ITCN - Router Basics
This lecture is divided into hyperlinked sections
Introduction
Router Components
RAM
NVRAM
Flash Memory
ROM
Interfaces
Router Software
Router Situations
Routers in WANs
Router Interfaces
Connecting a PC
to a Router for Configuration
Conclusion
Common Data Rates in LANs and
WANs
Introduction
A router is a computer that is placed at junctions
of network connections and it is dedicated to selecting the most
appropriate paths for packets of information whilst they cross computer
networks. A router can be thought of as the Internet's equivalent of a
postal sorting office where arriving packets of data are examined to
discover their destination and then directed to the most appropriate
netowrk connection.
In terms of a road network, a router can be thought
of as a roundabout to allow incoming traffic to access a set of
different roads (networks) .
Likening a
roundabout to a router1
Router
Components
The components of a modern router differ very
slightly from the PC architecture. The most obvious difference
internally is that there is no hard disk. The router has non-volatile
Flash memory to hold the operating system whilst power is off. It also
has another type of non-volatile memory known as NVRAM to hold files
containing the setup details for the router once it has been
configured. In common with the PC, the router has RAM and ROM, a
motherboard and ports through which it can be accessed.
Externally, the router has no monitor nor keyboard
attached during normal operation. To access the router, it is necessary
to use a PC and the appropriate program to interface with the router.
When a router is first purchased, it is necessary to use a PC running a
terminal emulator such as hyperterminal to set up the initial
configuration, however once the router is in operation in a live
network, it is possible to access the router either by TELNET or by
using a modem to make a direct connection via POTS.
RAM, also called dynamic RAM (DRAM), has the
following characteristics and functions:
• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering (shared RAM)
• Maintains packet-hold queues
• Provides temporary memory for the configuration
file of the router while the router is powered on
• Loses content when router is powered down or
restarted
NVRAM has the following characteristics and
functions:
• Provides storage for the startup configuration file
• Retains content when router is powered down or
restarted
Flash memory has the following characteristics and
functions:
• Holds the operating system image (IOS)
• Allows software to be updated without removing and
replacing chips on the processor
• Retains content when router is powered down or
restarted
• Can store multiple versions of IOS software
• Is a type of electronically erasable, programmable
ROM (EEPROM)
Read-only memory (ROM) has the following
characteristics and functions:
• Maintains instructions for power-on self test
(POST) diagnostics
• Stores bootstrap program and basic operating system
software
• Requires replacing pluggable chips on the
motherboard for software upgrades
Interfaces have the following characteristics
and functions:
• Connect router to network for frame entry and exit
• Can be on the motherboard or on a separate module
Router Software
In order to co-ordinate the hardware components of a router, it
needs an operating system. The operating systems that most people have
heard of are the Windows family (e.g. Windows 2000, XP, Vista, Windows
7) and UNIX, LINUX
and their variants. Cisco routers use a proprietory operating system
known as Internetworking Operating System (IOS). This operating
system offers a command-line interface and owes much of its
syntax to UNIX, the operating system upon which it is loosely
based.
Router Situations
Routers are found in the OSI model at layer 3, the Network
Layer. They can be used in Local Area Networks (LANs) to segment the
network, having the
advantage that they do not forward Layer 2 broadcasts e.g. ARP
requests. This gives increased performance in large LANs.
Routers can also be used at the boundary of a LAN and a Wide Area
Network (WAN). In this
type of situation, the router performs protocol translation between the
WAN and the LAN. An example of this could be an Ethernet LAN with a
Frame Relay connection to the Internet. The router would have to strip
off the address and control information in the header of arriving
frames from the LAN and replace this information with address and
control information to suit the Frame Relay connection. The data
content of the frames remains unchanged. The "wrapper" placed around
the data crossing a network is known as encapsulation. Encapsulation
differs between all LAN and WAN technologies.
Routers are also used within WANs at the junctions of communication
lines to forward data packets onward towards their destinations. The
type of router used at the heart of a WAN must have very high speed
switching in order not to cause delays in forwarding millions of
packets per second.
Routers in WANs
A WAN is said to operate at the physical layer and at the data link
layer. This does not mean that the other five layers of the OSI model
are not found in a WAN. It simply means that the characteristics that
separate a WAN from a LAN are typically found at the physical layer and
the data link layer. In other words, the standards and protocols used
in WANs at Layer 1 and Layer 2 are different from those used in LANs at
the same layers.
The WAN physical layer describes the interface between the data
terminal equipment (DTE) and the data circuit-terminating equipment
(DCE). Generally, the DCE is the service provider and the DTE is the
attached device. In this model, the services offered to the DTE are
made available through a modem or a CSU/DSU.
The principal function of a router is finding suitable paths and
switching arriving data packets along those paths. Routing occurs at
the network layer, Layer 3, but if a WAN operates at Layers 1 and 2, is
a router a LAN device or a WAN device? The answer is both, as is so
often the case in the field of networking. A router may be exclusively
a LAN device, it may be exclusively a WAN device, or it may sit at the
boundary between a LAN and a WAN and be a LAN and WAN device at the
same time.
One of the roles of a router in a WAN is to route packets at Layer 3,
but this is also a role of a router in a LAN. Therefore routing is not
strictly a WAN role of a router. When a router uses the physical and
data link layer standards and protocols that are associated with WANs,
it is operating as a WAN device. The primary WAN roles of a router are
therefore not routing, but providing connections to and between the
various WAN physical and data-link standards. For example, a router may
have an ISDN interface using PPP encapsulation and a serial interface
terminating a T1 line using Frame Relay encapsulation. The router must
be able to move a stream of bits from one type of service, such as
ISDN, to another, such as a T1, and change the data link encapsulation
from PPP to Frame Relay.
Many of the details of WAN Layer 1 and Layer 2 protocols will be
covered later in the course, but some of the key WAN protocols and
standards are listed here for reference.
WAN physical layer standards and protocols:
• EIA/TIA-232
• EIA/TIA-449
• V.24
• V.35
• X.21
• G.703
• EIA-530
• ISDN
• T1, T3, E1, and E3
• xDSL
• SONET (OC-3, OC-12, OC-48, OC-192)
WAN data link layer standards and protocols:
• High-level data link control (HDLC)
• Frame Relay
• Point-to-Point Protocol (PPP)
• Synchronous Data Link Control (SDLC)
• Serial Line Internet Protocol (SLIP)
• X.25
• ATM
• LAPB
• LAPD
• LAPF
Router Interfaces
The interfaces are the router's connections to the outside. The three
types of interfaces are local-area network (LANs), wide-area network
(WANs), and Console/AUX. The LAN interfaces are usually one of several
different varieties of Ethernet or Token Ring. These interfaces have
controller chips that provide the logic for connecting the system to
the media. The LAN interfaces may be a fixed configuration or modular.
The WAN interfaces include serial, ISDN, and integrated Channel Service
Unit (CSUs). As with LAN interfaces, WAN interfaces also have special
controller chips for the interfaces. The WAN interfaces may be a fixed
configuration or modular. The modules are known as Wide
Area Network Interface Cards
(WICs). Modern routers have a modular chassis, allowing flexibility and
future proofing of the hardware of the router.
Connecting a PC to a Router
for Configuration
When connecting a router to a PC for configuration purposes, it is
necessary to use a terminal emulator program such as Hyperterminal so
that the PC can communicate with the router's console port. When
setting up Hyperterminal, it is necessary to set various parameters as
described below.
Configure terminal emulation software on the PC for the following:
The appropriate com port e.g. COM1, COM2 etc
9600 baud
8 data bits
1 stop bit
No parity
No flow control
A physical connection with the router is also necessary. This is
accomplished using a special type of cable known as a rollover cable.
The connections of a rollover cable are totally different from those on
a straight-through or a crossover cable. Do not try to use any other type of
cable - it will fail and may damage either the PC or the router.
Connect a rollover cable to the router console port (RJ-45 connector).
Connect the other end of the rollover cable to the RJ-45 to DB-9
adapter
Attach the female DB-9 adapter to a PC
Conclusion
In this lecture we have seen that a network is an
inter-connection
of nodes. The nodes may be repeaters, bridges, switches or routers.
These
nodes are connected using either guided or un-guided transmission media
that actually carry the data from one physical location (node) to
another.
Common Data Rates in LANs and WANs
OC-x Optical Carrier levels:
Used to
specify the data rate of fiber optic networks. The base rate (OC-1) is
51.84 Mbps. OC-2 runs at twice the base rate, OC-3
at three times the base rate (155.52 Mbps), etc. Planned rates are:
OC-1, OC-3, OC-12 (622.08 Mpbs), OC-24 (1.244 Gbps), and OC-48 (2.488
Gbps).
|
Term
|
Line Rate
|
Also known
as/Comparable to
|
|
E1
|
2.048 Mbps
|
|
|
T1
|
1.544 Mbps (24 x
64 kbps)
|
DS1
|
|
T2
|
6.312 Mbps
|
DS2
|
|
T3
|
44.736 Mbps
|
DS3
|
|
Ethernet
|
10 Mbps
|
|
|
Fast Ethernet
|
100 Mbps
|
|
|
Token Ring
|
4 or 16 Mbps
|
|
|
DS0
|
64 kbps
|
|
|
DS1
|
1.544 Mbps
|
T1
|
|
DS2
|
6.312 Mbps
|
T2
|
|
DS3
|
44.736 Mbps
|
T3
|
|
OC-1
|
51.84 Mbps
|
STS-1
|
|
OC-3
|
155.52 Mbps
|
STS-3 & STM-1
|
|
OC-12
|
622.08 Mbps
|
STS-12 & STM-4
|
|
OC-24
|
1244.16 Mbps
|
STS-24 & STM-8
|
|
OC-48
|
2488.32 Mbps
|
STS-48 & STM-16
|
|
OC-96
|
4976.64 Mbps
|
STS-96 & STM-32
|
|
OC-192
|
9953.28 Mbps
|
STS-192 &
STM-64
|
|
ISDN Basic (2B+D)
|
2 x 64 kbps &
1 x 16 kbps
|
|
|
ISDN B Channel
|
64 kbps
|
|
|
ISDN D Channel
|
16 kbps
|
ISDN Signalling
Channel
|
|
ISDN Primary
(24B+D)
|
24 x 64 kbps &
1 x 16 kbps
|
|
|
Sonet
|
51.84 Mbps - 10
Gbps
|
|
|
SDH
|
155 Mbps - 10 Gbps
|
|
|
PDH
|
T1/E1 - 565 Mbps
|
|
1 Image
courtesy of http://www.transport.wa.gov.au/cycling/images/cycling_roundabout1.jpg
(c) MM Clements
