send data directly to each other.

Two types of packet-switching service are very popular today:

Chapter 5. Fundamentals of IPv4 Addressing and Routing

Path selection sometimes is used to mean:

Overview of Network Layer Functions

Today, the only Layer 3 protocol that is used widely is the TCP/IP network layer protocol, specifically, IP. IP does not require any overhead agreements or messages before sending a packet, making IP a connectionless protocol, with no error recovery.

Routing (Forwarding)

Routing focuses on the end-to-end logic of forwarding data.

The routing table for any particular network layer protocol contains a list of network layer address groupings. Instead of a single entry in the routing table per individual destination network layer address, there is one routing table entry per group.

The routing process forwards the packet, and only the packet, end-to-end through the network, discarding data-link headers and trailers along the way.

Routers build new data-link headers and trailers and because the new headers contain data-link addresses, the PCs and routers must decide what data-link addresses to use. The Address Resolution Protocol (ARP) is used to dynamically learn the data-link address of an IP host connected to a LAN.

Routing as covered so far has two main concepts: