Introduction to digital networks

1
Introduction to digital networks

• B. Macq (macq_at_tele.ucl.ac.be)

2
The digital transmission media
01110001
01110001
01110001
t0t1t2t3t4t5t6t7
Axelle
Benoit
time

• The bit rate of the media determines the time
  duration of a bit (the
• duration between two colors in the stream)
• Digital transmission implies
• digital coding of the sources
• modulation of the streams to be in the right
  frequency on the media

3
Multipoint-to-multipoint communications
Axelle
Cathy
Brute force no networks
Diana
Benoit
Cathy
Axelle
Local Area Network Media access control Ethernet
or Token ring
Diana
Benoit
4
Multipoint-to-multipoint communications (cont.)
Cathy
Axelle
Switch or router
Diana
Benoit
Wide Area Networks and modern LANs (Switched
Ethernet) Switch connection oriented
network Router connectionless (datagram)
oriented network
5
LAN local area networks

• Addressing physical devices
• Defining frames
• Media Access Control
• Logical link control
• LAN performances and usefulness
• LAN interconnection

6
Addressing physical devices
Add. A
Add. B
Add. C
Axelle
Benoit
Cathy

• The address is a unique number
• N digits in telephony
• 48 bits for the Ethernet address
• Network Service Access Point for ATM (NSAP),
  between 7 and 20 bytes

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Framing

• The available bit rate is divided into frames
  HDLC (High Level Data Link Control) used in ISDN
  and Frame Relay
• Ethernet

X8bits
8bits
8 to 24bits
8 to 24bits
16bits
8bits
...0010001111110
0111111011...
Flag
Address
Control
Information
FCS
Flag
Information frame (NS,NR) Supervisory
frame Unnumbered frame
7 bytes 1 byte 6 bytes 6 bytes 2
bytes lt1518 bytes 4 bytes 96bytes
Start delimiter
Source address
Destination address
Information
Preamble
Length
FCS
Delay
8
Media access control (MAC)

• Ethernet carrier-sense multiple access with
  collision detection (CSMA/CD), 3 states of the
  network (idle, busy, jamming)
• Token ring two types of frames
• control token
• frame format
• FDDI temporized token (Fiber Distributed Data
  Interchange)

9
Evolution towards switched Ethernet, towards
Gbit/s
Switch
Twisted pair 100 Base-T 100Mbit/s
Axelle
Benoit
Cathy
Diana
10
Logical link control (LLC)

• Flow and error control-each data frame sent has a
  frame number
• Sequencing of data-large amount of data are
  spliced and sent with frame numbers.

Start delimiter
Source address
Destination address
Information
Preamble
Length
FCS
Delay
LLC
data
11
Performances and usefulness of local area networks

• A major concern is the behavior of the network
  under heavy loads, in computer environments,
  Ethernet is suited for 10-1OO users
• Different types of traffics
• file transfers
• voice
• multimedia
• Why networks ?
• Sharing resources (storage space, printers, )
• easier administration of users and tools (e.g.
  software licenses)
• group-wares (mailing, computer supported
  cooperative working, ...)

12
LAN interconnection
LAN2
bridge
LAN1
Internet
router
router
LAN3
gateway
router
router
router
13
Internet Protocol

• Transmission of datagrams over physical networks
  (encapuslation into frames address resolution
  protocol)
• Routers store and forward towards a router
  closer to the destination
• Routing information updated regularly
• IP address are related to a naming system (domain
  name server, DNS)
• IP address are a combination of a network address
  and a host address

IPa
IPb
data