The physical layer, hubs, and ethernet

The main functions of the physical layer is to send bits and receive bits, as well as state transitions.

• Bits come in only values of 1 or 0 (morse code with numerical values).
• State transitions – changes in voltage from high to low and vice-versa.

Hubs at the physical layer

What is a hub?

• A hub is really a multiple port repeater.

Hubs in a network

• All devices in the same collision domain.
• All devices in the same broadcast domain.
• Devices shares some bandwidth.

What is a Physical Star Network?

• A physical star network is where the hub is a central device and cables are extended in all directions from it.

Note: Hubs and repeaters can be used to enlarge the area covered by a single LAN segment, however, this is NOT RECOMMENDED.

Ethernet at the physical layer

• Created by a group called DIX (Digital, Intel, Xerox)
• There are three types of ethernet
  • 802.3u – Fast Ethernet
  • 802.3ab – Gigabit Ethernet on Category 5
  • 802.3ae – 10Gbps over fiber and coax

Electronics Industries Association and The Newer Telecommunications Industry Alliance (EIA/TIA)

• The standard body that creates the physical layer/specifications for ethernet.
• EIA/TIA specifies that ethernet use a registered jack (RJ) connector with a 4 5 wiring sequence on unshielded twisted pair (UTP) cabling (RJ45).

Cables

• Cables are measured in decibals (dB)
• Cabling in corporate and home markets is measured in categories.
  • A higher quality cable will have a higher rated category and lower attenuation.
    • ex. category 5 > category 3

Key term:

• Crosstalk – the unwanted signal interference from adjacent pairs in the cable.

A closer look at the Data-Link layer

The Data-Link layer provides physical transmission of the data and handles error notifications, network topology, and flow control.

Lets take a closer look at it’s function

1. Ensures messages are delivered to the proper device on LAN using hardware addresses and will translate messages from the Network layer to transmit.
2. Transmit messages into pieces called a data frame and adds a customized header containing the hardware destination and source address.
3. Data-Link uses binary values and hexadecimal values.

Binary Values

Nibble Values	Byte Values
8  4  2  1	128  64  32  16  8  4  2  1

Binary to Decimal Chart

Binary Value	Decimal Value
10000000	128
11000000	192
11100000	224
11110000	240
11111000	248
11111100	252
11111110	254
11111111	255

Hexadecimals

Hex is short for hexadecimal, which is a numbering system that uses the first six letters of the alphabet (A through F) to extend beyond the available ten digits in the decimal system. Hexadecimal has a total of sixteen digits.

Hex to Binary to Decimal Chart

Hexadecimal	Binary	Decimal
0	0000	0
1	0001	1
2	0010	2
3	0011	3
4	0100	4
5	0101	5
6	0110	6
7	0111	7
8	1000	8
9	1001	9
A	1010	10
B	1011	11
C	1100	12
D	1101	13
E	1110	14
F	1111	15

Six major points of Routers

There are six major points to remember or keep in mind when working with routers or layer 3 devices. You should probably commit these to memory.

Here are the six major points

1. Routers, by default, will not forward any broadcast or multi-task packets.
2. Routers use the logical address in the network layer header to determine the next hop router to forward the packet to.
3. Routers can use access list, created by an administrator to control security on the types of packets that are allowed to enter or exit an interface.
4. Routers can provide layer 2 bridging functions if needed, and can simultaneously route through the same interface.
5. Routers provide connections between virtual LANS (VLANS).
6. Routers provide Quality of Service (QoS) for specific types of network traffic.

A closer look at the Network Layer

The network layer manages devices addressing, tracks location of devices on the network, and determines the best way to move data, which means the Network layer must transport traffic between devices that aren’t locally attached.

Lets take a closer look at it’s function

1. Checks a packet when it is first received on a router interface.
2. If the packet isn’t destined for that particular router, it will look up the destination network address in the routing table.
3. Once the router chooses an exit interface, the packet will be sent to that interface to be framed and sent out on the local network.
4. If the router is unable to locate an interface; the packets will be dropped.

Key Terms

Router – a router is a network layer device (layer 3 device) and provides the routing services within an internetwork.

Data packets – are used at the network layer to transport data through the internetwork.

Routed Protocols – are used to support data traffic.

Route Update Packets – are used to update neighboring routers about the networks connected to all routers within the internetwork.

Routing Protocols – are used to send out route updates packets.

Network Address

1. A protocol specific network address.
2. A router must maintain a routing table for individual routing protocols because each routing protocols keep track of a network with a different addressing scheme (ex. IP, IPv6, and IPX).

Interface – is the exit interface a packet will take when destined for a specific network.

Metric – is the distance to the remote network.

Visual of Windowing

 

Windowing Diagram

Window size of 1 = sending machine waits for an acknowledgement for each data segment it transmit before transmitting another segment.

Window size of 3 = allowed to transmit three data segments before an acknowledgement is actually received.