comunication devices

1. •1
Lecture#07
Computer Communications
& Networks CS-576

2. Type of Transmission
Unicast: Unicast is the term used to describe communication where a
piece of information is sent from one point to another point. In this
case there is just one sender, and one receiver
Multicast: Multicast is the term used to describe communication
where a piece of information is sent from one or more points to a set of
other points. In this case there is may be one or more senders, and the
information is distributed to a set of receivers (there may be no
receivers, or any other number of receivers).
Broadcast:Broadcast is the term used to describe communication
where a piece of information is sent from one point to all other points.
In this case there is just one sender, but the information is sent to all
connected receivers.

3. Type of Transmission

4. Introduction
There are many different devices for interconnecting networks
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5. Hub
Connects a group of Hosts,
multiple LANs.
It is physical layer device.
Broadcast
Collisions may occur if two hosts
send data at same time.

6. Hub
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7. Switch
Switches add more intelligence to data transfer management.
Multiple devices communicate simultaneously
It is layer 2 device

8. Switches
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9. Switch MAC Address Table

10. Switch MAC Address Table
The following six steps describe the process used to populate the
MAC address table on a switch
1. The switch receives a broadcast frame from PC1 on Port 1, as
seen in Figure
2. The switch enters the source MAC address and the switch port
that received the frame into the address table.
•Port1:PC1 MAC
Port2:Empty
•Port3:Empty

11. Switch MAC Address Table
3. Because the destination address is a broadcast, the switch floods
the frame to all ports, except the port on which it received the
frame.
4. The destination device replies to the broadcast with a unicast
frame addressed to PC1.

12. Switch MAC Address Table
5. The switch enters the source MAC address of PC2 and the port
number of the switch port that received the frame into the
address table. The destination address of the frame and its
associated port are found in the MAC address table
6. The switch can now forward frames between source and
destination devices without flooding, because it has entries in the
address table that identify the associated ports

13. Router
A router is a device that sends packets from one network to
another network
Routers receive packets, read their headers to find addressing
information, and send them on to their correct destination on the
network or Internet
Routers can forward packets through an internetwork by
maintaining routing information in a database called a routing
table
The routing table typically contains the address of all known
networks and routing information about that network such as:

14. Router
 Interface
 Routing Path
 Next Hop
 Route Metric (Cost)
 Route Timeout
Routers build and maintain their routing database by periodically
sharing information with other routers. The exact format of these
exchanges is based on the routing protocol. The routing protocol
determines
 The information contained in the routing table
 How messages are routed from one network to another
 How topology changes (i.e. updates to the routing table) are communicated
between routers
Convergence is used to describe the condition when all routers
have the same (or correct) routing information.

15. Router

16. External Components of a 2600 Router

17. Overview of Router Modes

19. Bridge, LAN switch, Ethernet switch
There are different terms to refer to a data-link layer interconnection
device:
The term bridge was coined in the early 1980s.
Today, the terms LAN switch or (in the context of Ethernet) Ethernet
switch are used.
Convention:
Since many of the concepts, configuration commands, and protocols for
LAN switches were developed in the 1980s, and commonly use the old
term `bridge’, we will, with few exceptions, refer to LAN switches as
bridges.
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20. 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.
•20
HighSpeed
Backplane
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Output
Buffers
Input
Buffers
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Hub Switch

21. Dual Speed Ethernet hub
Dual-speed hubs operate
at 10 Mbps and 100
Mbps per second
Conceptually these hubs
operate like two Ethernet
hubs separated by a
bridge
•21
Dual-Speed
Ethernet Hub

22. Gateways
The term “Gateway” is used with different meanings in different contexts
“Gateway” is a generic term for routers (Level 3)
“Gateway” is also used for a device that interconnects different Layer 3
networks and which performs translation of protocols (“Multi-protocol
router”)
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23. Bridges versus Routers
An enterprise network (e.g., university network) with a large
number of local area networks (LANs) can use routers or
bridges
1980s: LANs interconnection via bridges
Late 1980s and early 1990s: increasingly use of routers
Since mid1990s: LAN switches replace most routers
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24. A Routed Enterprise Network
•24
Internet
Router
Hub
FDDI
FDDI

25. A Switched Enterprise Network
•25
Internet
Router
Bridge/
Switch

26. Bridges versus Routers
Routers
Each host’s IP address must
be configured
If network is reconfigured,
IP addresses may need to be
reassigned
Routing done via RIP or
OSPF
Each router manipulates
packet header (e.g., reduces
TTL field)
Bridges/LAN switches
MAC addresses of hosts are
hardwired
No network configuration
needed
Routing done by
learning bridge
algorithm
spanning tree algorithm
Bridges do not manipulate
frames
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27. Bridges
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Overall design goal: Complete transparency
“Plug-and-play”
Self-configuring without hardware or software changes
Bridges should not impact operation of existing LANs
Three parts to understanding bridges:
(1) Forwarding of Frames
(2) Learning of Addresses
(3) Spanning Tree Algorithm