Okay, work has been hectic this week so once again I am only getting to this on a Friday! Gotta do it now as I expect to be in Forries before 3!
Quick recap:
802.3 – Ethernet
802.5 – Ring
802.11(x) – Wireless
Key Acronyms:
• FHSS – Frequency Hopping Spread Spectrum
• DSSS - Direct Sequence Spread Spectrum
• Multiplexing – send and receive on same channel
Wireless:
• 802.11
o Has transmission speeds of 1 or 2 Mbps
o Radio frequency of 2.4GHz
o One variation uses FHSS and one uses DSSS
• 802.11.a
o Speeds up to 54Mbps
o 5 GHz band
o Transmission typically at 6, 12 or 24Mbps
o Uses orthogonal frequency division multiplexing (OFDM)
o Range roughly 20 metres
• 802.11b
o Speed – 11Mbps
o 2.4GHz
o Range 100 metres
• 802.11g
o Speed 54Mbps
o 2.5GHz
o Average speeds 20-25Mbps
o Range – 100 metres
• 802.11n
o Speeds up to 300Mbps
o Two streams so 600Mbps
o 2.4GHz or 5GHz frequency
Channels in 802.11b/g Implementation:
Overlap in the radio signals or bleeding can occur when these overlap between adjacent accesses points and clients. The best channels to use to avoid this are channels: 1, 6 and 11.
802.11 Modes:
You get infrastructure and ad hoc modes
Infrastructure mode is when the WAP is hardwired to the network (server) and the workstations connect wirelessly. Uses BSS (basic service set) or ESS (Extended service set) as the protocol
Ad hoc is when the workstations connect to the WAP wirelessly and there is no hardwired backbone. This uses the protocol IBSS (independent basic service set)
WLAN Architecture: see page 190, not important.
Protocols:
• BSS (Basic Service Set)
o One access point connected to a wired network and has one or more wireless stations or clients. A BSS is also referred to as Infrastructure mode as it requires an access point in order to transmit data
• ESS (Extended Service Set)
o The ESS comprises two or more BSS’s that are connected to a common distribution system such as a wired network. An ESS must contain at least two wireless access points operating in infrastructure mode, which allows clients to use their AP depending on the locations of the wireless station or point.
• ESSID (Extended Service Set ID)
o The ESSID is the name that identifies a wireless network or wireless access point so that you can identify the networks clearly.
• IBSS (Independent Basic Service Set
o IBSS does not contain an AP. It is a peer to peer network in which each wireless station or client has its own wireless network adapter and each acts as both client and wireless AP.
802.11 BEACONS
Beacon frames are management frames that are only 50 bytes long. They are used to start and maintain wireless communication. They contain information about the communication process such as the STA and SSID, channel number and security protocol.
Wire Equivalent Privacy (WEP)
• To prevent eavesdropping, IEEE 802.11 WEP defines and algorithm that gives authorised users the same level of security they would on a wired network.
WIFI Protected Access (WPA)
• WPA is a Wi-Fi standard that was designed to improve upon the security flaws of WEP. WPA2 is an upgrade and includes all the 802.11 mandatory standards.
• It improves data encryption through the Temporal Key Integrity Protocol (TKIP).
• WPA adds AES a cipher based CCMP encryption for even greater security.
• User authentication, considered poor over WEP through the EAP (Extensible Authentication Protocol). WEP regulates access to the wireless network based on the computers specific MAC address which is relatively easy to sniff or spoof.
• EAP is built on a more secure public key encryption system to ensure that only the authorised network users can access the network.
Wireless Authentication Methods:
Authentication Method Description
Open System This uses null authentication
Shared Key This authentication method verifies the identity of a station by using a WEP key. Both the station and the AP must be configured to use the data encryption and the same WEP key
802.1x and EAP This authentication method authenticates the user and not the station. A RADIUS server is used – Remote Authentication Dial In Server – the user dials in to the server which disconnects the call and will return the call and reconnect.
Bluetooth
• This is not a network service, it is peer-to-peer
• Bluetooth 1.1 is a wireless protocol used to communicate form device to device in a small area up to 10 metres, transfer is approximately 1Mbps.
• Bluetooth 1.2 increases speed to 2.1-3Mbps and the range increases to about 30 metres.
Networking with TCP/IP
Families and Protocols:
• Successful network communications hinges on them sharing a network protocol to perform various functions related to that communication.
• Network protocols are classified into general families that share common features.
Network Protocols:
Network communications between computers is provided by a network protocol – rules by which network operations are conducted.
There are the following Protocols:
• Network and transport layer protocols
• Application – Presentation- and session layer Protocols
Protocol Bindings:
Assigning a protocol to a network interface card (NIC) is referred to as protocol binding. Multiple protocols can be bound to a single NIC. The NIC can use any of the protocols that are bound to it to communicate with other nodes on the network.
Binding order:
A network interface bound with multiple protocols attempts to connect to a receiving node by testing its available protocols, one by one until it finds a protocol that the receiving node answers.
The TCP/IP Protocol:
This is a protocol suite which works at the network and transport layers of the OSI model. Providing the protocol family functions of reliability, connection and routing.
TCP/IP
The transmission Control/Internet protocol (TCP/IP) is a non-proprietary, routable network protocol suit that enables computers to communicate over all types of networks. TCP/IP is the native protocol of the Internet and is required for Internet connectivity.
IPv4 Addresses:
An IPv4 Address is a 32 bit binary number assigned to a computer on a TCP/IP Network. The 32 bit binary IP Address is usually separated by dots into four 8 bit octets.
• Binary = 11010000.01111011.00101101.00010010
• Decimal = 208.123.45.18
Dotted decimal Notation – this is how TCP/IP address is usually displayed!
An IP address is like a mailing address:
Some of the numbers in the IP address identify the network segment on which computer resides, just as a person mailing address uses a street name etc.
Binary and Decimal Conversion:
Binary is a base 2 numbering system in which any bit in the number is either 1 or 0
Standard Speed Frequency Approximate Range
802.11 1-2Mbps 2.4GHz 100 metres
802.11a 54Mbps 5GHZ 20 metres
802.11b 11Mbps 2.4GHz 100 metres
802.11g 54Mbps 2.4GHz 100 metres
802.11n 300Mbps 2.4 or 5MHz 70 metres
Well that is it for now - see you all tomorrow.
Friday, February 19, 2010
Friday, February 12, 2010
Lesson 9
Quick Lesson 8 Re-cap – HOT TIP
Ethernet Frames can have a total size of 1526 bytes – the data at a maximum or 1500 bytes
The minimum can be 72 bytes where the data is at the minimum of 46 bytes
Lesson 9
IEEE Standards
• A family of Networking standards developed by the IEEEE in 1980 to address the rapid changes in networking technology. The 802.x standards are divided into several sub-categories to address the different requirements.
We work with 3 main standards:
1. 802.3 – Ethernet (CSMA/CD)
2. 802.5 – Ring
3. 802.11 - Wireless
802.3 Standard
• IN addition to the media type (cable) 802.3 also defines the transmission speed and signal method.
10 Base standard
• This describes the type of media used and the speeds at which each type of media operates. The cable specification contains three components
• A number indicating the speed
• The signal type in baseband or broadband
• The code for either copper or fiber
The codes and physical media are as follows:
T – UTP or STP (cat3,4,5,5e)
F – Fiber optic
FL – Fiber optic link
FX – Fiber optic using 2 pairs of multimode fiber
10Mbps table below:
10Base-2
Standard Specification Medium Distance
10Base-2 IEEE 802.3a Thinnet Coax 185 metres
10Base-5 IEEE 802.3 ThickNet 500 metres
Fast Ethernet
Standard Specification Medium Distance
100Base-T IEEE 802.3u Cat 5 UTP 100 metres
Gigabit Ethernet (1000 Mbps)
Standard Specification Medium Distance
1000Base-T IEEE 802.3ab Cat 5e or Cat 6 UTP 100 metres
Token Ring Networks
Token-based Media Access
• Media access method in which computers pass a special sequence of bits called a token between them. Only the node holding the token can transmit on the network.
Token Ring Standards – these use a MAU NOT a Hub
• There are two token ring standards that are very similar. IBM Token Ring and IEEE 802.5
Token Ring Characteristics IBM Token Ring IEEE 802.5
Transmission Speed 4/16 Mbps 4/16Mbps
Media Type Star Not Specified
Signalling Baseband Baseband
Access Method Token Passing CSMA/CA Token Passing
Token States
Token State Description
Available There is no data in the payload and the token may be captured for use
Captured There is a valid data payload
Acknowledged Notification of either ACK or NACK positive or negative
Reserved A priority system, in which the token can be reserved
Token ring Access Priority
• Token ring networks use a priority system that permits certain user designated high priority stations to use the network more frequently. They have a priority and reservation field.
MAU (Multi-station Access Unit) – sometimes called a MSAU
• MAU’s can be wired together to form a ring using patch cables and lobe cables for connections.
• A MAU has the same function as a switch.
Token Ring Failure Recovery
• Token ring networks take on a star configuration when a device is turned off or disconnected from the network. IN this situation the MAU simply bypasses the disconnected device, when the device is online but not responsive the MAU will keep the connection live but breaks the ring because the device is not passing data.
Fiber Distributed Data Interface (FDDI) – 802.5 (ring)
Ring 802.5 has an IN and OUT port on the NIC, for FDDI has 2X IN and 2X OUT
This is not very important but nice to know.
It is a networking ring technology that uses double single mode or multimode fiber that operate at 100Mbps.
• Dual Ring
o FDDI ring does not have a device such as a MAU to remove stations when they are turned off. Second fiber transceivers cannot be configured to reverse, there is receive hardware and transmit hardware.
• FDDI Connection Devices
o Nodes are connected to the FDDI in one of two ways.
In dual attached stations (DAS) nodes are connected directly to both the primary and secondary rings
IN single attached stations (SAS) node are connected to a concentrator which is connected to both rings. – Here the concentrator gives the SAS access to the primary ring.
FDDI Failure recovery
• DAS nodes provide fault tolerance measure that detects loss if connectivity and then loop back the signal.
Ethernet Frames can have a total size of 1526 bytes – the data at a maximum or 1500 bytes
The minimum can be 72 bytes where the data is at the minimum of 46 bytes
Lesson 9
IEEE Standards
• A family of Networking standards developed by the IEEEE in 1980 to address the rapid changes in networking technology. The 802.x standards are divided into several sub-categories to address the different requirements.
We work with 3 main standards:
1. 802.3 – Ethernet (CSMA/CD)
2. 802.5 – Ring
3. 802.11 - Wireless
802.3 Standard
• IN addition to the media type (cable) 802.3 also defines the transmission speed and signal method.
10 Base standard
• This describes the type of media used and the speeds at which each type of media operates. The cable specification contains three components
• A number indicating the speed
• The signal type in baseband or broadband
• The code for either copper or fiber
The codes and physical media are as follows:
T – UTP or STP (cat3,4,5,5e)
F – Fiber optic
FL – Fiber optic link
FX – Fiber optic using 2 pairs of multimode fiber
10Mbps table below:
10Base-2
Standard Specification Medium Distance
10Base-2 IEEE 802.3a Thinnet Coax 185 metres
10Base-5 IEEE 802.3 ThickNet 500 metres
Fast Ethernet
Standard Specification Medium Distance
100Base-T IEEE 802.3u Cat 5 UTP 100 metres
Gigabit Ethernet (1000 Mbps)
Standard Specification Medium Distance
1000Base-T IEEE 802.3ab Cat 5e or Cat 6 UTP 100 metres
Token Ring Networks
Token-based Media Access
• Media access method in which computers pass a special sequence of bits called a token between them. Only the node holding the token can transmit on the network.
Token Ring Standards – these use a MAU NOT a Hub
• There are two token ring standards that are very similar. IBM Token Ring and IEEE 802.5
Token Ring Characteristics IBM Token Ring IEEE 802.5
Transmission Speed 4/16 Mbps 4/16Mbps
Media Type Star Not Specified
Signalling Baseband Baseband
Access Method Token Passing CSMA/CA Token Passing
Token States
Token State Description
Available There is no data in the payload and the token may be captured for use
Captured There is a valid data payload
Acknowledged Notification of either ACK or NACK positive or negative
Reserved A priority system, in which the token can be reserved
Token ring Access Priority
• Token ring networks use a priority system that permits certain user designated high priority stations to use the network more frequently. They have a priority and reservation field.
MAU (Multi-station Access Unit) – sometimes called a MSAU
• MAU’s can be wired together to form a ring using patch cables and lobe cables for connections.
• A MAU has the same function as a switch.
Token Ring Failure Recovery
• Token ring networks take on a star configuration when a device is turned off or disconnected from the network. IN this situation the MAU simply bypasses the disconnected device, when the device is online but not responsive the MAU will keep the connection live but breaks the ring because the device is not passing data.
Fiber Distributed Data Interface (FDDI) – 802.5 (ring)
Ring 802.5 has an IN and OUT port on the NIC, for FDDI has 2X IN and 2X OUT
This is not very important but nice to know.
It is a networking ring technology that uses double single mode or multimode fiber that operate at 100Mbps.
• Dual Ring
o FDDI ring does not have a device such as a MAU to remove stations when they are turned off. Second fiber transceivers cannot be configured to reverse, there is receive hardware and transmit hardware.
• FDDI Connection Devices
o Nodes are connected to the FDDI in one of two ways.
In dual attached stations (DAS) nodes are connected directly to both the primary and secondary rings
IN single attached stations (SAS) node are connected to a concentrator which is connected to both rings. – Here the concentrator gives the SAS access to the primary ring.
FDDI Failure recovery
• DAS nodes provide fault tolerance measure that detects loss if connectivity and then loop back the signal.
Friday, February 5, 2010
Lesson 8
So, it has been a busy week - have only had a chance to do this now. But here it is. Thanks for all the comments and feedback guys! NOT....
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!
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!
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