MAN (Metropolitan Area Network)
MAN is a computer network usually spanning a campus or a city, which typically
connect a few local area networks using high speed backbone technologies. A MAN
often provides efficient connections to a wide area network (WAN). There are
three important features which discriminate MANs from LANs or WANs:
- The network size falls intermediate between LANs and WANs. A MAN typically
covers an area of between 5 and 50 km range. Many MANs cover an area the size of
a city, although in some cases MANs may be as small as a group of buildings.
- A MAN (like a WAN) is not generally owned by a single organization. The MAN,
its communications links and equipment are generally owned by either a
consortium of users or by a network service provider who sells the service to
the users.
- A MAN often acts as a high speed network to allow sharing of regional
resources. It is also frequently used to provide a shared connection to other
networks using a link to a WAN.
MAN adopted technologies from both LAN and WAN to serve its purpose. Some legacy
technologies used for MAN are ATM, FDDI, DQDB and SMDS. These older technologies
are in the process of being displaced by Gigabit Ethernet and 10 Gigabit
Ethernet. At the physical level, MAN links between LANs have been built on fiber
optical cables or using wireless technologies such as microwave or radio.
Medium Access Control (MAC)
The Media Access Control is often said to be a sub-layer of the OSI data Link
layer. On every network interface adaptor card there is a set of computer chips
that handle communication with the physical media (copper wire, fiber optic
cable or the air) by controlling the communication signal (electricity, light or
radio frequencies) over the physical media. In plain english, the computer chips
that control the electricity transmitted and received on a copper wire are
MAC-related hardware. The MAC sublayer provides the means to access the the
physical medium used for communication. The MAC sublayer also communicates with
the Logical Link Control (LLC) sub-layer above it allowing it to access and
speak to the upper layer network protocols such as IP.
In a centralized scheme, a controller is designated that has the authority to
grant access to the network. A station wishing to transmit must wait until it
receives permission from the controller. In a decentralized network, the
stations collectively perform a medium access control function to dynamically
determine the order in which stations transmit. A centralized scheme has certain
advantages as listed below.- It may afford greater control over access for providing such things as
priorities, overrides and guaranteed capacity.
- It enables the use of relatively simple access logic at each station.
- It avoids problems of distributed coordination among peer entities.
Centralized scheme has following disadvantages as listed below.
- It creates a single point of failure; that is, there is a point in the network
that, if it fails, causes the entire network to fail.
- It may act as a bottleneck, reducing performance.
In general, we can categorize access control techniques as being either
synchronous or asynchronous. With synchronous techniques, a specific capacity is
dedicated to a connection; this is the same approach used in circuit switching,
frequency division multiplexing (FDM) and synchronous time division multiplexing
(TDM). The asynchronous approach can be further subdivided into three
categories.- Round Robin
In round robin each station in turn is given the opportunity to transmit. During
that opportunity, the station may decline to transmit or may transmit subject to
a specific upper bound, usually expressed as a maximum amount of data
transmitted or time of this opportunity. In any case, the station, when it is
finished, relinquishes its runs, and the right to transmit passes to the next
station in logical sequence. Control of sequence may be centralized or
distributed. Polling is an example of a centralized technique.
- Reservation
In stream traffic reservation techniques are well suited. In general, for these
techniques time on the medium is divided into slots, much as with synchronous
TDM. A station wishing to transmit reserves future slots for an extended or even
an indefinite period. Again, reservations may be made in a centralized or
distributed fashion.
- Contention
For busty traffic, contention techniques are usually appropriate. With these
techniques, no control is exercised to determine whose turn it is; all stations
contend for time in a way that can be, as we shall see, rather rough and tumble.
These techniques are, of necessity, distributed by nature. Their principal
advantage is that they are simple to implement and under light to moderate load
of data traffic, they are efficient. For some of these techniques, however,
performance tends to collapse under heavy load.
MAC Frame Format Structure
MAC layer receives a block of data from the LLC (Logical Link Control) layer and
is responsible for performing functions related to medium access and for
transmitting the data. As with other protocols layers, MAC implements there
function, making use of a protocol data unit at its layer; in this case, the PDU
(Protocol Data Unit) is referred to as a MAC frame. The exact format of the MAC
frame differs somewhat for the various MAC protocols in use but in general we
have the following format.
Here is list of fields in detailed.
- MAC Control: This field contains any protocol control information needed for
the functioning of the MAC protocol. For example, a priority level could be
indicated here.
- Destination MAC Address: The address of the destination device on the LAN for
this frame.
- Source MAC Address: The address of the source device on the LAN from which
this frame is being transmitted.
- LLC: The LLC data from the next higher layer.
- CRC: The Cyclic Redundancy Check field also known as the Frame Check Sequence
(FCS). This is an error-detecting technique.
Logical Link Control (LLC)
The LLC is part of the data link layer in a protocol stack. The data link layer
controls access to the network medium and defines how upper-layer data in the
form of packets or datagrams is inserted into frames for delivery on a
particular network. The underlying physical layer then transmits the framed data
as a stream of bits on the network medium.
The IEEE 802.2 standard defines LLC, which is positioned in the protocol stack.
Note that LLC resides on the upper half of the data link layer. The MAC (Medium
Access Control) sub-layer is where individual shared LAN technologies such as
Ethernet are defined. Early on, the data link layer contained only LLC-like
protocols; but when shared LANs came along, the IEEE positioned the MAC
sub-layer into the lower half of the data link layer.
Basically, LLC provides a common interface, and provides reliability and
flow-control features. It is a subclass of HDLC (High-level Data Link Control),
which is used on wide area links. LLC can provide both connection-oriented and
connectionless services.
The LLC acts like a software bus, allowing multiple higher-layer protocols to
access one or more lower-layer networks. For example, a server may have multiple
network interface cards (and an Ethernet and a token ring card). The LLC will
forward packets from upper-layer protocols to the appropriate network interface.
This scheme allows upper-layer protocols to operate without specific knowledge
of the lower-layer network in use.
LLC Services
LLC specifies the mechanism for addressing stations across the medium and for
controlling the exchange of data between two users. The operation and format of
this standard is based on HDLC. Three services are provided as alternative for
attached devices using LLC. - Unacknowledged connectionless service
This service is a datagram style service. It is very simple service that does
not involve any of the flow and error-control mechanisms. Thus, the delivery of
data is not guaranteed. However, in most devices, there will be some higher
layer of software that deals with reliability issues. - Connection-mode service
This service is similar to that offered by HDLC. A logical connection is set up
between two user exchanging data and flow control and error control are
provided.
- Acknowledged connectionless services
This is a cross between the previous two services. It provides that datagrams
are to be acknowledged, but no prior logical connection is set up.
Typically, a vendor provides these services as options that the customer can
select when purchasing the equipment. Alternatively, the customer can purchase
equipment that provides two or all three services and select a specific service
based on application.
LAN Systems
The medium access control technique and topology are key characteristics used in
the classification of LANs and in the development of standards. There are
following system we will be discussing.
- Ethernet and Fast Ethernet (CSMA and CD)
Ethernet is a standard communications protocol embedded in software and hardware
devices, intended for building a local area network (LAN)Ethernet was designed
by Bob Metcalfe in 1973, and through the efforts of Digital, Intel and Xerox
(for which Metcalfe worked), "DIX" Ethernet became the standard model for LANs
worldwide. The term Ethernet refers to the family of local-area network (LAN)
products covered by the IEEE 802.3 standard that defines what is commonly known
as the CSMA/CD protocol. Three data rates are currently defined for operation
over optical fiber and twisted-pair cables: - 10 Mbps-10Base-T Ethernet
- 100 Mbps-Fast Ethernet
- 1000 Mbps-Gigabit Ethernet
As mentioned earlier, Ethernet uses Carrier Sense Multiple Access with Collision
Detection (CSMA/CD). When an Ethernet station is ready to transmit, it checks
for the presence of a signal on the cable i.e. a voltage indicating that another
station is transmitting. If no signal is present then the station begins
transmission, however if a signal is already present then the station delays
transmission until the cable is not in use.
History of CSMA/CD
The original Ethernet was developed as an experimental coaxial cable network in
the 1970s by Xerox Corporation to operate with a data rate of 3 Mbps using a
carrier sense multiple access collision detect (CSMA/CD) protocol for LANs with
sporadic but occasionally heavy traffic requirements. Success with that project
attracted early attention and led to the 1980 joint development of the 10-Mbps
Ethernet Version 1.0 specification by the three-company consortium: Digital
Equipment Corporation, Intel Corporation, and Xerox Corporation. The original
IEEE 802.3 standard was based on, and was very similar to, the Ethernet Version
1.0 specification. The draft standard was approved by the 802.3 working group in
1983 and was subsequently published as an official standard in 1985 (ANSI/IEEE
Std. 802.3-1985). Since then, a number of supplements to the standard have been
defined to take advantage of improvements in the technologies and to support
additional network media and higher data rate capabilities, plus several new
optional network access control features.
Ethernet Network Elements
Ethernet LANs consist of network nodes and interconnecting media. The network
nodes fall into two major classes:- Data terminal equipment (DTE) - Devices that are either the source or the
destination of data frames. DTEs are typically devices such as PCs,
workstations, file servers, or print servers that, as a group, are all often
referred to as end stations.
- Data communication equipment (DCE) - Intermediate network devices that receive
and forward frames across the network. DCEs may be either standalone devices
such as repeaters, network switches, and routers, or communications interface
units such as interface cards and modems.
Ethernet Network Topologies and Structures
LANs take on many topological configurations, but regardless of their size or
complexity, all will be a combination of only three basic interconnection
structures or network building blocks. - The simplest structure is the point-to-point interconnection as shown in
figure below. Only two network units are involved, and the connection may be
DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in point-to-point
interconnections is known as a network link. The maximum allowable length of the
link depends on the type of cable and the transmission method that is used.
- The original Ethernet networks were implemented with a coaxial bus structure,
as shown in figure given below. Segment lengths were limited to 500 meters, and
up to 100 stations could be connected to a single segment. Individual segments
could be interconnected with repeaters, as long as multiple paths did not exist
between any two stations on the network and the number of DTEs did not exceed
1024. The total path distance between the most-distant pair of stations was also
not allowed to exceed a maximum prescribed value. Although new networks are no
longer connected in a bus configuration, some older bus-connected networks do
still exist and are still useful.
Since the early 1990s, the network configuration of choice has been the
star-connected topology as shown in figure given below. The central network unit
is either a multi-port repeater (also known as a hub) or a network switch. All
connections in a star network are point-to-point links implemented with either
twisted-pair or optical fiber cable.
CSMA/CD MAC Frame
The diagram given below describes the structure of the standard 802.3 Ethernet
frames. - Preamble Field: A 7 octet pattern of alternating 0s and 1s used by the
receiver to establish bit synchronization.
- Start Frame Delimiter: Sequence 10101011 in a separate field, only in the
802.3 frame.
- Destination Address: Hardware address (MAC address) of the destination station
(usually 48 bits i.e. 6 bytes).
- Source Address: Hardware address of the source station (must be of the same
length as the destination address, the 802.3 standard allows for 2 or 6 byte
addresses, although 2 byte addresses are never used).
- Length: Specifies the length of the data segment, actually the number of LLC
data bytes, (only applies to 802.3 frame and replaces the Type field).
-
LLC: Data unit supplied by LLC.
- Data Unit: Actual data which is allowed anywhere between 46 to 1500 bytes
within one frame.
- Pad: Zeros added to the data field to 'Pad out' a short data field to 46 bytes
(only applies to 802.3 frame).
- FCS: Frame Check Sequence to detect errors that occur during transmission
(802.3 version of CRC). This 32 bit code has an algorithm applied to it which
will give the same result as the other end of the link, provided that the frame
was transmitted successfully.
Token Ring
Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from
one machine to the next and so on around the ring until it ends up back where it
started. It also uses a token passing protocol which means that a machine can
only use the network when it has control of the Token; this ensures that there
are no collisions because only one machine can use the network at any given
time.
Token Ring Operation
Token Ring and IEEE 802.5 are two principal examples of token-passing networks.
Token-passing networks move a small frame, called a token, around the network.
Possession of the token grants the right to transmit. If a node receiving the
token has no information to send, it passes the token to the next end station.
Each station can hold the token for a maximum period of time.
If a station possessing the token does have information to transmit, it seizes
the token, alters 1 bit of the token (which turns the token into a
start-of-frame sequence), appends the information that it wants to transmit, and
sends this information to the next station on the ring. While the information
frame is circling the ring, no token is on the network (unless the ring supports
early token release), which means that other stations wanting to transmit must
wait. Therefore, collisions cannot occur in Token Ring networks. If early token
release is supported, a new token can be released when frame transmission is
complete.
The information frame circulates the ring until it reaches the intended
destination station, which copies the information for further processing. The
information frame continues to circle the ring and is finally removed when it
reaches the sending station. The sending station can check the returning frame
to see whether the frame was seen and subsequently copied by the destination.
Unlike CSMA/CD networks (such as Ethernet), token-passing networks are
deterministic, which means that it is possible to calculate the maximum time
that will pass before any end station will be capable of transmitting. This
feature and several reliability features, which are discussed in the section
"Fault-Management Mechanisms," later in this chapter, make Token Ring networks
ideal for applications in which delay must be predictable and robust network
operation is important. Factory automation environments are examples of such
applications.
A good gif example on internet here:
Simple Token Ring:
http://www.datacottage.com/nch/anigifs/trani.gif
Hub Token Ring:
http://www.datacottage.com/nch/anigifs/trhubani.gif
Token Ring MAC Frame Format
Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command
frames. Tokens are 3 bytes in length and consist of a start delimiter, an access
control byte, and an end delimiter. Data/command frames vary in size, depending
on the size of the Information field. Data frames carry information for
upper-layer protocols, while command frames contain control information and have
no data for upper-layer protocols. Both formats are shown figure given below.
It consist the following fields:- Start Deliminater (SD): Indicates start of the frame.
- Access Control (AC): Indicates the frame's priority and whether it is a token
or a data frame.
- Frame Control (FC): Contains either Media Access Control information for all
computers or "end station" information for only one computer.
-
Destination Address (DA): Indicates the address of the computer to receive the
frame.
- Source Address (SA): Indicates the computer that sent the frame.
-
Data Unit (DU): Contains the data being sent.
- Frame Check Sequence (FCS): Contains CRC error-checking information.
-
End Deliminator (ED): Indicates the end of the frame.
-
Frame Status (FS): Tells whether the frame was recognized, copied, or whether
the destination address was available.
Fiber Distributed Data Interface (FDDI)
FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on
fiber optic lines in that can extend in range up to 200 km (124 miles). The FDDI
protocol is based on the token ring protocol. In addition to being large
geographically, an FDDI local area network can support thousands of users.
An FDDI network contains two token rings, one for possible backup in case the
primary ring fails. The primary ring offers up to 100 Mbps capacity. If the
secondary ring is not needed for backup, it can also carry data, extending
capacity to 200 Mbps. The single ring can extend the maximum distance; a dual
ring can extend 100 km (62 miles).
FDDI is a product of American National Standards Committee X3-T9 and conforms to
the open system interconnect (OSI) model of functional layering. It can be used
to interconnect LANs using other protocols. FDDI-II is a version of FDDI that
adds the capability to add circuit-switched service to the network so that voice
signals can also be handled. Work is underway to connect FDDI networks to the
developing Synchronous Optical Network.
Function of FDDI
The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing,
dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed
backbone technology because of its support for high bandwidth and greater
distances than copper. It should be noted that relatively recently, a related
copper specification, called Copper Distributed Data Interface (CDDI) has
emerged to provide 100-Mbps service over copper. CDDI is the implementation of
FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on
FDDI specifications and operations, but it also provides a high-level overview
of CDDI.
FDDI uses dual-ring architecture with traffic on each ring flowing in opposite
directions (called counter-rotating). The dual-rings consist of a primary and a
secondary ring. During normal operation, the primary ring is used for data
transmission, and the secondary ring remains idle. The primary purpose of the
dual rings, as will be discussed in detail later in this chapter, is to provide
superior reliability and robustness. Figure shows the counter-rotating primary
and secondary FDDI rings.
Bridges
A LAN bridge connects two or more LANs at layer two in the OSI network model.
The LAN bridge receives packets from a LAN segment connected to one port and
forwards them to another LAN segment connected to a different port. While a LAN
bridge serves the purpose of extending network range, it also relieves the
problem of congestion that multiple devices can cause on a single Ethernet
segment. LAN bridges employ varying mechanisms to deliver their functionality. A
simple LAN bridge regulates the transmission of frames to avoid congestion on
the network. A learning LAN bridge remembers (learns) the Ethernet address of
each frame it receives, in order to record which devices are connected to each
port. The learning bridge can then examine the destination address of each
received frame to determine whether or not it should be forwarded to another
part of the network. This selective forwarding improves the efficiency of
communications across the network. While bridges provide services similar to
those offered by routers and repeaters, there are some significant differences.
Routers, like LAN bridges, act as agents to receive and forward messages. Unlike
a router, however, a LAN bridge has no network-layer address. The LAN bridge is
transparent to both client and server workstations. Repeaters, are like LAN
bridges in that they also transmit information across an Ethernet network. But
having no memory, a repeater will retransmit all the data it receives, including
any frames that cause collisions. Unlike a repeater, A LAN bridge has the memory
and intelligence to alleviate collisions when forwarding Ethernet frames.
Bridges can be grouped into categories based on various product characteristics.
Using one popular classification scheme, bridges are either local or remote.
Local bridges provide a direct connection between multiple LAN segments in the
same area. Remote bridges connect multiple LAN segments in different areas,
usually over telecommunications lines. The figure illustrates these two
configurations.
Remote bridging presents several unique internetworking challenges, one of which
is the difference between LAN and WAN speeds. Although several fast WAN
technologies now are establishing a presence in geographically dispersed
internetworks, LAN speeds are often much faster than WAN speeds. Vast
differences in LAN and WAN speeds can prevent users from running delay-sensitive
LAN applications over the WAN. Remote bridges cannot improve WAN speeds, but
they can compensate for speed discrepancies through a sufficient buffering
capability. If a LAN device capable of a 3-Mbps transmission rate wants to
communicate with a device on a remote LAN, the local bridge must regulate the
3-Mbps data stream so that it does not overwhelm the 64-kbps serial link. This
is done by storing the incoming data in onboard buffers and sending it over the
serial link at a rate that the serial link can accommodate. This buffering can
be achieved only for short bursts of data that do not overwhelm the bridge's
buffering capability.
There are several reasons for the use of multiple LAN's interconnected:- Geography
- Performance
- Reliability
- Security
Note: This is last part of this article series.
HAVE A HAPPY CODING!