Capitol 9 ETHERNET
The first Ethernet standard was published in 1980
Ethernet operates in the lower two layers of the OSI model: the Data Link layer and the
Physical layer.
Standards for LANs start with the number 802. The standard for Ethernet is 802.3.
Logical Link Control (LLC)
Layerul Data Link e separat in 2:
Logical Link Control (LLC)
Media Access Control (MAC)
Logical Link Control (LLC)
Handles the communication between the
upper layers and the networking
software, and the lower layers, typically
the hardware
Implemented in software (driver NIC)
Media Access Control (MAC)
lower Ethernet sublayer of the Data Link
implemented by hardware –NIC
responsibilities: Data Encapsulation &
Media Access Control
Data Encapsulation
Frame delimiting
Addressing
Error detection
Acesta formeaza sau dezasambleaza frameul
MAC layer adauga headerul & trailerul la PDU din Layer3
Incapsularea permite adaugarea de adresa MAC pentru livrare
Procesul de creare a frameurilor e benefic pentru sincronizare
Trailerul contine CRC pentru Error correction
Underlying logical topology of Ethernet is a multi-access bus
Ethernet provides a method for determining how the nodes share access to the media. The
media access control method for classic Ethernet is Carrier Sense Multiple Access with
Collision Detection (CSMA/CD).
The success of Ethernet is due to the following factors:
Simplicity and ease of maintenance
Ability to incorporate new technologies
Reliability
Low cost of installation and upgrade
Moda veche-topologie bus
Thicknet, (10BASE5)-coaxial-500m
Thinnet (10BASE2)-coaxial- 200m
half-duplex- unul comunica restul astepata
Moda noua –topologie star (logical multi acces (bus))
UTP
Se introduce switchul, trecerea la full duplex
Frame pt ethernet
2 modele:
DIX Ethernet standard (Ethernet II)
IEEE 802.3
Dimensiunea de 64 B-> 1518 B
Preamble (7 bytes) & Start Frame Delimiter (SFD) (1 byte) sunt folosite pentru
sincronizare. Atrag atentia destinatarului ca urmeaza sa primeasca un frame.
Destination MAC Address field (6 bytes)
Source MAC Address field (6 bytes)
Length/Type Field –lungimea segmentului Data
Data and Pad fields (46 - 1500 bytes) contine datele incapsulate pentru layerele
superioare. Frame-ul trebuie sa fie minim 46B, altfel se completeaza
Frame Check Sequence (FCS) field (4 bytes)-detectia de erori, foloseste CRC
Adresa MAC
Adresa unica data de IEEE, cod 3 bytes ce formeaza Organizationally Unique Identifier (OUI).
Numita si burned-in address (BIA) – se scrie in ROM
IEEE requires a vendor to follow two simple rules:
All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's
assigned OUI as the first 3 bytes.
All MAC addresses with the same OUI must be assigned a unique value (vendor code or
serial number) in the last 3 bytes.
Network layer address enables the packet to be forwarded toward its destination.
Data Link layer address enables the packet to be carried by the local media across each
segment.
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 is 48 ones displayed as Hexadecimal FF-FF-FF-FF-FF-FF.
Multicast -multicast addresses allow a source device to send a packet to a group of devices.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
detect
handle collisions
manage the resumption of communications.
Carrier Sense-device-urile asculta reteaua, daca detectecteaza un semnal el asteapta pana
cand se termina transmisia apoi transmite si el, in timp ce mesajul este transmis asculta in
continuare
Multi-access- din cauza distantei dintre device-uri, unul dintre acestea se poate sa nu detecteze
transmisia altuia si sa emita la randul lui un mesaj, cele 2 mesaje se intalnesc pe media si se
ciocnesc iar resturile mesajelor se transmit in continuare
Collision Detection-cand un device asculta, el poate detecta o coliziune intrucat se produce o
crestere in amplitudinea semnalului. In urma coliziunii deviceurile din retea nu mai transmit
pentru o perioada de timp.
Jam Signal and Random Backoff- in urma ulei coliziuni deviceurile transmit un semnal de blocaj
pentru a atentiona celelalte deviceuri, acestea realizand un backoff algorithm(impiedica
transmiterea pentru o perioada de timp).
Collsion domain=network segment-hub & repetor
Latency (intarziere)-timp necersar de propagare a semnalului pe media
In half-duplex mode, daca o coliziune nu a avut loc, deviceul ce vrea sa transmita va emite un
semnal de 64b de informatie pt sincronizare(PREAMBLE),apoi va transmite frame-urile ramase.
Ethernet asincron (asynchronous) – lent, <10Mbps, deviceul ce primeste informatia va folosi 8B
de timing information pt sincronizare iar apoi ii va elimina
Ethernet sincron (synchronous)-rapid,>100Mbps, imformatia de timing nu este necesara, dar
contine campurile Preamble and Start Frame Delimiter (SFD).
Bit Time-timpul necesar ca un bit sa fie trimis pe media
Slot Time- parametru pentru cate deviceuri pot fi pe media, daca o coliziune are loc acesta
aceasta va fi detectata intr-o anumita cantitate de date receptionare din frame.
Slot time is an important parameter for the following reasons:
The 512-bit slot time establishes the minimum size of an Ethernet frame as 64 bytes.
Any frame less than 64 bytes in length is considered a "collision fragment" or "runt
frame" and is automatically discarded by receiving stations.
The slot time establishes a limit on the maximum size of a network's segments. If the
network grows too big, late collisions can occur. Late collisions are considered a failure
in the network because the collision is detected too late by a device during the frame
transmission to be automatically handled by CSMA/CD.
Interframe Spacing-distanta minima dintr 2 frme-uri ce nu au fost intr-o coliziune
Jam Signal(semnal de blocare)-in urma unei coliziuni device-urile vor emite un
semnal de blocare de 32b ce va implifica coliziunea pt ca toate device-urile sa
sesizeze blocajul
Backoff Timing- timpul de asteptare de dupa coliziune, este mai mare pentru
device-ul care a creat coliziunea
