Network Hardware, Packets and Error Detection

What a packet carries. A message is split into packets, and each packet carries headers added by successive layers: the MAC addresses of the source and next device (layer 2), the IP addresses of the original sender and final destination (layer 3), the port numbers and sequence number of TCP or UDP (layer 4), then the payload — a fragment of the file, page or audio — and finally a checksum or cyclic redundancy check for error detection. Headers are overhead: small packets waste a higher proportion of capacity on headers, while large packets lose more data to a single bit error because the whole packet must be resent. Typical sizes of about 1 500 bytes are a compromise between the two.

Hub, switch, router: each reads one layer. A hub reads nothing; it repeats every incoming signal to every port, so all devices share the medium, collisions occur, and any machine can listen to any traffic — which is why hubs are obsolete. A switch reads the layer-2 MAC address and forwards the frame to the one port where that address was last seen, learning the table automatically; two pairs of machines can talk at full speed simultaneously. A router reads the layer-3 IP address and chooses the next hop between different networks using a routing table; it is the device that connects a home or school network to the internet. A wireless access point is a switch port that uses radio. A modem converts between the digital signal of the network and the form used by the telephone, cable or fibre line. Because each device reads only its own layer's header and ignores the rest, a router need not understand what the payload is — which is why the internet carries kinds of content that did not exist when it was designed.

Media. Copper twisted pair (Ethernet cable) is cheap and easy to install but limited to about 100 m per run and vulnerable to electrical interference. Fibre optic carries light, so it has far higher bandwidth, runs for kilometres, and is immune to interference and much harder to tap, but it costs more and needs skilled installation. Wireless needs no cabling and supports mobility, but shares its channel among all users in range, is slowed by walls and distance, and can be received by anyone nearby, which turns security into an encryption problem. Choosing a medium is a question of distance, required speed, mobility, interference and budget.

Detecting errors. Noise flips bits in transit, so receivers check. A parity bit makes the number of 1s in a byte even (or odd); it catches any single-bit error but misses two flipped bits, and it cannot say which bit is wrong. A checksum adds the values of the data and sends the total; the receiver recalculates and compares. A cyclic redundancy check uses polynomial division and catches burst errors that checksums miss. All of these detect; correction is done by asking for retransmission, which is what TCP does using acknowledgements and sequence numbers. Detection costs extra bits per packet — another reason header overhead matters.