1. Computer networks

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

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

4. 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

5. 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

6. 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

7. 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.)

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

9. Packet switching

10. 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

11. Hierarchy

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

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

14. 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 „

15. 802.3 (Ethernet) Nodes identified by MAC address CSMA/CD
10 / 100 Mbps ; Gbps
Manchester-code
differential transmission

16. Ethernet frame

17. 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

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

19. 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

20. 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

21. 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

22. 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

23. 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

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

27. Internet
framing (ethernet-ip-tcp)