Physical Layer

Physical layer deals with communication media. This layer receive frame from data link layer and convert them in bits. It load these bits on actual communication media.

Depending on media type these bit values are converted in single. Some use audio tones, while others utilize state transitions—changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to explain the proper bit patterns to be used, how data is encoded into media signals.

Fiber Cabling

Two types of fiber are used for connections: multimode and single-mode.

Multimode fiber

o   Multimode fiber has thickness of either 850 or 1300 nm (nanometres).

o   Light signals are provided by an LED.

o   Multimode fiber can achieve speeds in hundreds of MBPS.

o   Multiple signals can be transmitted per fiber.

Single-mode fiber

o   Single mode fiber has a thickness of 1300 or 1550 nm.

o   Light signals are provided by lasers.

o   Single mode fiber can achieve speed up 100Gbps.

o   Only one signal cab be transmitted per fiber.

Ø  Loss factor

Loss factor is used to describe any signal loss in the fiber before the light source gets to the end of the fiber.

Ø  Connector loss

Connector loss is a loss that occurs when a connector joins two pieces of fibres.

Ø  Attenuation

Attenuation is a signal loose due to distance.

Ø  Microbending

Microbending describe distortion in light source due to wrinkle in the fiber, typically where the cable is slightly bent.

Ø  Macrobending

Macrobending describe leakage of the light source from the fiber, usually from a bend in the fiber cable.

Ø  Transmission of signals across the fiber are defined by two standards SONET and SDH. Both of these standards define the physical layer framing used to transmit light sources, which also includes overhead for the transmission.

Data link layer

Main functions of data link layer are

•Defining the Media Access Control (MAC) or hardware addresses

•Defining the physical or hardware topology for connections

•Defining how the network layer protocol is encapsulated in the data link layer frame

•Providing both connection-less and connection-oriented services

•Defines hardware (MAC) addresses as well as the communication process that occurs within a media.

MAC Address

MAC address is a 48 bit long layer two address. It is also known as hardware address. This address is burnt with device by manufacturing company.

•The first six hexadecimal digits of a MAC address represent its manufacture company.

•MAC addresses only need to be unique in a broadcast domain.

•You can have the same MAC address in different broadcast domains.

Frame

Data link layer receive packet from network layer and wrap it with layer two header that is known as frame. There are two specifications of Ethernet frame.

1. Ethernet II

2.802

Key points to remember:-

o   Ethernet II does not have any sub layers, while IEEE 802.2/3 has two: LLC and MAC.

o   Ethernet II has a type field instead of a length field (used in 802.3).

o   802.2 use a SAP or SNAP field to differentiate between encapsulated layer-3 payloads.

o   With a SNAP frame, the SAP fields are set to 0xAA and the type field is used to indicate the layer-3 protocol.

o   802.2 SAP frame is eight bits in length and only the first six bits are used for identifying upper-layer protocols, which allows up to 64 protocols.

o   802.2 SNAP frame support of up to 65,536 protocols.

Network Layer

Network layer is responsible for providing logical address know as IP address. Router works on this layer. Main functions of this layer are following:-

o   Define IP address

o   Find routes based on IP address to reach its destination

o   Connect different data link type together like as Token Ring, Serial, FDDI, Ethernet etc.

IP packet

Network layer receive segment from transport layer and wrap it with IP header that is known as packet.

Datagram

Datagram is just another name of packet. Network layer use datagram to transfer information between machines.

Two types of packets are used at the Network layer: data and route updates.

Data packets

Data packets are used to transport the user data across the network. Protocols used by data packets are known as routed protocol. For example IP and IPv6

Route update packets

These packets are used to update routers information within internetwork. Protocols that send route update packets are called routing protocols; for example RIP, RIPv2, EIGRP, and OSPF.

IP addresses are broken into two components:

o   Network component: - Defines network segment of device.

o   Host component :- Defines the specific device on a particular network segment

IP Classes

Class A addresses range from 1-126.

Class B addresses range from 128-191.

Class C addresses range from 192-223.

Class D addresses range from 224-239.

Class E addresses range from 240-254.

o   0 [Zero] is reserved and represents all IP addresses;

o   127 is a reserved address and is used for testing, like a loop back on an interface:

o   255 is a reserved address and is used for broadcasting purposes.

Class A, B, C address are known as public address that can be used to access devices in other public networks, such as the Internet.

Private IP and ISP

Private IP address can be used to configure private network. You can use private IP to build your network without paying a single penny. But one biggest problem with private IP is that with private IP you cannot access the internet. This is the point where ISP comes from. ISP purchase a bulk of public IP address and provide them on rent. Whatever you pay to ISP for accessing internet is actually the charge of using public IP address.

Private IP address: - Not routable in public network

Class A: 10.0.0.0 - 10.255.255.255

Class B: 172.16.0.0 - 172.31.255.255

Class C: 192.168.0.0 - 192.168.255.255

Network layer protocols that you should know for exam

Protocol           Description

IP        IP of TCP/IP, featuring routable 32-bit addressing.

IPX     The equivalent of IP in Novell Netware.

ICMP  Internet Connection Management Protocol. Incorporates Ping and Traceroute, which are layer 3 link-testing utilities.

OSPF, IGRP, EIGRP, RIP, ISIS.   Dynamic routing protocols that learn about remote networks and update them other neighbour routers running the same protocol.

ARP, RARP   Address Resolution Protocol (and Reverse ARP). ARP learns what MAC address is associated with a given IP address. Reverse ARP learns an IP address given a MAC address.