Network interconnection:
Repeaters, bridges, routers, gateways
Repeaters: the simplest
interconnection devices,
connect networks of the
Repeater
same architecture at the
Segment A
physical layer, and more
Segment B
higher layers.
Ethernet
Amplifier
segments
Transceiver
Any signal in one segment is
repeated in another segment,
Segm. B and vice versa. No filtering at
Segm. A
all.
Bridges: connect LANs at the data link layer.
Reasons for using bridges:
1. Connect departmental small LANs into a layer (campus)
LAN.
2. Connect geographically spread LANs (LANs in different
buildings).
3. Splitting a load of the LAN among its different parts.
4. Too long distance between workstations (more than 2.5
km for 802.3).
5. Reliability aspect.
6. Isolating sensitive traffic (security aspect).
Backbone LAN
B
LAN
1
Bridge
LAN
2
B
LAN
3
1
Operation of a LAN bridge from 802.3 to 802.4
Host A
From higher
layer
PKT
LLC
Phys.
LLC
Bridge
PKT
PKT
PKT
PKT
MAC
Host B
For 2 ports
802.3
PKT
802.3
PKT
802.4
PKT
802.4
PKT
802.3
PKT
802.3
PKT
802.4
PKT
802.4
PKT
CSMA/CD
LAN
802.4
Token bus
LAN
PKT
LLC header
MAC header
802.4
802.4
PKT
Data
PKT
Frame
Problems:
1. Each of the 802.x LANs uses its own frame format.
2. Interconnected LANs run at different data rate (buffering
is necessary).
3. Most important: three 802.x LANs have different
maximum length.
No solution in 802.x: large frames are discarded!!!
4. There are 9 combinations of 802.x to 802.y bridges.
2
More details about bridges
Architecture:
DB
Bridge
protocol &
buffers
Station ddr.
. . . . . .
. . . . . .
. . . . . .
MAC
chipset
LAN 1
Port1
Port2
Port
. . . .
. . . .
. . . .
LAN 2
Stations
3
An example:
All stations on different LANs have unique address
DB
DB
Station Port
Station Port
addr. numb.
addr. numb.
1
1
1
1
Bridge1
Bridge2
2
1
2
1
3
2
3
2
4
2
4
2
5
2
5
2
6
2
6
2
Ethernet
Ethernet
Port1 Port2
Port1 Port2 Ethernet
LAN A
LAN B
LAN C
1
2
3
Stations
4
Stations
5
6
Stations
Logically, this is one large LAN
Functions of bridge 1(A-to-B):
1. Read all frames on LAN A, accept those addresses to
stations on LANs B, C.
2. Using the MAC protocol for LAN B, retransmit accepted
frames on LAN B.
3. The same for B-to-A traffic.
4
Transparent (spanning tree) bridge
LAN 1
Bridge1
A
LAN 2
Port1
Port2
Bridge2
B
Port1 Port2
Port3
LAN 3
D
C
E
1. Initially, a bridge contains no info about stations.
2. A bridge accepts every frame transmitted on all the LANs
to which it is attached.
3. When a frame arrives, a bridge must discard or forward
it.
If forward, then: to which LAN ?
A frame arrived
on some port
Destination and source
LANs are the same ?
No
No
Destination LAN is unknown ?
Forward the frame to
the corresponding port
Yes
Discard the
frame
Yes
Forward the frame to all
ports except the port at
which it arrived
5
Operation of bridge 2
(initially bridge table is empty)
Bridge table
Transmissions
Frame Port At which the
from
#
frame arrived
host
1. C  E
C
3
Forward frame to ports 1, 2
2. E  C
E
3
Discard frame
3. B  D
B
1
Forward frame to ports 2, 3
4. A  D
A
1
Forward frame to ports 2, 3
5. D  A
D
2
Time
Forward frame to port 1
6. A  D
Forward frame to port 2
6
Firewalls (see textbook, pp. 410-411)
A castle
A bridge
A deep moat
High-level
checking
Application
gateway
A network
being guarded
by the firewall
Packet filtering
router (for
incoming
messages)
A firewall
Packet filtering
router (for outgoing
messages)
7
Routers (sometimes called gateways).
Main purpose: to interconnect different networks at the
network layer.
Using: in WANs.
Two classes of routers (according to the OSI model):
 Connection-oriented
 Connectionless
Connection oriented routers
A full
router
Net 1 to
Net 2 to
internet Buffers internet
Internet
Internet
to net 1
to net 2
Network
1
Network
2
Machine for both
networks
Two halfNetwork
routers
1
Machine 1,
within
network 1
Host A
Net 2 to
internet
Internet
to net 1
Internet
to net 2
Communication line
(X.75 protocol or X.25)
Router 1
Router 2
3
3
3
2
2
1
1
Network1
Net 1 to
internet
X.75
X.75
X.75
Network
2
Machine 2,
within
network 2
Host B
3
2
2
1
1
Network2
8
Network 2
Network 1
A
VC1
VC2
R
R
R
VC3
R
R
Source
host
VC4 Network 3
R
B
VC5
R
Concatenated virtual circuits
between hosts A and B via a
number of routers in three
networks.
All routers are connectionoriented.
Source host
datagram
R
A
A router
R
A table of
virtual
circuits
R
R
R
R
R
Destination
host
datagram
R
R
R
B
Connectionless routing
(using datagrams)
Destination
host
9
Datagrams moving through different networks
Host A
4
msg
3
IP msg
2
frame 1
Router 1
Router 2
IP msg
IP msg
frame 2
frame 3
frame 2
Host B
msg
IP msg
frame 3
frame 3
1
Network 1
Network 2
frame 1:
MAC1 IP msg
frame 2:
:
frame 3:
MAC2 IP msg
Network 3
 msg – a message from the
transport layer
 IP – layer 3 header (IP
header)
 MACi – data link header
for the network i
MAC3 IP msg
Problems:
Different networks have different maximum size of a packet.
So the fragmentation (and reverse process) is necessary.
Two approaches to fragmentation:
1. Transparent fragmentation
Network
packet
R
To next
network
R
This router
reassembles the
fragments (one
router for all
fragments!)
This router
fragments a
large packet
2. Non-transparent fragmentation
Large
packet
Network
R
R
To next
network
R
10
Software architecture for bridges and routers
Main requirement is high performance.
A bridge
Let there be a
continuous flow
LAN1
of packets from
LAN1 to LAN2
10 Mbps,
A packet
64 bytes in packets
generated
1
51 sec
51 sec
LAN2
51 sec
 20 000 packets/sec
time
A bridge (or a router) as a set of processes
Processes:
1, 3, 5 – with the
highest priority.
8, 9 – with the
lowest priority.
Bridge
LAN1
Design issues:
1. Using the single
address space.
2. IPC – by use of LAN
1
shared memory.
3. Scheduling: let
each process run
to completion.
4. Disable
interrupts (use a
polling
approach).
LAN3
LAN2
Statistics
  Management





LAN3

LAN2
11