Shared Ethernet

CSMA/CD

→ Carrier Sense : The LAN device listens to the Ethernet network to sense the carrier signal on the network, ensures that line is idle to reduce the likelihood of collision.

→ Multiple Access : Any datagram can be received by multiple devices

→ Collision Detection : If there's a collision, the LAN device will wait a random amount of time to transmit again.

The Hub does not have a MAC address and it only forwards data without filtering them.

Disadvantage of Hub : serious collision, broadcast flooding, no security guarantee.

Structure of Ethernet Frame

| DMAC | SMAC | Length/Type | DATA/PAD | FCS |

[6] [6] [2] [46~1500] [4]

If Length/Type > 1500 → represent type of frame → can submit to protocol upper layer without going through the LLC sub-layer (Ethernet II)

If Length/Type <= 1500 → represent length of frame (Ethernet_SNAP / 802.3)

Structure of Ethernet II Frame

… | Length/Type | DATA/PAD | …

0x0800 IP datagram → 0x800 (in hex) is bigger than 1500 (in decimal)

0x0806 ARP request/response

0x8035 RARP request/response

Three Switch Modes

Cut-Through

→ Forward as soon as it receives destination MAC address

→ Low delay

→ No error check

Store-and-forward

→ Forward after it receives all the packets

→ Delay lies on frame length

→ Switch checks error, the error packet will be discarded

Fragment-free → inherit the advantages of cut-through and store-and-forward modes

→ Switch receives the first 64 bytes of the packet, then lookups the address table by header and forwards

→ Switch checks the first 64 bytes, if it finds error, the packet will be discarded

Weakness of L2 Switch : Broadcast flooding, No security guarantee

what is the working theory of the L2 switch?

→ learning based on the source MAC address and forwarding based on the destination MAC address