Computer networks

Topologies Point to point Bus (rail) Ring Tree, star, etc.

Direction of information simplex half duplex duplex (full-duplex)

Addressing P-P: no need for addressing Star with central controller (master): each client (slave) has own cable M->S, S->M directions needs no address in message S->S needs address Bus, ring, tree: target address is part of message, everyone gets message, only target reads it

Network access Random access systems can have collisions Solving these means a delay - not necessarily fixed time - can give max limit for delay Time slot (TDM) and token ring systems have no collisions can guarantee transfer speed, timing, priorities both kinds have different optimal utilization scenarios

Network access AlohaNet (1971, originally radio system for Hawaii islands): random access; M-S with frequency duplex (one freq for terminals, another frequency for central station) Collision Detection: if didn’t get answer, decided there was a collision, then send message again after random time interval

Network access Slotted Aloha: can only transmit in fixed time slots (TDM) CSMA (Carrier Sense Multiple Access): listens to channel before sending message (eg. Ethernet AX.25, etc.)

Routing (path selection) Fix line Subscriber line (on a public phone system eg.) Packet switching (on a public phone/network system)

Packet switching

Hierarchy systems can build upon each other (in layers) (look up ISO-OSI network layers model, Internet), each has different properties/functions Eg. bottom Ethernet or Token ring, on top of it IP, on top of it TCP or UDP all have own data frame and addressing

Hierarchy

LAN standards (Local Area Network) 802.3 (Ethernet) 802.5 Token ring 802.4 Token bus 802.11 Wireless LAN (Wi-Fi) 802.15.1 Bluetooth 802.15.4 (->ZigBee, WirelessHART, MiWi stb)

802.x - medium coaxial cables (RG58) (needs terminator resistor!) UTP (Unshielded Twisted Pair) Optical fiber Radiowaves (wireless)

802.x - Addressing MAC: Media Access Control 6byte address, eg. 01:23:45:67:89:ab physically coded address of network cards (NIC,network interface controller) it’s not the IP address! if many network cards in a computer -> each has its MAC address can be multicast or broadcast address „The EUI-48 is expected to have its address space exhausted by the year 2100. „

802.3 (Ethernet) Nodes identified by MAC address CSMA/CD 10 / 100 Mbps ; ... 100Gbps Manchester-code differential transmission

Ethernet frame

Token ring, token bus Logical ring can be made over a physically bus topology (each node knows the address of neighbours) empty frame goes around the network the host owning the token can write into this frame, then passes it

TCP/IP - IP Internet Protocol packet switched,unreliable, connection-less service „datagram” (packet)

TCP/IP - IPv4 IPv4 4 byte, eg. 192.168.1.1 max 2^32 address practically all used up to ease problems, sub-networks were developed where same address intervals are re-used; this makes addressing btw sub-networks harder

TCP/IP - IPv6 128 bit – about 3.4×1038 address eg. 2001:0db8:85a3:0042:1000:8a2e:0370:7334 will last for a longer time harder to remember  already used but switchover still didn’t happen

TCP/IP - TCP puts message back together from the packets (can arrive in different order) asks again for missing pieces reliable can have several logical ports on one physical connection

TCP/IP - UDP no error correction,no asking again for missing frames unreliable but faster than TCP eg. audio/video „stream” – speed („real-time”) is more important than bit error rate

Internet Basic idea: decentralized network, has to work even if several nodes fail originally military uses – ARPANET universities and research centers joined soon has a main backbone network with overseeing organizations ISP (internet service providers) are at end points of backbone to provide connection for end users

Internet DNS (Domain name server) URL (universal resource locator)

Internet framing (ethernet-ip-tcp)