Transcript Data Link Layer

Data Link Issues
Relates to Lab 2.
This module covers data link layer issues, such as local area networks
(LANs) and point-to-point links, Ethernet, and the Point-to-Point Protocol
(PPP).
1
Data Link Layer
• The main tasks of the data link layer are:
• Transfer data from the network layer of one machine to
the network layer of another machine using hop by hop
transmission over single links.
• Convert the raw bit stream of the physical layer into
groups of bits (“frames”)
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
2
Types of Single Segment Networks
• There are two types of communication networks:
– Broadcast Networks: All stations share a single
communication channel
– Point-to-Point Networks: Pairs of hosts (or routers) are
directly connected
Broadcast Network
Point-to-Point Network
• Typically, local area networks (LANs) are broadcast and wide area
networks (WANs) are point-to-point
3
Local Area Network
• Local area networks (LANs) typically connect computers
within a building or a campus
• Almost all LANs are broadcast networks
• Typical topologies of LANs are bus or ring or star
• We will work with Ethernet LANs. Ethernet has a bus or star
(Wifi) topology.
Bus LAN
Ring LAN
4
MAC and LLC
Data Link
Layer
• In any broadcast network, the stations must ensure that only
one station transmits at a time on the shared communication
channel
• The protocol that determines who can transmit on a broadcast
channel is called the Medium Access Control (MAC) protocol
• The MAC protocol is implemented
in the MAC sublayer which is the
to Network Layer
lower sublayer of the data link layer
Logical Link
Control
• The MAC is physical layer/topology
Medium Access
dependent
Control
• The higher portion of the data link
to Physical Layer
layer is often called the Logical Link
Control (LLC)
5
IEEE 802 Standards
• IEEE 802 is a family of standards for LANs, which defines
an LLC and several MAC sublayers
IEEE 802 standard
IEEE
Reference
Model
802.1
Logical Link
Control
802.2
802.6
802.5
802.4
802.3
Medium
Access
Control
Physical
Layer
Higher
Layer
Data Link
Layer
Physical
Layer
6
Ethernet
• Speed:
• Standard:
10-1000 Mbps
802.3, Ethernet II (DIX)
• Most popular physical layers for Ethernet:
•
•
•
•
•
10Base-T
10Base2
100Base-TX
100Base-FX
1000Base-FX
10 Mbps Twisted Pair
Thin Ethernet: 10 Mbps thin coax cable
100 Mbps over Category 5 twisted pair
100 Mbps over Fiber Optics
1Gbps over Fiber Optics
7
Bus Topology
• 10Base5 and 10Base2 Ethernets have a bus topology
Ethernet
8
Star Topology
• With 10Base-T and higher bit rates, stations are connected to
a hub in a star configuration.
• Wifi, basically wireless Ethernet, is also a star, Multiple
stations connected to one Access Point (AP).
Hub
9
Ethernet Hubs vs. Ethernet Switches
• An Ethernet switch is a packet switch for Ethernet frames
• Buffering of frames prevents collisions.
• Each port is isolated and builds its own collision domain
• An Ethernet Hub does not perform buffering:
• Collisions occur if two frames arrive at the same time.
Hub
Switch
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
HighSpeed
Backplane
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Input
Buffers
Output
Buffers
10
Ethernet II, (RFC 894)
1. Minimum Ethernet Frame size (legacy) has to be 64bytes
2. Minimum Ethernet data payload = 64 – 6 – 6 – 2 - 4 = 46bytes
3. Padding is used to fill up the data field if the IP datagram is
less than 46 bytes
802.3 MAC
Ethernet
destination
address
source
address
type
data
CRC
6
6
2
46-1500
4
0800
IP datagram
2
38-1492
0806
ARP request/reply
PAD
2
28
10
0835
2
RARP request/reply PAD
28
Padding
Padding
10
11