The 5-4-3 Rule is very important to remember and there will be a question in the exam related to this rule!
Simple: 5 segments, 4 Repeaters / hubs and only 3 Hubs can be populated
This rule can be used to create collision domains
The hub broadcasts on a network
Subnet Mask defines the broadcast domains
We need to be able to define certain instances of a device in to the following criteria:
1. The level on the OSI Model
2. Define it
3. The physical architecture (draw it)
So a hub would be:
1. On the Physical Layer
2. It Broadcasts network media
3. Physical Star, logical bus (csma/cd)
The difference between a Passive and an Active hub:
• Passive has no power (electricity and it broadcasts)
• Active has power so can effectively repeat / amplify
The Hub functions on the physical layer, and is a networking device used to connect nodes in a physical star topology network into a logical bus topology. There are active and passive hubs as mentioned above!
A passive hub simply receives the data transmitted from a device into one port and then broadcasts it out to the devices connected to all the other ports on the hub. The Active hub does the same but boosts the signal much like a repeater. Hubs are cheap and easy to manage but have a high degree of contention as the broadcast on all the ports creating a contention domain so do not provide the best performance.
Managed Hub
This is a hub which includes functions enabling you to monitor and configure its operation. You connect to the hub using specific software or via a dedicated management port. Can also be called and intelligent hub.
Switching Hub
(this could be called a switch) The switching hub reads the destination address of the packet and directs it to the correct port. Switching hubs are slower as they have to process the information and router to the correct port. These hubs can also support load balancing allowing them to address ports dynamically.
Hub Speeds
Hubs transmit at speed or either 10Mbps or 100 Mbps – they are typically auto sensing which means that should you have various devices with different transmission speeds the hub will sense this and operate on the slowest speed.
Switches
A switch is a networking device used to connect the drops in a physical star topology network into a logical bus topology. They work with pairs of ports connecting segments together creating contention domains which are isolated. The ultimate purpose of a switch is to transport data from A to B in the in the fastest possible manner.
Bridges
A bridge is a network device that divides a logical bus topology into segments. The bridge uses the MAC address and not TCP/IP thus it has no internet and it bridges domains.
Routers
A router is a networking device which connects multiple networks that use the same protocol such as TCP/IP – Routers can work only with routable protocols.
The physical topology of a router is the Mesh Topology and Logical bus topology with CSMA/CD
Routable protocol is TCP/IP and non routable is like MAC
VPN – virtual private network – this tunnels thru the internet to get to the other sites
Gateway – this is a device which translates between different protocols.
Wireless Access Points
This is a device which connects wireless devices to the device which can connect to wired networks. Like my Huawei router at home! Wired and wireless.
Gateways
A gateway is a device, software or a system that converts data between incompatible systems.
The OSI Model
The OSI model is the Open System Iinterconnection
This is a system developed by the International Organisation for standardization (ISO) for: communication in open system networks – NB NB!!
The model has 7 layer or steps which are:
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK (type of network i.e. 802.3 or 5 or 11)
DATA-LINK (how it sends)
PHYSICAL (getting it on the line or wireless)
Layer 7 The Application Layer
The application layer provides services and utilities that enable the programmes to access the network and its resources. This layer also defines the protocols for transferring files, sending emails, and saving data to the networat this layer are:
• HTTP – hyper text transfer protocol
• DNS – domain name service
• FTP – file transfer protocol
• SMTP – simple mail transfer protocol
• Telnet
Layer 6 The presentation Layer
This layer is responsible for encoding data into a standard network compatible format.
The presentation layer also adds services such as data compression and encryption, examples of technologies at this layer are:
• MME
• SSL
• TLS
• GIF
• Jpeg
• TIFF
Layer 5 The Session Layer
This layer is responsible for establishing the connection between network devices and applications, maintaining the connection and then terminating or restarting it when needed.
This layer controls how, when and for how long a device can transmit or receive and specifies procedures for the connection, terminating or restarting of sessions.
It also specifies the procedure for synchronising data transfer between two devices with different data transmission rates.
Examples of these technologies:
• TCP
Layer 4 The Transport Layer
The transport layer accepts data from the upper layers and breaks it into smaller units known as segments or packets. These packets/segments are passed onto the lower layers and ensures that all the pieces arrive correctly at the other end.
Transport layer is also responsible for carrying out error correction and sending acknowledgements at the network level.
Gateways can operate at this layer and the higher levels of the OSI model, examples are as follows:
• TCP
• UDP
• IPsec (IP Security)
Layer 3 The Network Layer
The network layer address data packets, it specifies how the packets are router from a source to a destination through the network and ensures the delivery of those packets. Layer 3 is also where the protocol address is attached to the data packet.
The network layer make their decisions based on the protocol address and not the MAC address. Examples of the technologies that function at this level are:
• IP
• ARP
• ICMP
• DHCP
• RIP
• OSPF
• BGP
• IGMP
Layer 2 The Data-Link Layer
The data-link layer is responsible for transferring data packages between adjacent network nodes without error.
The data-link layer is responsible for grouping the data bits into frames and attaching the address of the receiving node to each frame, thus forming a data packet.
Bridges and switches are some of the devices and PPP and SLIP are protocols that operate at the Data-Link layer.
The Data-Link layer can be divided into two sub layers:
• The Logical Link control (LLC) – the LLC sub-layer is responsible for identifying layer protocols and for encapsulating those protocols so that multiple upper-layer protocols can share the same media. The LLC checks the CRC and either ACKS or NAKCS the data.
• The Media Access Control (MAC) – The MAC sub-layer defines how packets are placed on the media. It is contention based network, the MAC sub-layer is responsible for the careier sense; in a token passing network it is responsible for the token.
Layer 1 The Physical Layer
The Physical Layer provides the means of transporting the data bits over a physical medium.
Examples of technologies in the Physical ALyer:
• Ethernet
• Fast Ethernet
• ATM
• Token Ring
• FDDI
The OSI Data Communication Process: (data transmission through the OSI reference involves the following stages)
1. When data is transmitted, it is first added to the application layer of the OSI reference model
2. Data is then forwarded down to the next layer and so on until it is placed on the network media by the Physical Layer
3. When Data is received, it is first added to the physical layer of the OSI Reference model
4. Data is forwarded upward to the next layer and so on until it reaches the Application Layer.
Insert image
Ethernet Frames
• Preamble – A pattern of 1’s and 0’s used to signal the start of the frame, 7 bytes in size
• Start of frame Delimiter (SFD) – The start of the frame delimiter is 1 byte and identifies the beginning of the data field
• Destination Address – The address to which the frame is being sent; I t can be in unicast, multicast or broadcast address. The destination address is 6 bytes in size
• Source Address – The address of the node sending. The frame is always in unicast address. The source address is 6 bytes in size
• Frame Type – The frame type tells which the upper-layer protocol should receive the data after Ethernet processes it. This frame type is 2 bytes in size
• Data – the payload of the frame (the information being sent). This must be at least 46 bytes in size or it won't be sent, this can also not be bigger than 1500 bytes.! NB NB!!
• CRC – A 4 byte word generated by the sending node, enabling the receiving node to the quality of the data received.
More tomorrow!
No comments:
Post a Comment