LOCAL AREA NETWORKS
-- Network Configurations and Components
-- Transmission Techniques and Media
-- Access Control Protocols
-- Performance Analysis
-- Virtual LAN
• Fast Ethernet
• Ethernet Switch
• Gigabit Ethernet
• Token Ring
In general, a local area network can have the one of the following four
configurations: (1) bus, (2) ring, (3) star, and (4) ring-star hybrid.
Different configurations use different network access techniques and
yield different performance.
Ring C entral
C on trol
Bus and ring configurations use shared transmission media for
message transfer between user nodes. The star configuration uses
dedicated transmission medium. LANs of bus configuration (e.g.,
Ethernet) and ring configurations (e.g., IBM Token Ring) are very
popular. However, recent LAN technology such as ATM LAN uses the
LAN FUNCTIONAL ELEMENTS
There are four major functional elements for a LAN. They are:
1. A Transmission Medium
In the bus and ring topologies, all messages sent by the users are
transported on a common transmission medium (a bus or ring).
The protocol used to detect and mediate contention among users is
called the medium access protocol.
2. Network Access Stations
An network access station is responsible for (1) implementing the
medium access protocol, (2) placing user messages onto the
transmission medium, (3) inspecting the header of the messages
received to select those intended for local reception, (4) performing
error and flow control, and (5) buffering messages to be sent and
3. Network Controller
A network controller is used to perform admission control and call
processing in the connection oriented network. In star configuration
LANs, this performs the switching function.
Gateways are used to connect the LAN to external users via
another LAN, and/or wide area networks. A gateway could be a
bridge or a router.
TRANSMISSION TECHNIQUES AND MEDIA
Two transmission methods have been used for LANs.
It uses analog technologies. Analog signals are multiplexed onto to
the same transmission medium using frequency modulation
technique. It requires amplifiers and modems. The advantages of
broadband method are (1) it has large capacity, (2) it can
broadcast over large areas using amplifier, and (3) it can be
adapted to existing CATV technology. The disadvantages are its
complexity and cost.
Baseband transmission is totally digital. The entire frequency
spectrum is used for transmission. Baseband LANs are limited in
distance due to signal attenuation. Repeaters can be used to join
different LAN segments to extend the distance.
Transmission media include:
(1) Twisted pair:
These are the existing wires in the building. So they are
economical. However, they are limited in bandwidth to few
megabits per second and are susceptible to noise.
(2) Coaxial cable:
Coaxial cable has better performance, provides higher capacity,
can support a larger number of devices, and can span greater
distance. It can be used by both broadband and baseband
IEEE has standardized protocols for local area networks. They are
equivalent to the first (physical) and second (data link) layers in the
OSI 7-layer reference model.
IEEE 802.2 Logical Link Control (LLC)
802.3 802.4 802.5 802.6 802.11 Link
CSMA/CD Token-Bus Token-Ring MAN CSMA
MAC MAC MAC MAC MAC
Physical Physical Physical Physical Physical Physical
Medium Medium Medium Medium Medium Layer
802.2 specifies the logical link control protocol which is applicable to
all the network configurations. Functions performed include error
control, flow control and sequencing. 802.3 defines the media access
control (MAC) protocol and the physical medium specification for the
bus configuration LAN. 802.4, 802.5, 802.6 and 802.11define the MAC
protocols and the physical medium specifications for the token-bus,
token ring LANs, metropolitan area networks, and wireless LANs
receptively. The MAC protocols define the procedures and message
formats for the network access stations to access the transmission
The combination of the MAC protocols of 802.x and 802.2 is
equivalent to the second layer (i.e., data link layer) of the OSI
reference model; the physical medium specifications of 802.x are the
physical layer of the OSI reference.
802.2 LLC PROTOCOL
802.2 specifies two alternatives of service to higher layer entities: (1)
connectionless service and (2) connection-oriented service. These
services are defined by specifying the service primitives and
parameters exchanged between an LLC entity and its users. Two
primitives (L_DATA.request, L_DATA.indication) for connectionless
services and fourteen primitives (e.g., L_DATA_CONNECT.request,
L_DATA_CONNECT.indication, L_DATA_CONNECT.confirm) for
connection-oriented services are supported.
The LLC protocol has similar formats and functions to those of the
HDLC. The LLC frame consists of four fields: (1) destination service
access point (SAP) address, (2) source SAP address, (3) control field,
and (4) data field.
Octets 1 1 1 or 2 => 1
DSAP SSAP Control Data
Some examples of SAPs are:
SAP Value (Hexadecimal) Assignment
E0 Novell Netware
AA subnet access protocol (SNAP)
00 null SAP
7F ISO 802.2
FE OSI protocol
BC Banyan VINE
Depending on the type of frame, the length of the control field can be
either 1 or 2 octets. There are three types of frames: (1) information
transfer frame, (2) supervisory frame, and (3) unnumbered frame.
Formats of the control fields for these three types of frames are:
Bit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0 N(S) P/F N(R) Information
1 0 SS XXXX P/F N(R) Supervisory
1 1 MM P/F MMM Unnumber
where N(R)= transmitter send sequence number,
N(S)= transmitter receive sequence number,
S= supervisory function bit used to indicate RR (00) , RNR (10)
or REJ (01),
M= modifier function bit used to indicate unnumbered frames,
such as, unnumbered information frame for connectionless
service, set asynchronous balanced mode extended frame
(SABME) (1111P110) and unnumbered acknowledgment frame
(UA) (1100F110) for connection-oriented service, etc.
P/F= poll/final bit,
The type of frame is recognized by examining the first two bits.
SLOTTED ALOHA SYSTEMS
In the late 1960s, University of Hawaii developed a system, called
Aloha, to connect computers. It is a packet-switched radio
communication network. The central node listens to packets
transmitted by other nodes at the radio frequency f0=407 Mhz and
broadcasts the received packets at the radio frequency f1=413 MHz.
F 1 = 413 M Hz F 0 = 407 M Hz
The Aloha system was originally designed to be unslotted and then
later improved to be slotted.
In the slotted Aloha system, time is divided into contiguous fixed -
length intervals called time slots, each of duration T seconds. Each
user sends packets with fixed length corresponding exactly to the
duration of one time slot. Each user knows its propagation delay to the
central node. Whenever a user seeks to transmit a packet, it
synchronizes its transmission such that, at the central node, the
packet falls precisely into one time slot, and no user does a packet
overlap two time slots.
When two or more packets arrive at the central node at the same time,
a collision occurs, and the central node can not broadcast any of the
packets collided. To avoid repeated collisions among the same set of
users, each user waits some random time slots before attempting to
Let p be the probability that a given user attempts to access a given
time slot, i.e., p is the probability that a user has a least one packet
ready for transmission. There could be more than one user wants to
access the same time slot. Their probabilities are independent and
identical. The number of users, Q, is a binomial distributed random
variable. The probability that k out of the total N users are competing
for the same slot is:
P(Q = k ) = ⎜ ⎟ p k (1 − p ) N −k
If k=0, there is no transmission, the time slot is wasted. If k >1,
collision occurs and again the time slot is wasted. A successful
transmission is when only one user attempt to transmit at a given time
slot. Thus, the probability of a successful transmission is when k=1,
ps = Pr ob( success) = P (Q = 1) = ⎜ ⎟ p (1 − p) N −1
= Np(1 − p) N −1
To find the probability, p, that maximizes the probability of a successful
transmission, ps , we have
= − N ( N − 1) p(1 − p) N − 2 + N (1 − p) N −1 = 0
Thus the maximum success probability is
p( Max ) = N (1 / N )(1 − 1 / N ) N −1 = (1 − 1 / N ) N −1
lim p( Max ) = lim (1 − 1 / N ) N −1 = 1 / e = 36.8%
N →∞ N →∞
For a slot Aloha system, at most only 36.8% of the available time slots
can be effectively used. This is the efficiency of the network.
RESERVED SLOTTED ALOHA SYSTEM
The reservation ALOHA protocol is for nodes that use the same
channel for the reservation and for the transmission of packets. The
R.ALOHA protocol begins with a reservation phase. During this phase,
the nodes use the slotted ALOHA protocol to attempt to access the
channel. At the end of the reservation phase, the node which made
the reservation transmits the packet. After the transmission phase is
finished, the reservation phase begins again.
In the reservation phase, the utilization efficiency is 36.8%, and the
efficiency at the transmission phase is 100%.
Treservation + Ttransmission
2. 72 * +1
where TSreservation = time slot for reservation (fixed length)
TStransmission = time slot for transmission (fixed length)
Usually, TSreservation << TStransmission . If TSreservation = 0.05* TStransmission
efficiency = 88%, much higher than pure slotted ALOHA system.
UNSLOTTED ALOHA SYSTEM
In the unslotted Aloha system, a user can attempt to transmit a packet
at any time. We assume each packet takes T seconds to be sent.
Thus, packets may arrive at the central node while another user is
Let p be the probability that a given user attempts to begin transmitting
within an arbitrarily chosen window of width T. If two or more packets
begin transmission within the window, collisions occur.
An arbitrarily chosen T-second test window will see the start of a
successful transmission if the three following conditions are met:
1. Only one of the N users begins access within that window;
2. Once the first user begins transmission, no other user transmits
within the T-second window needed to complete the transmission of
3. No user attempted to begin transmission within the T-second
window preceding the beginning of the test window.
These three conditions are independent, with probabilities p1, p2, and
p3, given by
p1 = ∑ p = Np
p2 = (1 − p) N −1
p3 = (1 − p) N −1
Thus, the probability of a successful transmission is
ps = p1p2 p3 = Np(1 − p) 2 N − 2
The optimizing value of p is
= − N ( 2 N − 2 ) p (1 − p ) 2 N − 3 + N (1 − p ) 2 N − 2 = 0
we get p =
2N − 1
And, we have
p ( Max) = (1 − ) 2 N −2
2 2N − 1
( Max )
= 1 / 2e = 18. 4%
Thus, the efficiency of unslotted ALOHA is only half of that of the
eff( N ) 0.3
0 10 20 30 40
Ethernet was first invented by Xerox company in the 70s. In the 80s,
the IEEE 802.3 standard defines a group of LANs with various
physical layer standards (i.e., physical media and rates) and the media
access control (MAC) protocol based on Ethernet.
The standard designates the 802.3 LANs by short-hand notations.
10Base5 means 10Mbps, baseband, and 500-meter segment. This
type of LANs uses 50-ohm coaxial-cable. It is called thick Ethernet (3/8
10Base2 means 10Mbps, baseband, and 200-meter segment. It uses
thin flexible coaxial cable and is called thin Ethernet (3/16 inch).
10BaseT means 10Mbps, baseband, and unshielded twisted pair
(UTP). The length is 100 meters and the impedance is 100 ohms.
10BaseF means 10Mbps and uses fiber.
10Broad36 means 10Mbps, broadband, and up to 1800 meter
The MAC for IEEE 802.3 is called carrier sense multiple access/
collision detection (CSMA/CD).
There are three variations of CSMA: non-persistent, 1-persistent, and
In non-persistent CSMA, the user listens to the medium before
transmission. If the medium is idle, it can transmit. If the medium is
busy, wait an amount of time drawn from a probability distribution.
These procedures are repeated.
In the 1-persistent CSMA, the user listens to the medium,
1. If the medium is idle, transmit.
2. If the medium is busy, continue to listen until the medium is idle,
then transmit immediately.
In the p-persistent CSMA, the user listens to the medium, and
1. If the medium is idle, transmit with probability p, and delay one
time unit with probability (1-p).
2. If the medium is busy, continue to listen until the medium is idle
and repeat step 1.
3. If transmission is delayed for one time unit, repeat step 1.
IEEE 802.3 adopts the 1-persistent CSMA. The CSMA/CD protocol
adopted by IEEE 802.3 is as follows:
1. When a node has data to send, it first sets the backoff factor
2. The node monitors the medium and waits for an idle.
3. Once the medium becomes free, the node waits for an inter-
frame gap (IFG) period of time and then sends its frame. The
IFG is the time needed to send 96 bits.
4. While transmitting, the node also monitors for any collision. If a
collision is detected during the transmission, the node
immediately ceases transmitting data, and transmits a brief
jamming signal to assure that all other nodes are aware that
there has been a collision.
5. The node increases the backoff factor k by 1. If K is greater than
10, the node aborts the transmission. Otherwise, it goes back to
step 2 to retransmit the data after waiting for a period of backoff
time. It uses a truncated binary exponential backoff algorithm
to calculate the backoff time. It randomly chooses a variable r,
where 0 <= r <= (2k – 1). The backoff time is r x (slot time).
The length of a time slot is at least twice the propagation delay in
the bus, i.e., 2τ = 2 × (length of the bus)/(electrical wave speed).
The standard specifies the time slot as 512-bit time.
The original Ethernet frame format is as following:
Octets 8 6 6 2 46 - 1500 4
Preamble DA SA Type Data FCS
Where Preamble is used to establish bit synchronous and to locate the first bit of
Destination Address (DA) specifies the unique physical address (MAC
address) of the receiving device.
Source address (SA).
Type indicates the type of network layer protocol used in the data field.
Frame check sequence (FCS) is a 32-bit CRC based on all field starting
The MAC addresses are uniquely assigned by the Ethernet device
(e.g., NIC card, router port) manufacturers, and are hard-coded into
The Ethernet MAC address is 6 bytes long. The format is as follows:
Organizational Unique Identifier Network Interface Controller
(OUI) (NIC) Specific Identifier
Bit (MSB) 0: Global unique
1: Locally administered
The address space is grouped into two parts. The first three bytes
contains the organizational unique identifier (OUI), which identifies the
manufacture. The second group is the NIC specific identifier, which is
assigned by the manufacture to each NIC card. The least significant
bit (LSB) of the first byte indicates whether this address is for unicast
(=0) or multicast (=1). The second least significant bit of this byte
indicates whether this address is globally unique (=0) or locally
administration (=1). If the MAC address is all 1’s, then it is a broadcast
Some examples of the Ethernet type assignments are as follows:
Type Value (Hex)1 Assignment
06-00 Xerox Network Service
08-00 Arparnet Internet Protocol (IP)
08-05 X.25 level 3
08-06 Address resolution protocol (ARP)
0B-AD Banyan VINE
80-35 Reverse address resolution protocol (RARP)
80-38 Digital Equipment Corp
80-F3 AppleTalk ARP
90-00 Loop Back
The IEEE 802.3 Ethernet frame format is different from that of the
original Xerox Ethernet. The 802.3 Ethernet frame format is as
Octets 7 1 6 6 2 1 1 1 3 2 =< 1492 4
Preamble SFD DA SA Length DSAP SSAP Control OUI Type Data FCS
Preamble is used to establish bit synchronous and to locate the first bit of the
frame. This field contains 7 bytes of alternating 0s and 1s.
Start Frame Delimiter (SFD) indicates the start of the frame. This byte has a
bit pattern of 10101011.
Destination Address (DA) specifies the receiving device MAC address.
Source Address (SA) specifies the source device MAC address.
Length indicates the number of octets in the data field starting from DSAP to
before the FCS field.
DSAP and SSAP are the destination and source service access points.
Control is the control field of the LLC.
OUI is the organization unique identifier
Type indicates the type of network layer protocol used in the data field.
Frame Check Sequence (FCS) is a MAC frame 32-bit CRC based on all field
starting from DA.
In the above frame format, the field from DSAP to Control is the IEEE
802.2 LLC field. The combination of the OUI and Type fields is called
the sub-network access protocol (SNAP).
Examples of OUI assignment are as follows:
OUI Value (Hex) Assignment
1. For complete listing, see IETF RFC-1340 or RFC-1700.
00-00-F8 Digital Equipment Corp.
00-80-C2 IEEE 802 committee
00-A0-3E ATM Forum
The reason for adding SNAP field to the 802.3 frame is that the Type
field used by the Ethernet frame has been assigned by IEEE802.3
committee to be the length field. A field is needed to identify the
network layer protocol (i.e., the Type field). The Type field requires two
octets, SAP field is only one octet long. So, SNAP is used.
If the 802.3 Ethernet frame contains an IP packet, then the LLC and
SNAP fields are defined as:
DSAP-SSAP-Control : AA-AA-03 (hex)
OUI-Type: 00-00-00-08-00 (hex).
This special LLC (DSAP=AA, SSAP=AA) indicates a SNAP to follow.
In the SNAP field, OUI of 00-00-00 indicates that Ether type to follow,
and Type=08-00 identifies the network layer protocol used is IP.
CSMA/CD Performance Analysis:
Let the probability that a user has data to send = p and there are N
users. Then the probability, ps, that a successful transmission is
initiated when the bus becomes idle is the probability that only one
user attempts access. So,
p s = Np (1 − p) N −1
Let M be number of time slots which elapse when a successful
transmission is initiated. If M=k, then there are k-1 unsuccessful or
wasted time slots before the successful attempt. Thus, we have
p( M = k ) = p S (1 − p s ) k −1 , k≥1
Let the time needed to transmit a packet is T seconds. The total
elapsed time F from completion of the last successfully transmitted
packet to completion of the next successful transmitted packet is:
F = 2τM + T-2τ
and the average elapsed time is
<F> = 2τ<M> + T-2τ
where <M> = ∑ kp ( M = k ) = p s ∑ k (1 − p s ) k −1
k =1 k =1
−d ∞ k
= ps ∑ (1 − p s )
dp s k = 0
[1 − (1 − p s )]2 ps
Thus, <F> = + T − 2τ
Since τ and T are fixed, <F> is minimized if Ps is maximized.
To obtain the p which yields the maximized ps, we have
ps = 0 .
This yields p=1/N. Thus,
p s MAX ) = (1 − ) N −1
When N→∞, ps
( MAX )
= 1/ e .
Thus, <F>= 2τ(e-1) + T.
The channel efficiency η of the Ethernet using CSMA/CD is 2
η = (Duration needed to transmit a packet) / (Total time required)
T + 2τ (e − 1) 1 + (e − 1) / a
where a= T/2τ.
The throughput, Th, can be defined as
size _ of _ packet _(in _ Bits )
time _ required _ to _ send _ the _ packet
Let the size of the packet be s bits, the transmission speed in the
Ethernet be t bits/second, the length of the shared bus be l meters,
and the propagation delay in the transmission media be c meters/sec.
T = s/t, and τ = l/c.
2 * (e − 1) +
Note that the shorter the length of the bus, l, the higher the throughput.
This is the reason that the length of the bus is limited and the number
of users on each bus has been limited too. This is called micro-
2 This is the theoretical value. In reality, the measured efficiency is η=
1 + 2.5 / a
In the analysis of shared media Ethernet, it has been shown that the
higher the transmission speed, t, the higher the throughput. Ethernet,
most popularly known as 10baseT, operates at 10 Mb/s. To increase
the speed, fast Ethernet was proposed and standardized by IEEE as
802.3u Ethernet uses shard bus configuration and operates at 100
Mb/s data rate. The transmission media used are as follows:
100BaseT4 100BaseTX 100BaseFX
Medium Four pairs; UTP Two pairs; UTP Optical fiber,
Category 34 Category 5 multi-mode
Max. Length 100m 100m 2Km
To maintain compatibility with Ethernet (10baseT), fast Ethernet uses
the same frame format as that for the Ethernet. It also uses CSMA/CD
to access the bus.
Many fast Ethernet can support both 10Mb/s and 100Mb/s interfaces
with the host computers. Fast Ethernet performs auto-negotiation with
the NIC cards to detect the speed.
Exercise: Compare the throughput of 10baseT and 100baseT.
3. IEEE 802-3u-1995, “Media Access Control (MAC) Parameters, Physical Layer,
Medium Attachment Units and Repeater for 100 Mbps Operations, Type 100Base-T,”
IEEE Press, Piscataway, NJ, 1995.
4. Category 3 unshielded twisted pair (UTP) wire is used for regular phone line.
With the increasing demand of bandwidth of many applications and
the increasing processing power of the new computers, there appears
to be a need for faster Ethernets, i.e., Ethernets with bandwidth higher
than fast Ethernet (100Mb/s). IEEE 802.3z task force has defined the
specification of the gigabit Ethernet, which operates at 1000Mb/s.
Highlights of differences between 10Mb/s, 100Mb/s and Gigabit
Ethernet Fast Ethernet Gigabit Ethernet
Data rate 10Mb/s 100Mb/s 1Gb/s
Cat 5 UTP 100m 100m 100m
STP/Coax 500m 100m 25m
Multimode Fiber 2Km 412m (half duplex) 220-550m
2Km (full duplex)
Single Mode Fiber 25Km 20Km 5Km
In the gigabit Ethernet physical layer, four physical media have been
specified. They are (1) long reach single mode fiber (denoted as
1000BaseLX), (2) short reach multimode fiber (1000BaseSX), (3) 150-
ohm balanced copper cable (1000BaseCX), and (4) category-5
unshielded twisted pair (1000BaseT). The first three physical layers
have been defined by 802.3z (denoted as 1000Base-X), while 802.3ab
committee has defined specifications for the category-5 twisted pair
In 1000base-X, the coding of the Ethernet frames in the physical
media is 8B/10B coding. In this coding, an eight bit data is coded into
a 10-bit code group. This 8B/10B coding has excellent properties,
such as transition density, run-length limiting, DC balance, and error
The physical layer also performs AutoNegotiation to determine the
data rate, i.e., 10, 100 or 1,000 Mb/s.
5 H. Frazier and H. Johnson, “Gigabit Ethernet: From 100 to 1,000 Mbps”, IEEE
Internet Computing,pp. 24-31, January/February 1999.
The frame format is similar to that for the 10 and 100 Mb/s Ethernets.
It is as follows:
/I/ /I/ /S/ Preamble sfd MAC Header Upper Layer Header + Data CRC /T/ /R/ /I/ /I/
Idle Gigabit Ethernet Frame Idle
Where: /I/ is the idle code, /S/ signifies the start. Each frame ends with
a pair of code groups, /T/ and /R/.
In the link layer, the operation can be either half-duplex or full duplex.
If half-duplex is used, the media access control (MAC) protocol is
CSMA/CD with two extensions. These two extensions are:
o The carrier extension appends a set of special symbols to the
end of short frames so that the resulting frames are at least 4096
bits, instead of the 512-bit imposed by the 10 and 100 Mb/s
Ethernets. This extension is used to overcome the inherent
limitation of the CSAM/CD algorithm that mandates the round-
trip signal propagation delay between any two host computers
not to exceed the time required to transmit the smallest
o An optional frame bursting was defined. Frame bursting allows
multiple short frames to be transmitted consecutively up to a
limit, without relinquishing control of the signaling control.
If full duplex is used, CSMA/CD is disabled. A link level flow control is
used to prevent receiver buffer overflow. A pause protocol is adopted
that a congested receiver can request the transmitter to pause its
transmission. The congested receiver can sent to the transmitter a
pause frame which contains a timer value expressed as a multiple of
512 bit-time. Once the pause frame is received, the transmitter should
then stop transmitting. If the receiver becomes uncongested before the
timer expires, the receiver can send another pause frame with timer
value set to zero. The transmitter then resumes transmitting.
The physical and link layer diagram of gigabit Ethernet is summarized
Media Access Control (MAC)
Gigabit Media-Independent Interface (GMII)
1000Base-X PHY Physical
8B/10B AutoNegotiation 1000Base-T
Phy. Coding Sunlayer Sub-Layer
1000Base-LX 1000Base-SX 1000Base-CX Physical
Fiber-Optic Fiber-Optic Copper Phy. Medium Attachment Attachment
Xcvr Xcvr Xcvr Sub-layer
Single mode Fiber Multimode Shielded Categorr-5
(10μ 5Km) Fiber Copper Unshield
Multimode Fiber (50 μ 500m Cable (25m) Twisted pair (100m)
(50 μ 550m 62.5μ 220m)
A gigabit media independent interface is defined so that the MAC layer
can interface with various physical media. GMII defines independent
8-bit- parallel transmit-and-receive synchronous data interface.
Ethernet and fast Ethernet use shared medium to transmit packets
from different computers. Because all computers share the same
transmission medium, collisions do occur which reduce the throughput
as we have already seen. In addition, all computers shared the total
bandwidth (10Mb/s or 100 Mb/s). Thus, the more computers, the less
bandwidth used by each computer.
Switched Ethernet or Ethernet switch has been developed to eliminate
the collision and to improve the throughput. It is a packet switch, and
users do not share the transmission medium. Just like other switches,
the Ethernet switch has buffers to perform store-and-forward functions
to send packets from one user to another user based on the
destination MAC address.
The users can keep using their network interface cards (NIC) which
have already been installed in their PCs. These NICs perform the
CSMA/CD algorithm as before and send packets to the Ethernet
switch using the Ethernet packet format. Because in the Ethernet
switch, there is no shared medium, the transceiver in the NIC always
detects no activity in the Ethernet. Thus, the NIC can always send
packets to the switch.
The switching can be done by either store-and-forward or cut-through
method. In the store-and-forward method, the switch does not switch
until the whole packet is received and examined. In the cut-though
method, the switch starts to send the packet to the destination once it
receives the source and destination MAC addresses.
6. M. Molle and G. Watson, “100BaseT/ IEEE 802.12/ Packet Switching,” IEEE
Communications Mag., pp. 64-73, August, 1996.
Gateways are used to connect users of one LAN to external users via
another LAN, and/or wide area networks. A gateway could be a bridge
or a router. Bridges interconnect networks based on link layer
addresses (e.g., MAC addresses), while routers interconnect networks
based on the network layer addresses (e.g., IP addresses). In general,
bridges are used for LAN-LAN interconnection, and routers are used
for LAN-WAN interconnection. We will discuss routers later when we
discuss Internet protocols.
In a shared media LAN, such as Ethernet, the distance is limited.
Bridged LAN can be used in a campus environment to extend LANs
or to interconnect existing LANs that are deployed separately by
different departments. An example of two LANs connected by a bridge
is shown in the following figure.
1 2 H4
In the above network configuration, two Ethernets are connected using
a bridge. If host, H1 in LAN1 wants to send a message to host H3, the
message is broadcasted to all hosts of LAN1 and the bridge. By
examining the destination MAC address at the header of the message
frame, H2 and the bridge discard the message received, and H3
accepts it. If H1 sends a message to H7, the message is broadcasted
to LAN1, and both H2 and H3 will discard it. The bridge receives the
7. F. Backes, “Transparent Bridges for Interconnection of IEEE 802 LANs,” IEEE
Network, vol. 2, no. 1, pp. 5-9, January, 1988.
message. After examining the destination MAC address, the bridge
determines that the destination is on LAN2. It then broadcasts the
message to LAN2 through port 2. Thus, H4 to H7 of LAN2 all receive
the message. However, only H7 accepts it, other hosts will discard it.
In the above example, the bridge not only extends the LAN, it also
knows which user is on which LAN. This type of bridges is called
Transparent bridges were defined by IEEE 802.1d committee. They
are called transparent, because the users are unaware of the
existence of the bridges. Thus, the introduction of the bridge does not
require the hosts to be configured. A transparent bridge performs the
following three basic functions:
- Forwards frames from one LAN to another.
- Learns where hosts are attached to the LAN.
- Prevents looping in the topology.
Each transparent bridge has a forwarding table. When a frame arrives
on one of its interface ports, the bridge has to perform table look up to
decide whether or not to forward the received frame to another port
based on the destination MAC address.
Each entry of the forwarding table has at least two fields: the MAC
address of a host and the associated port of the bridge. Initially, the
forwarding table is empty. The bridge builds the table by learning.
Before the forwarding table is completely filled, when a frame is
received with the destination MAC address not on the table, the bridge
will broadcast the frame to all its ports except the port that it receives
In the above network configuration, the bridge has two ports (ports 1
and 2) attached to LAN1 and LAN2. When the bridge receives a frame
broadcasted by H1, it learns from the source MAC address that H1
MAC address is associated with port 1. In the same manner, the
bridge learns that H7 MAC address is associated with port 2 after it
receives a frame broadcasted by H7 to LAN2. After a while, a
forwarding table as shown below is established.
MAC address Port
If H5 sends a frame to H2, the bridge receives the frame from port 2.
By examining the destination MAC address and the table look up, the
bridge routes the frame to port 1 and broadcasts the frame to LAN1.
The LAN environment is dynamic, hosts may be added, removed, or
moved. The bridge needs to adapt to the dynamics of the network.
First the bridge adds a timer associated with each entry. The timer is
decremented periodically. When the timer reaches zero, the entry is
removed. When the bridge receives a frame with the source MAC
address matches with the one in the table, the entry is refreshed.
Secondly, when the bridge receives a frame and finds a match in the
source address but the port number in the entry is different from the
port number on which the frame arrives, the bridge updates the entry
with the new port number.
Show the forwarding tables of bridges A and B of the following network
configuration. How do these bridges build these two tables?
LAN1 LAN2 LAN3
H1 1 Bridge 2
1 2 B
Spanning Tree Algorithm8
One potential problem of the learning process is that it does not detect
looping. Looping can cause flood of frames and bring down the
network completely. To remove loops in a network, a spanning tree
algorithm (STA) has been specified by IEEE 802.1D committee.
The spanning tree algorithm requires that each bridge have a unique
bridge ID number, each port within a bridge have a unique port ID, and
all bridges on a LAN recognize a unique MAC group address.
The algorithm is as follows:
1. Select the bridge with lowest bridge ID as the root bridge for all the
bridges of all the LANs.
2. Calculate the cost of each path to the root bridge. The cost is
assigned according to some pre-defined criteria. The path cost is
the sum of the costs along the path. An example is each LAN costs
1. If a path traverses through four LANs, the path cost is 4.
3. Determine the root port for each bridge except the root bridge. The
root port is the port with the least-cost path to the root bridge. In
case of ties, the root port is the one with the lowest port ID.
4. Select a designated bridge for each LAN. The designated bridge is
the one that offers the least cost path from the LAN to the root
bridge. The port that connects the LAN and the designated bridge is
called designated port.
5. Place all root ports and all designated ports in the forwarding states
in the spanning tree. They are the ports that are allowed to forward
frames. The other ports are placed into a blocking state, and are
not allowed to forward frames.
The algorithm is implemented using a distributed algorithm. Each
bridge exchange special messages called configuration bridge
protocol data units (configuration BPDUs). Each configuration BPDU
contains the bridge ID of the transmitting bridge, the root bridge ID,
and the cost of the least cost path from the transmitting bridge to the
root bridge. The following shows a Configuration BPDU.
8. IEEE 802.1D, IEEE Standard for Local Area Network MAC (Media Access Control)
Octets 2 1 1 1 8 4 8 2 2 2 2 2
Protocol Protoccol BPDU Flags Root Root Bridge Port Message Maximum Hello Forward
ID Version Type Bridge Path ID ID Age Age Time Delay
ID ID Cost
Protocol ID: All zeros for STP
Protocol version ID: All zeros for current STP version
BPDU type: All zeros for configuration BPDU
Flags: Only the least and the most significant bits are used. When the
least significant bit is set to 1, it indicates this BPDU as a
topology change message. When the most significant bit is set to
1, the BPDU is a topology change acknowledgement.
Root Bridge ID: Bridge ID for the root bridge. The first two bytes
identify the priority of the bridge, and the last six bytes are the
MAC address of a port of the root bridge.
Root path cost: The cost of the shortest path from this bridge to the
Bridge ID: The ID of this bridge.
Port ID: The ID of the port that sends this configuration BPDU. It
consists of two parts. Six bits are used to indicate the priority and
the remaining ten bits are used to indicate the port number.
Message age: The time elapsed from the generation of the
configuration BPDU by the root bridge and its receipt by the
bridge processing the BPDU. It has a 1/256 second increment.
Maximum age: The maximum amount of time the configuration BPDU
can be used.
Hello time: The time interval that the root bridge should send the
Forward delay: The time that a bridge must remain in each
intermediate processing state before transition from blocking to
Each bridge records the best configuration BPDU it has so far. A
configuration BPDU is the best if it has the lowest root bridge ID. If
there is a tie, the configuration BPDU is best if it has the lowest cost to
the root bridge.
The configuration BPDU uses the ordinary Ethernet frame format. The
destination MAC address is a special multicast address assigned to all
bridges. The source MAC address is the address of the port. The SAP
value is assigned to be 0x42 (i.e., 01000010 in binary).
Each bridge initially assumes that it is the root bridge. Each bridge
transmits configuration BPDUs periodically on each of its ports. When
a bridge receives a configuration BPDU from a port, the bridge adds
the path cost to the cost of the LAN that this BPDU was received from.
The bridge then compares the configuration BPDU with the one
recorded. If the bridge received a better configuration BPDU, it stops
transmitting on that port and save the new configuration BPDU.
Eventually, only one bridge, the designated bridge, on each LAN will
be transmitting configuration BPDU on that LAN.
Each bridge maintains a timer for the saved configuration BPDU. The
timer is reset when the bridge receives a configuration BPDU. If the
timer expires due to some bridge failure, the bridge starts the spanning
tree algorithm again.
The following is an example of six LANs connected by five bridges.
LAN3 1 1
2 2 1
Note that some LANs are connected by two bridges. This could be
constructed to provide redundancy. The corresponding spanning tree
configuration is as follows:
LAN3 D 1 R 1
D 2 2 R 1
R 1 3
D 3 LAN5
The ports with dashed lines to the bridges are in the blocking state.
STP can take a long time to converge. The Rapid Spanning Tree
Protocol (RSTP) was introduced by IEEE 802.1w. RSTP basically is
the same as STP, but it provides faster convergence time from
topology changes. RTSP provides faster recovery by monitoring the
link status of each port and generating a topology change after the link
status change. RTSP improve recovery time by adding alternate port
for a port that acts as a backup to the root port. In addition, RTSP
reduces the number of port states.
Explain how the above spanning tree configuration is constructed.
Typically, an organization that has many nodes (or stations) deploys
separated LANs, which are connected by routers. Nodes on different
geographic areas or in different groups are physically connected to
different LANs. However, this physical association between a node
and a specific LAN is not flexible. An employee may be moved to
another floor and is still in the same group, thus wants to be in the
same LAN. One way to solve this problem is to re-wire, which is costly
and time consuming. Another problem arises that nodes in the
different group but in the same area may want to share the same LAN
to share cost. In this case, nodes from a group do not want to receive
any broadcast from the other group.
Virtual LAN (VLAN) has been developed to allow logical partition of
nodes in the LAN into communities of interest called VLAN groups.
Members of a VLAN group are not constrained by the physical
location. The following figure shows an example of VLAN groups
supported by two Ethernet switches.
Ethernet Switch A Ethernet Switch B
9. IEEE 802.1Q-1998, IEEE Standard for Local and Metropolitan Area Networks: Virtual
Bridged Local Area Networks.
Note that nodes that are attached to the same Ethernet switch are
assigned to different VLANs; while nodes from different Ethernet
switches can be in the same VLAN.
IEEE 802.1Q defines the architecture and protocol for VLAN. It
specifies the MAC frame format as follows:
Octets 7 1 6 6 4 2 4
Preamble SFD DA SA TAG LLC + Data FCS
Bits 16 3 1 12
TPID (= 0x8100) Priority F VLAN ID
A new field, called Tag, of four bytes lenth is introduced. The Tag
Protocol ID (TPID) field has a value of 0X8100, which indicates the
following two byte field is Tag Control Information (TCI). Note that
0X8100 (hexadecimal) is greater than 1500 (decimal). Thus, the
switches can easily identify that it is not a length field.
The TCI contains a three-bit priority field, which is defined in IEEE
802.1p, a one-bit canonical format identifier (CFI), and a twelve-bit
VLAN ID. The priority bits support up to eight different priority levels.
The CFI field is used for IEEE 802.5 token ring LANs. The VLAN ID
field can support up to 4094 VLANs. (Two possible values are
The membership of a VLAN can be defined based on
- Ethernet switch port;
- MAC address;
- IP address;
- Policy based.
VLAN is useful within an enterprise. Network service providers who
are providing network service to connect LANs from different customer
sites find that VLAN is not enough. This is because the VLAN tags
defined in IEEE 802.1q is controlled by the customer. Thus, Ethernet
frames from different customers that are transmitted in the service
provider’s network may have the same VLAN tag. In order for the
customers to share the same public network, another tag is needed in
order to identify the customers.
IEEE 802.1ad (Provider Bridges) is an amendment to IEEE standard
IEEE 802.1Q. It is also called Q-in-Q or Stacked VLANs. IEEE
802.1ad defines the insertion of a S-VID (service VLAN ID, aka S-tag)
by the service provider’s edge Ethernet switch (or bridge) at the UNI to
provide separate instances of the MAC services to multiple
independent customers of a public Bridged Local Area Network. With
IEEE 802.1ad, the Ethernet frames from different customers in the
service provider’s network can be easily identified, and it does not
require cooperation among the customers, and requires a minimum of
cooperation between the customers and the service provider.
The following figure illustrates the operation of 802.1ad:
10 IEEE 802.1ad-2005, Virtual Bridged Local Area Network – Amendment 4: Provider
The frame format of an Ethernet frame with IEEE 802.1ad is as
Octets 7 1 6 6 4 4 2 4
Preamble SFD SA S-TAG C-TAG LLC + Data FCS
Bits 16 3 1 12 Bits 16 3 1 12
TPID (=0x88a8) PCP E S - VLAN ID TPID (=0x8100) PCP F C - VLAN ID
Note that the Service –TAG and the customer-TAG have the same
format. Inside the customer Ethernet network, the Ethernet frames
have only the customer tag. When the frames enter the service
provider’s network, at the UNI, the provider Ethernet switch inserts the
service tag. Inside the service provider’s network, the switches forward
the Ethernet frames based on the S-TAG and the destination MAC
address (DA). At the egress switch, the S-tag is removed before the
Ethernet frame is transmitted to the customer’s network.
S-Tag, in general, is administered by the service provider.
IEEE 802.1ad is useful for a metropolitan Ethernet network that
connects Ethernet local area networks from different sites for a
customer. However it has limitations: (1) The S-VID field can only
accommodate 4096 customers. For a large network, especially a
backbone network, this number may not be enough. (2) Most Ethernet
control protocols, such as bridge protocol data units (B-PDUs) used by
the customer networks can not interact with the service provider’s
Ethernet switches/ bridges. These control protocol data units must be
tunneled through the service provider’s network.
IEEE 802.1ah specifies the tunneling of the customer’s Ethernet
frames through the service provider’s backbone bridged network
(PBBN). This protocol is also called MAC-in-MAC.
The following figure illustrates the Ethernet frame format defined in
Octets 7 1 6 6 4 18 4 4 2 4
Preamble SFD B-DA B-SA B-TAG I-TAG S-TAG C-TAG Length LLC + Data FCS
Bits 16 3 1 12 Bits 16 3 1 1 1 2 24 48 48
D D U
TPID (=0x88a8) B-PCP E B - VLAN ID TPID (=0x88e7) I-PCP E C Res1 Res2 I-SID C-DA C-SA
I I A
Note: B-DA: Backbone Destination Address,
B-SA: Backbone Source Address,
B-TAG: Backbone Tag,
TPID: Tag Protocol Identification
B-PCP: Backbone Priority Code Point,
DEI: Discard Eligible Indicator,
I-PCP: Service Instance Priority Code Point,
UCA: Use Customer Address,
I-SID: Backbone Service Instance Identifier,
11 IEEE 802.1ah, Virtual Bridged Local Area Networks – Amendment 6: Provider Backbone
C-DA: Customer Destination Address,
C-SA: Customer Source Address
The value of TPID for B-TAG is 0x88a8, and that for I-TAG is 0x88e8.
Since I-SID field has 24 bits, it can support up to 16million service
instances. The number of backbone MAC addresses is much smaller
than that for the customers. So it is much easier for the backbone
bridges to learn these addresses. The transport of the Ethernet frames
in PBBN uses the B-DA and B-VLAN ID.
The following figure shows an example of a network which constists of
the customer networks, provider bridge networks (PBN), and a
provider backbone bridge (PBB) network.
The token ring LAN is based on the use of a single token that
circulates around the ring when all users are idle. A user wishing to
transmit must wait until it detects token passing by. It changes the
token from “free token” to “busy token”. The user then transmit a frame
immediately following the busy token. The frame makes a round trip
around the ring. Every user on the ring will examine the destination
address in the header of the frame. Only the intended user will receive
the frame. When the transmitting user receives the frame it sent, it
purges that frame, and transmits a free token to the ring.
The format of the token is
O c te ts 1 1 1
SD AC ED
and the frame format is
Octets 1 1 1 2,6 2,6 1 1
SD AC FC DA SA Data ED FS
where SD is an 8-bit pattern as starting delimiter.
Access control (AC) identifies the frame priority and indicates whether this
is a token or a data frame.
Frame control (FC) indicates whether this is an LLC data frame. If not, bits
in this field control operation of the token ring MAC protocol.
Destination address (DA).
Source address (SA).
Frame check sequence (FCS).
Ending delimiter (ED).
Frame status (FS) contains the address recognized (A) and frame copies
In the above Token Ring architecture, only one token is available, only
one packet is outstanding in the ring, and only the transmitting user
can generate a new token. Thus, this architecture wastes valuable
time on the ring.
An alternative is to let the user who has just transmitted a packet
immediately reinsert a token to the ring, rather than waiting for the
packet to travel completely around the ring. The packet is relayed
around the ring and is copied by the receiver. The trailing token is also
relayed until it arrives at a user having a packet waiting for
transmission. There, the token is removed, the new packet inserted,
and a new token is immediately reinserted following the new packet.
Again, only the transmitting user can remove the packet from the ring.
This architecture allows more than one packet in the ring and is more
IEEE 802.5 specifies this alternative as the medium access method.
In a token ring LAN, there are two factors that affect the delay
performance. One is the propagation delay in the ring, which is
determined by the length of the ring. The other factor is the processing
delay by each access station. Each access station has to examine the
circulating packets. While the processing delay for each station is
fixed, as the number of access stations increases, the processing
delay starts to dominate.
Assume every node send a packet with duration T (i.e., the insertion
time of the packet), and let’s denote
Dx,y the propagation between nodes x and y;
δ the duration (or the insertion time) of the token;
DH the processing time at each node;
N total number of nodes.
Data Token Stripping
1 2 3 .... N 1'
Station 2 1 2
0 D1,2+DH D1,2+DH+2T
1 2 3
0 D1,2+DH+D2,3+DH D1,2+D2,3+2DH+3T
1 2 3 N-1 N
D p = D1, 2 + D2 , 3 + ... + DN −1, N + DN ,1
Then the token ring efficiency is
NT + NDH + D p
N →∞ DH
Thus, the larger the T, the higher the efficiency, η.