CCNA Expl 1. Chapter 9. ETHERNET

До сих пор я вела конспекты своих занятий только в тетрадке. Но возможно кому-то пригодятся мои  "Chapter  Resume.".... Да и мне самой, со временем будет проще заглянуть в блог, нежели копаться в тетрадках. Открываю серию постов "CCNA Exploration. " Начнем с 9-ой главы. Если хватит сил, отцифрую предыдущие.

CCNA Exploration. 
Network Fundamentals.
Chapter 9. Resume. ETHERNET

Ethernet operates in the lower two layers of the OSI model:                                                                      9.1
1) the Data Link layer 
Ethernet separates the functions of the Data Link layer into two distinct sublayers:
    1.1 the Logical Link Control (LLC) sublayer 
          functions: a. makes the connection with upper layers
                           b. frames the Netwrk layer(NL) packet
                           c. identifies the NL protocol
                           d. remains relatively independent of the phyzical equipment.
    1.2 the Media Access Control (MAC) sublayer. 

          functions: a. data encapsulating  
                                - Frame delimiting
                                - Addressing
                                - Error detection
                           b. media access control
                                - controls the placement of frames on the media
                                - media recovery 
2) the Physical layer.
    The success of Ethernet is due to the following factors: 1) Simplicity and ease of maintenance,2) Ability to incorporate new technologies,
3) Reliability, 4) Low cost of installation and upgrade.
                                                                                                   9.2

   half-duplex communication -  only one station could successfully transmit at a time. 

   full-duplex communications - having a connection that can carry both transmitted and received signals at the same time.

   MAN (Metropolitan Area Network ) - Ethernet was initially limited to LAN cable systems within single buildings, and then extended to between buildings.                                                           

                                                                                                                                                         9.3

         Dropped frames are likely to be the result of collisions or other unwanted signals and are therefore considered invalid.

The Frame Check Sequence (FCS) field (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). 

 MAC addressing is added as part of a Layer 2 PDU. An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits. 

MAC Address Structure:=  (3-byte) Organizationally Unique Identifier [OUI] +  NIC [vendor code or serial number] (3 bytes)

          The Network layer address enables the packet to be forwarded toward its destination.

          The Data Link layer address enables the packet to be carried by the local media across each   segment.

    In Ethernet, different MAC addresses are used for Layer 2 -

- unicast MAC address is the unique address used when a frame is sent from a single transmitting device to single destination device.

- broadcast MAC address- the packet contains a destination IP address that has all ones (1s) in the host portion.(DHCP, ARP protocols).On Ethernet networks, the broadcast MAC address is 48 ones displayed as Hexadecimal FF-FF-FF-FF-FF-FF. 

- multicast addresses allow a source device to send a packet to a group of devices. 

                                                                                                                                                              9.4

   Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to detect and handle collisions and manage the resumption of communications

• Carrier Sense.( all network devices that have messages to send must listen before transmitting. )
• Multi-access.
• Collision Detection (When a device is in listening mode, it can detect when a collision occurs on the shared media.)
• Jam Signal  (The jamming signal is used to notify the other devices of a collision, so that they will invoke a backoff algorithm.)
• A random backoff period ensures that the devices that were involved in the collision do not try to send their traffic again at the same time, which would cause the whole process to repeat.  

    Hubs and repeaters are intermediary devices that extend the distance that Ethernet cables can reach. Because hubs operate at the Physical layer, dealing only with the signals on the media, collisions can occur between the devices they connect and within the hubs themselves.

   A collision domain is  referred to as a network segment. 

Latency.

   An Ethernet frame takes a measurable time to travel from sendind device to he receiver. Each intermediary device contributes to the overlay latency.

   In half-duplex mode, if a collision has not occurred, the sending device will transmit 64 bits of timing synchronization information, which is known as the Preamble. 

                     Ethernet with throughput speeds of 10 Mbps and slower are asynchronous. 

                     Ethernet with throughput speeds of 100 Mbps and higher are synchronous. 

Bit Time- a period of time is required for a bit to be placed and sensed on the media. 

Slot time- time that a transmitting station waits before attempting to retransmit following a collision.

Interframe spacing - this time is measured from the last bit of the FCS field of one frame to the first bit of the Preamble of the next frame.                                                                                                                      9.5

                                                                                                                                                               9.6

    Because of hubs do not perform any type of traffic filtering, this type of Ethernet LAN has limited use in today's networks. Ethernet implementations using hubs are now typically used only in small LANs or in LANs with low bandwidth requirements.

    Scalability. With each increase in the number of devices on the media, performance is degraded. 

   Latency. Increasing the length of media or the number of hubs and repeaters connected to a segment results in increased latency.

    Network Failure.If any device connected to the hub generates detrimental traffic, the communication for all devices on the media could be impeded. 

    Switches allow the segmentation of the LAN into separate collision domains. 

  Each port of the switch represents a separate collision domain and provides the full media bandwidth to the node or nodes connected on that port. 

    In a LAN where all nodes are connected directly to the switch, the throughput of the network increases dramatically. The three primary reasons for this increase are:

    1. Dedicated bandwidth to each port

    2. Collision-free environment

    3. Full-duplex operation

Forwarding is Based on the Destination MAC
The switch maintains a table, called a MAC table, that matches a destination MAC address with the port used to connect to a node. If a match is found, the port number in the table that is paired with the MAC address is used as the exit port for the frame.
The MAC table - switch table\ bridge table. 

Switch Operation 
To accomplish their purpose, Ethernet LAN switches use five basic operations: 
~ Learning 
~ Aging 
~ Flooding 
~ Selective Forwarding 
~ Filtering 

The ARP protocol provides 2 basic functions: 
1. Resolving IPv4 addresses to MAC addresses; 2. Maintaining a cache of mappings
ARP cache- table in node's memory to find the Data Link layer address that is mapped to the destination IPv4 address. 
The ARP table is maintained dynamically.


Все... пока больше не могу. осталось немного.Постараюсь доделать завтра.