~DRAFT~ CHAPTER 5: Link layer

~DRAFT~ CHAPTER 5: Link layer
~DRAFT~
CHAPTER 5: Link layer
Contents
Network notes – Link layer..................................................................................................................... 1
1
Context of Chapter .................................................................................................................. 2
2
Introduction ............................................................................................................................. 2
3
Objectives of Chapter/module ................................................................................................ 3
4
Services of the link layer ......................................................................................................... 4
5
Error detection ........................................................................................................................ 4
a.
Parity check............................................................................................................................. 4
b.
Cyclic Redundancy Check ...................................................................................................... 5
6
Error Correction ...................................................................................................................... 5
7
MAC ....................................................................................................................................... 6
a.
Fixed assigned MAC............................................................................................................... 8
b.
Demand assigned MAC .......................................................................................................... 9
c.
Random Access MAC........................................................................................................... 10
8
Ethernet and switched Local Area Networks ........................................................................ 11
a.
Link layer addressing ............................................................................................................ 12
b.
Ethernet ................................................................................................................................. 16
c.
Virtual LANs ........................................................................................................................ 18
d.
Multiprotocol Label Switching ............................................................................................. 20
9
Conclusion ............................................................................................................................ 21
10
Review Questions ............................................................................................................. 21
11
Further reading: ................................................................................................................. 23
1
1
Context of Chapter
The data-link, or simply, link layer is the second layer in the OSI model and performs the final stage
of encapsulation before passing down packets to the physical layer to be converted into bits. Of the
layers it is the lowest that is fully implemented and controlled inside routers, and as such is where
hardware meets software. It takes a network layer datagram and encapsulates it into a link layer frame
by adding a header with source and destination addresses and a footer (or trailer) with a value for
error checking, and prepares data for the physical layer. The error checking mechanism is for errors
occurring in the physical layer. Figure 1 shows the encapsulation into a link layer frame relative to the
encapsulation at the network and transport layers. A data-link protocol specifies the structure of the
frame, as well as a channel access protocol that specifies the rules by which a frame is transmitted
onto the link. We will see that some features of the link layer overlap with the network layer and their
interaction enables a number of different functions.
FIGURE 1: DATA ENCAPSULATION AT THE LINK LAYER
2
Introduction
To introduce the data-link layer, we must define what a link is. A link is a communication channel
that connects adjacent nodes along the communication path from source to destination host. In order
to move a datagram from source to destination node (any host, router, switch or WiFi access point that
runs a link layer protocol), datagrams must be moved over each of the individual links along the path
by framing them into data-link layer frames. This is the main purpose of the link layer protocol in a
device, operating within a local area network (LAN) or wide area network (WAN). In order to move
datagrams effectively over physical links, the link layer protocol performs the functions of local
2
delivery to adjacent nodes, addressing, and media arbitration or access control. The link layer also
prepares a packet for the physical layer and may perform detection and correction of errors occurring
in the physical layer. In many standards, the functionality is divided into two sublayers: the Link
Layer Control (LLC) and Medium Access Control (MAC) sublayers.
Link layer protocols are typically implemented in a router’s network interface card(s) (NIC), also
known as the network adapter. The NIC contains the electronic circuitry required to communicate
using a wired connection (e.g. Ethernet) or a wireless connection (e.g. WiFi). The software
components of the link layer implement higher level link layer functionality such as assembling link
layer addressing information and activating the controller hardware. On the receiving side, link layer
software responds to controller event notifications such as frame arrival, handles error conditions and
passes a datagram up to the network layer.
3
Objectives of Chapter/module
By the end of this Chapter you should be able to:

Note the services provided and functions performed by the data-link layer

Name and be able to explain the functioning of different error detection techniques in the link
layer

Name and describe some error correction techniques

Explain link layer control in wired networks

Know functions of the MAC sublayer

Name medium access features in wireless networks such as WiFi and explain how three main
classes of medium access work, their similarities and differences

Explain briefly how ALOHA works and its evolution to slotted ALOHA

Know what CSMA is and be able to name two different variants of CSMA and their
application areas

Understand aspects of Ethernet

Be able to name different Ethernet media and identify the speed and cable technology from a
media type name

Be able to explain how ARP is used in Ethernet networks in different topologies within
subnetworks, and what is required for interconnecting subnets.

Know how Virtual LANs work and name the different kinds

Understand the motivation and application for MPLS
3
4
Services of the link layer
A link layer protocol may offer the following services:

Framing – encapsulation of datagrams into link layer frames

Link access – performed by the medium access control (MAC) protocol, which specifies the
rules by which a frame gains access to a link when other frames are competing for the same
medium

Reliable delivery – not present in all link layer protocols, a reliable delivery service
guarantees to move each network layer datagram across the link without error.

Error detection and correction – physical layer bit errors are introduced by various factors
such as signal attenuation and electromagnetic noise. The link layer protocol in a receiving
node performs mathematical calculations to detect such errors.
5
Error detection
For a receiver to detect transmission errors the sender must add redundant data (extra bits) to the
outgoing frame as an error detection code. When the destination node receives a frame with an error
detection code, the link layer protocol at the receiving end recalculates the error code. If the received
error detection code matches the computed error detection code, the frame is considered valid. Link
layer error detection mechanisms may not eliminate all errors but this is a first line of defence. We
now detail some kinds of error detection.
a. Parity check
A single parity bit is the simplest form of error detection, which is able to detect a single bit error in a
frame, or detect an error if an odd number of bit flips occurred. A parity bit is a redundant bit added to
the data payload. A scheme may employ either even or odd parity, in some cases selectable by the
network configurator. In an even parity scheme, the sender simply includes one additional bit,
choosing its value (0 or 1) such that the total number of 1s in the original data bits plus a parity bit is
even. For odd parity schemes, the parity bit value is chosen such that there is an odd number of 1s.
The receiver then counts the number of 1s in the received bits. If an odd number of 1-valued bits are
found with an even parity scheme, the receiver knows that some odd number of bit errors have
occurred. If an even number of 1-valued bits is calculated in an even parity check scheme, an even
number of bit errors may have occurred but the parity check is unable to detect this. The converse is
4
true for odd parity schemes. This feature may be a particular problem for bursty errors i.e. errors that
are clustered together so the chances of multiple bits being corrupted within a single packet are high.
Figure 2 illustrates the parity check of a packet in an even parity scheme, where two bit errors
occurred within the channel. At the receiver end 0 bit errors were detected because the parity check
still found an even bit-sum.
FIGURE 2: EVEN BIT PARITY CHECK AT RECEIVER END
The single bit parity check may be extended to a 2-dimensional version, which is somewhat more
reliable but it is not common in practical systems. Instead, higher level error detection may be
employed instead.
b. Cyclic Redundancy Check
Cyclic Redundancy Check (CRC) codes are also known as polynomial codes, the reason for which
will become clear shortly. They are one kind of checksumming error detection. The idea is that a bit
frame 𝐷 consisting of 𝑘 data bits is represented as coefficients of a polynomial expansion with
exponents 0 to 𝑘 − 1. As an example a bit string such as 11011 is represented as the polynomial 𝑥 4 +
𝑥 3 + 𝑥 + 1. There is no 𝑥 2 term because the bit in that position has value 0. To produce the CRC
code, the XOR (logical exclusive OR) is performed on the bits. The sender and receiver must agree
beforehand on a generator polynomial 𝐺(𝑥), which must be smaller than the number of bits in the
frame. The checksum can be computed using the generator polynomial. If it divides without a
remainder, it is concluded that no error has occurred.
This CRC method can detect single bit errors, two isolated single bit errors, odd number bit errors
(provided the polynomial contains 𝑥 + 1) and burst errors of length less than or equal to the degree of
the polynomial. Other burst errors may be detected with varying probability.
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Error Correction
5
Error correction may be forward error correction (FEC) where the receiver both detects and corrects
errors by running an error-correcting code on board, and backwards error correction where the
receiver instead requests the sender to retransmit the data. To correct errors by FEC, a receiver must
identify which bit has been corrupted during transmission and reconstruct the original information
accurately. FEC is used widely in wireless environments that can tolerate occasional errors but where
retransmission may cause unacceptable delays, while retransmissions are more common when the
connection is fast and can tolerate retransmissions, such as in fibre optic connections.
Of fundamental importance to error correction is the Shannon capacity of a channel (transmission
medium, which can be wired or wireless). The Shannon capacity or Shannon limit is the maximum
information rate at which reliable communication is possible over a channel depending on its SINR,
which is the ratio of wanted signal power (S) to the power of interference (I) + noise (N). The SINR is
related to the probability of bit errors. Shannon’s theorem states that capacity
(1)
𝐶 = log 2 (1 + 𝑆𝐼𝑁𝑅)
for a channel with additive white Gaussian noise (that is, it is assumed that interference and noise
follow a Gaussian or normal distribution and may simply be added on top of the desired signal). The
code rate must be smaller than the channel capacity to ensure that reliable communication is possible.
The theorem says that error correcting codes are possible or can exist for a given channel capacity but
does not postulate the actual error correcting codes.
Hamming code is a single bit error detection mechanism that uses bit redundancy. The bits are
arranged such that different incorrect bits produce different error results and the corrupt bit can be
identified and its value corrected. Consider a frame consisting of 𝑘 data bits and 𝑟 check bits, with the
resulting 𝑛 = 𝑘 + 𝑟 bit unit being called a codeword. The Hamming distance is the number of bits by
which two codewords differ. To detect 𝑏-bit errors, requires 𝑏 + 1 Hamming distance between a pair
of codewords. We can now see that codewords with single parity have a minimum distance of two
(calculated as 1 parity bit + 1), so in order to correct 𝑏-bit errors requires a distance of 2𝑏 + 1. 𝑟
redundant bits can provide 2𝑟 combinations of information. In a 𝑘 + 𝑟 bit codeword, there is a
possibility that the 𝑟 bits themselves may get corrupted. So to correct single bit errors, the number of
𝑟 bits used must inform about 𝑘 + 𝑟 bit locations plus a bit indicating no error information, i.e. 𝑘 +
𝑟 + 1. This implies the inequality
2𝑟 ≥ 𝑘 + 𝑟 + 1
must hold, which can be used to calculate the number of required redundant bits.
7
(2)
MAC
6
The MAC sublayer controls how a node gains access to the transmission medium and permission to
transmit a frame. The sublayer is responsible for moving packets from one network interface card to
another across the shared channel. For point-to-point links that have a single sender on one end of the
link and a single receiver at the other end of the link, the link access protocol is simple (or nonexistent) - the sender can send a frame whenever the link is idle. The more interesting case is when
multiple nodes share a single broadcast link - the so-called multiple access problem. A MAC
broadcast protocol should ideally be decentralised so that there is no single node acting as a master
controlling transmission opportunity and presenting a single point of failure. However, there are many
useful centralised MAC protocols. A MAC protocol should also be simple and easy to implement so
that it is inexpensive and does not consume much computational power. If a single node alone has a
data rate of 𝑅 bits/s and there are 𝑀 nodes sharing a channel, the average throughput per node is 𝑅/𝑀
bits/s.
FIGURE 3: THE GENERALISED COMMUNICATION SYSTEM
At this point it may be useful to remind ourselves of the big picture of a communications system, and
the various steps required in getting a data stream from sender to a receiver. The blocks in the top half
of Figure 3 are inside the sender and the blocks in the bottom half represent elements in a receiver
node.
The source encoder converts the electrical signal from the information source or transducer to a
binary stream of bits. The channel encoder must then convert the bit stream to encoded information in
the form of a codeword. This is where redundant bits such as parity and CRC bits for error detection
and correction, discussed in Sections 5 and 6, are added. The modulator takes the codeword and
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converts it to a continuous electrical signal by modulating it onto a carrier signal so that it can be
transmitted through the channel. A carrier signal has a specific centre frequency and bandwidth
around the centre frequency and the majority of the signal power should lie within the channel
bandwidth. Amplitude modulation is where the amplitude of the signal power of the carrier is varied
according to the bit sequence to be transmitted. For example, a 0 could be transmitted at -A amplitude
and a 1 at A amplitude. In frequency modulation such as frequency shift keying (FSK) and phase shift
keying (PSK) the signal is varied by frequency. A 0 could be transmitted at centre frequency + offset
and a 1 could be transmitted at centre frequency – offset. Related to modulation is multiplexing, an
important part of the link layer. Multiplexing in this context refers to how different signals may be
modulated onto a common carrier signal or transmission medium. Understanding these elements of a
communication system is important to understand the functioning of MAC protocols as these are at
the interface of the physical layer (the channel) and the data link layer.
According to IEEE 802.3-2002 section 4.1.4, the functions required of a MAC are:

receive/transmit normal frames

retransmission and back-off in half-duplex connections

append/check FCS (frame check sequence)

inter-frame gap enforcement

discard corrupted frames

prepend or remove preamble, start frame delimiter, and padding bits

half-duplex compatibility: append/remove MAC address
a. Fixed assigned MAC
Frequency Division Multiple Access
Frequency Division Multiple Access (FDMA) use frequency division multiplexing. That is, the total
available bandwidth is partitioned into separate channels of equal channel bandwidth and each data
stream is assigned a certain channel for transmission at a given time. An example of FDMA systems
were the first-generation (1G) mobile phone systems, where each phone call was assigned to a
specific uplink frequency channel, and another downlink frequency channel. A related technique is
wavelength division multiple access (WDMA), based on wavelength division multiplexing (WDM).
In WDMA different data streams get assigned to different coloured fibres in an optical fibre bundle.
Time Division Multiple Access
8
In Time Division Multiple Access (TDMA) different data streams are assigned different constantlength time slots. 2G cellular systems were based on a combination of TDMA and FDMA where each
frequency channel was divided into eight time slots, seven used for phone calls and one for signalling
data.
Orthogonal Frequency Division Multiple Access
In Orthogonal Frequency Division Multiple Access (OFDMA), using principles of Orthogonal
Frequency Division Multiplexing (OFDM), the signal band is divided into many subcarriers that are
orthogonal to each another and subsets of subcarriers are assigned to individual users. This
orthogonality is achieved by having the carrier spacing equal to the reciprocal of the symbol
(codeword) period. The data to be transmitted on an OFDM signal is spread across the carriers of the
signal, each carrier taking part of the payload and thus reducing the data rate taken by each carrier.
Each data stream is then modulated onto a different subcarrier/time slot block. OFDM is actually a
two-dimensional modulation scheme since it modulates over both frequency and time. OFDMA is
used for the downlink and OFDM for the uplink in LTE networks. We will discuss OFDM in more
details in the chapter on wireless networks.
Code Division Multiple Access (CDMA)
Frequency hopping spread spectrum (FHSS) is a type of CDMA whereby the carrier is rapidly
switched among many different frequencies in a pseudo-random order that is known to both receiver
and transmitter, enabling the receiver successfully to distinguish and decode different data streams.
Direct sequence spread spectrum (DSSS) is a code division multiplexing scheme where the message
signal is used to modulate a bit sequence known as a pseudo-noise code. The pseudo-noise code
consists of a radio pulse that is much shorter in duration than the original message signal and so has a
larger bandwidth. The message signal scrambles and spreads the pieces of data resulting in a
bandwidth size nearly identical to that of the pseudo-noise sequence. Both DSSS and FHSS are part of
the IEEE 802.11 WiFi standard.
b. Demand assigned MAC
Demand assigned MAC protocols assign communication channels based on the capacity required by
nodes on the network (the demand). Channels are allocated to nodes for a specified amount of time,
which may vary from a fixed-time slot to the time it takes to transmit a data packet.
Polling
9
A prevalent centralised demand assignment scheme is polling. In this method a central device denoted
the master controls the channel access to the slave devices by checking with each slave in a specific
predetermined order whether it has data to transmit. If the polled node (the slave) has data to transmit
it informs the controller, which then allocates the (or a) channel to the ready node for a time slot. The
slave node then uses the full data rate to transmit. If the polled slave node has no data to transmit it
declines the controller’s request and the controller simply moves on to query the next node on the
network. Polling procedures may be configured so that higher priority nodes are polled more often or
are always polled first.
The main disadvantage of polling is the considerable overhead that is required by the large number of
messages the master generates to query the slave nodes and also that nodes must wait their turn before
transmitting even if the message is important (unless extra measures are employed to handle different
priority classes). The latter can be remedied by implementing a hybrid system where transmission
occurs mainly by polling but some high priority devices may interrupt the process in an unsolicited
manner when there is a change of state or it has a message to transmit. This kind of operation is
common in telemetry systems (Distributed Network Protocol v.3 or IEC 61850-7-2 based networks).
There is also a polled system defined in IEEE 802.11, called Point Coordination Function.
Reservation-based schemes
In reservation-based medium access, time slots (called mini slots because they are usually smaller
than data time slots) are set aside for carrying reservation messages. When a station has data to send,
it requests a data slot by sending a reservation message to the master in a reservation mini slot. On
receiving the reservation request, the master computes a transmission schedule and announces the
schedule to the slaves.
Polling or solicited communication is a taking turn protocol. Another taking turns protocol is the
token-passing. Here there is no master node but a certain special frame known as a token is exchanged
among the nodes in some fixed order. A node holding a token is allowed to transmit data up to a
certain limit and must pass on the token once it has no more data to send or has reached the limit for
the current round. If a glitch causes node not to forward the token some recovery procedure must be
invoked to get the token back in circulation. IEEE 802.5’s token-ring protocol is an example. IEEE
802.4’s token bus protocol implements token-passing over a "virtual ring" on a coaxial cable.
c. Random Access MAC
In random access MAC protocols nodes attempt to transmit data whenever they have data to send.
Each node sends at the full rate of the channel. If nodes experience a collision, they will all back off
10
and wait a pseudo-random time period before reattempting transmission. Since each node is likely to
choose a different delay period collisions can be overcome. However, a few or several attempts may
be necessary before all nodes get a chance to transmit their data. Some examples are now detailed.
ALOHA
The ALOHA protocol is one of the oldest MAC protocols. The principles of ALOHA are 1) A node
sends data when it has data to send without checking first, 2) If a node receives any data from another
station during that time, there has been a message collision. All transmitting nodes must try resending
"later". Nodes resend after waiting for the frame time duration or immediately with a certain
probability.
Note that the first step implies that Pure ALOHA does not check whether the channel is busy before
transmitting. Since collisions can occur and data may have to be sent again, ALOHA cannot use
100% of the capacity of the communications channel. How long a station waits until it transmits, and
the likelihood of collisions occurring are interrelated, and both affect how efficiently the channel can
be used.
Slotted ALOHA
Slotted ALOHA was the next iteration of ALOHA, introducing discrete timeslots and an increase in
the maximum throughput. A node can only transmit at the beginning of a timeslot, thus reducing
collisions while frames are in transit. Only transmission attempts within 1 frame time and not 2
consecutive frame times need to be considered, since collisions can only occur during each time slot
Carrier Sense Multiple Access (CSMA)
The CSMA protocol requires that nodes verify that there is no other traffic currently active on a
shared transmission medium before transmitting, i.e. nodes sense to verify there are no carrier signals
currently on the medium before transmitting. CSMA with collision detection (CSMA/CD) adds the
feature of terminating transmission as soon as a collision is detected to basic CSMA, shortening the
time required before a retry can be attempted. CSMA with collision avoidance (CSMA/CA) requires
that transmission be deferred for a random interval if the medium is sensed busy before transmission.
The IEEE 802.11 WLAN standard’s Distributed Coordination Function (the counterpoint to Point
Coordination Function) is based on CSMA/CA. Many other variations on CSMA exist.
8
Ethernet and switched Local Area Networks
In general, each NIC of a host or router has a link layer address used for link layer protocols, whether
it is an Ethernet interface, or WiFi card or other. Link layer switches, however, do not have link layer
11
addresses associated with their interfaces that connect to hosts and routers because a link layer switch
must carry datagrams between hosts and routers transparently, that is, without the host or router
having to address the frame explicitly to the intervening switch. This feature is an important feature
for implementing certain protocols, as we will see.
a. Link layer addressing
A link layer address is variously called a LAN address, a physical address, or a MAC address. MAC
addresses are 6-byte (48-bit) addresses typically expressed in hexadecimal notation, used to get link
layer frames from one interface to another physically connected interface on the same network. When
an adapter wants to send a frame to some destination adapter, the sending adapter inserts the
destination adapter’s MAC address into the frame and then sends the frame into the LAN. The
receiving adapter then checks whether the destination MAC address in the frame matches its own
MAC address. If it does match, the adapter extracts the enclosed datagram and passes the datagram up
the protocol stack. If not, the adapter discards the frame without passing the enclosed datagram up. If
the frame is to be broadcast to all adapters on the network, the MAC broadcast address is used. For
LANs with 6-byte addresses (e.g. Ethernet and 802.11), the broadcast address is a string of 48
consecutive 1s (that is, FF-FF-FF-FF-FF-FF in hexadecimal notation). MAC address allocation is
managed by the IEEE such that every single network adapter has a different MAC address. In addition
to being unique, MAC addresses are generally permanent although it is now possible to change an
adapter’s MAC address via software.
Address Resolution Protocol
Address Resolution Protocol (ARP) maps IP addresses of hosts and router interfaces on the same
subnet to MAC addresses (e.g. Ethernet addresses). As such, it sits at the interface of the network and
link layers. ARP client and server processes operate on all computers using IP over Ethernet, typically
implemented as part of the NIC’s driver. However, it is a generic protocol functioning by finding a
mapping between ordered pairs (𝑃, 𝐴) and arbitrary physical addresses, where 𝑃 is a network-layer
protocol and 𝐴 is an address of this protocol 𝑃. At the time of ARP’s development, different protocols
were used in the network layer.
Each host or router has an ARP table with the IP addresses, MAC addresses and time to live of the
current address mapping. ARP is self-learning and so nodes create their own ARP tables without
requiring the intervention of a network administrator. An ARP request includes a valid mapping
between the network layer protocol address and the MAC address of the request initiator, in addition
12
to the network address looked for, so one entry for the initiator is created in the ARP cache when the
request is received.
ARP operates in request/response transmission pairs on the local network, as illustrated in Figure 4.
The source node transmits a broadcast request message (to FF-FF-FF-FF-FF-FF) containing
information about the intended destination’s IP address. The destination then responds to the source
with the hardware address of the destination. It does this unicast. This process is illustrated in Figure
4.
FIGURE 4: ARP TRANSACTION PROCESS
FIGURE 5: FORMAT OF AN ARP MESSAGE
13
There are four types of ARP messages in the ARP protocol, identified by four values in the "operation
code" field of an ARP message, as seen in the second row of the second column in Figure 5. The
types of message are:
1.
ARP request
2.
ARP reply
3.
Return ARP request
4.
Return ARP reply
The terms source and destination apply to the same devices throughout the transaction since they refer
to the direction the payload data is travelling. However, there are two different protocol messages sent
in ARP, one from the source to the destination and one from the destination to the source. For each
ARP message, the sender is the one that is transmitting a message at the time and the target is the one
receiving it. The identity of the sender and target change for each message but source and destination
are constant according to the payload data to be sent (not the control message).
The format of an ARP message is shown in Figure 5, and request and response variations are shown
in Figure 7. ARP Request and ARP Reply messages have the same packet format but with different
operation field values. An ARP Request packet also differs from a subsequent reply by the missing
hardware address of the destination, so that it is easy to create a reply to a request. When receiving a
request packet, in which the desired station finds its network layer (e.g. IP) address, the following
steps are completed:

The MAC address of the network adapter is inserted in the field Hardware Destination
Address.

The two address fields for the sender and the destination are swapped.

The Operation field takes value 2 to mark the message as an ARP Reply.

Finally, the reply packet is sent.
14
FIGURE 6: ILLUSTRATION OF AN ARP TRANSACTION IN A LAN
ARP request to FF-FF-FF-FF-FF-FF
ARP reply to 49-72-16-08-64-14
FIGURE 7: REQUEST AND REPONSE ARP MESSAGE VARIANTS
An example is illustrated in Figure 6, with the message formats of the request and reply shown below
in Figure 7. In the example of Figure 6, computer A wishes to send a data packet to router R. In order
to do this it needs the link layer address and the IP address to be able to tell its data-link layer for
which computer the packet is destined. This is the purpose of request broadcast message 1. The
intended destination router R recognises its IP address in the target (i.e. destination) address specified
in the ARP message, as illustrated in Figure 7. This enables it to send a reply (message 2) to computer
A which includes its MAC address. With this information computer A informs its data-link layer of
the MAC address and can send the payload packet to the correct destination.
Gratuitous ARP is used when a node has selected an IP address and wishes to check that no other
node is using the same IP address. It can also be used to force a common view of the node's IP address
across the subnet (e.g. after the IP address has changed). This is common when an interface is first
15
configured, as the node attempts to clear out any stale caches that might be present on other hosts. To
do this the node sends an ARP request for itself.
The arp command can be used to output the ARP table (ARP cache) of a computer or to manipulate
the ARP table such as creating permanent entries or deleting entries. The option -a with the arp
command is used to view the ARP table, while -d <computer> can be used to delete the entry for
computer and the option -s <computer> <layer-2-address> is used to add computer layer-2-address
manually to the cache.
Sending a datagram to destination outside of the subnet
ARP performs address resolution to enable data transmission within a single subnet but to send a
packet across a router into another subnet an extra step is required. Proxy ARP is the name given
when a node responds to an ARP request on behalf of another node. This is commonly used to
redirect traffic sent to one IP address on another system, such as we now describe.
Suppose we have subnet 1, which has hosts with interfaces in the IP address range 111.111.111/24
and subnet 2 has the network address 222.222.222/24. Host 111.111.111.111 in subnet 1 must send a
packet to host 222.222.222.222 on subnet 2 so it creates a datagram with the destination address of
222.222.222.222. The packet must pass through the first hop router on the path to the destination in
subnet 2. Therefore host 111.111.111.111 must send the packet with the destination link layer address
of the router adapter interface facing subnet 1, which it acquires by ARP since the router is attached to
its subnet. Once the packet reaches the router the router uses its forwarding tables (constructed by the
network layer) to forward the packet via its correct interface to subnet 2. The router can then use ARP
in subnet 2 to obtain the correct MAC address for the actual intended destination host and send the
packet to the intended destination using the MAC address of the ultimate destination.
b. Ethernet
The most widely used network connection for personal computers is an Ethernet connection. Ethernet
is really a standard for computer network technologies that describes both hardware and
communication protocols, being both a link layer and physical layer technology. It was initially
developed at Xerox PARC in the 1970’s for sharing printers as a way to limit costs. It was intended to
be used in a bus topology, with computers attached to segments of coaxial copper cable limited to
500 m in length. The original work on Ethernet used 75 Ohm coaxial cable, and operated at 3 Mbps.
By the mid-1990’s the standard speed had increased to 100 Mbps. At present speeds up to 40 Gbps
are available (albeit for specialist networks) with 10 Gbps Ethernet (10GBASE-T) having been
standardised back in 2007.
16
Nowadays Ethernet is mainly used in a switch-based star topology using store-and-forward packet
switching. Earlier on hubs were used instead of switches, where the hosts and routers directly
connected to a hub with twisted-pair copper wire. A hub is a physical layer device that acts on
individual bits rather than frames. When a bit enters one interface of a hub it boosts the signal’s power
and broadcasts it on all its other interfaces. In the hub-based start topology Ethernet collision control
was required since a hub operates only at the physical layer, so MAC protocols were developed for
the purpose of collision detection and avoidance. If using switches instead for Ethernet LAN, there are
no collisions because switches operate up to the link layer and forwards frames selectively based on
the MAC address, and are usually used in full duplex connections so there is no need for a MAC
protocol. Carrier Sense Multiple Access protocol with Collision Detection (CSMA/CD) is used to
access a segment for transmission when a half-duplex device or connection is used.
A side note on half and full duplexing: Full-duplex transmission is achieved by setting switch
interfaces, router ports, and host NICs to full duplex, provided the host network card and the switch
port are capable of operating in full duplex mode. A dedicated switch port is required for each full
duplex node. Micro-segmentation, where each network device has its own dedicated segment to the
switch, ensures that full duplex will work properly. The network device has its own dedicated
segment so there are no collisions in full duplex transmission. With full duplex transmission, the
device can send and receive at the same time, effectively doubling the amount of bandwidth between
nodes and ensuring that no collisions occur.
Most modern computers have an Ethernet-enabled NIC built into the motherboard, such as the one
shown in Figure 8. RJ.45 is a common socket type used for Ethernet connections.
FIGURE 8: CISCO ETHERNET NIC WITH FOUR RJ.45 SOCKETS
Ethernet LANs may be implemented using a variety of media. Examples are:

10BASE-5 (10B5) Low loss coaxial cable (also known as "thick" Ethernet)

10B2 Low cost coaxial cable (also known as "thin" Ethernet)

10BT Low cost twisted pair copper cable (also known as Unshielded Twisted Pair (UTP),
Category-5)
17

10BF Fibre optic cable

100BASE-T Low cost twisted pair copper cable (also known as Unshielded Twisted Pair
(UTP), Category-5)

100BF Fibre Fast Ethernet

1000BT Low cost twisted pair copper cable (also known as Unshielded Twisted Pair (UTP),
Category-5)

1000BF Fibre Gigabit Ethernet

10000BT Category 6 (Unshielded Twisted Pair, a.k.a Cat-6)

10000BT Fibre 10 Gigabit Ethernet
The number at the beginning of the standard specifies the speed. “BASE” means that baseband
Ethernet only is carried on the physical medium. The final part refers to the physical medium, T for
twisted pair and F for fibre. Some design rules that should be observed when using different cable
technologies for Ethernet installations are given in Table 1. If the distance or maximum number of
systems per segment are exceeded performance is likely to be compromised with higher bit error rates
and more delay-prone network.
TABLE 1: NETWORK DESIGN RULES FOR DIFFERENT TYPES OF ETHERNET CABLE TECHNOLOGY
Segment type
10B5 (Thick Coax)
10B2 (Thin Coax)
10BT (Twisted Pair)
10BFL (Fibre Optic)
Max Number of systems per
cable segment
100
30
2
2
Max Distance of a cable
segment
500 m
185 m
100 m
2000 m
Ethernet has also evolved to provide network Operations and Management (OAM) functions. These
standards allow network operators to manage faults on the network and to validate the performance of
the service.
c. Virtual LANs
VLANs allow a network manager to segment a LAN into different broadcast domains logically so that
computers on the VLAN do not physically have to be located together. VLANs allow multiple virtual
local area networks to be defined over a single physical LAN infrastructure and it is possible to have
hosts that are connected together on the same physical LAN not being allowed to communicate
directly. This enables the network administrator to organise a network according to requirements
without needing the physical LAN to mirror the logical connection requirements of the organisation.
The purpose of a VLAN is to remove the limitation of physically switched LANs with all devices
18
automatically connected to each other. Each port on a switch can be configured into a specific VLAN,
and then the switch will only allow devices that are configured into the same VLAN to communicate.
In this way VLANs solve problems arising in LANs:

Lack of traffic isolation. In physical LANs broadcast messages will be sent to all devices on
the network. Limiting the scope of such broadcast traffic would improve LAN performance
and introduce improved security and privacy.

Inefficient use of switches if grouping and isolation were done physically and not through
VLANs

User management with organisational changes and movements of people. Rewiring every
time organisational changes occur that require restructuring of LANs would be expensive and
time-consuming. With VLANs this can be avoided.

Increased broadcast domain size when the number of devices on a LAN grows large that
makes inefficient use of resources, slows down the communication process and the hosts
themselves
VLAN membership can be classified by port, MAC address, protocol type or IP Subnet Address. The
802.1Q draft standard defines Layer 1 and Layer 2 VLANs only. Protocol type based VLANs and
higher layer VLANs are allowed, but not defined in this standard. As a result, these kinds of VLANs
are proprietary.
In order to set up a VLAN switches must include configuration tables indicating which VLANs are
accessible via which interfaces.
VLANs do not includes a mechanism for communication between VLANs so to do this layer 3
(network layer) devices must be introduced to the network. One way of interconnecting VLANs to
allow communication between them is to connect a VLAN switch port to an external router and
configure the port to belong to both (or all) the required VLANs. It is not always necessary to use a
separate switch as there are some devices on the market that contain both a VLAN switch and a
router. This is a type of VLAN created through an access link. It connects VLAN-unaware devices to
the port of a VLAN-aware bridge.
Another more scalable method to interconnect VLANs is called trunking or tagging. A trunk is a
single transmission channel between two points, each point being either the switching centre or the
node. VLAN trunking allows a single network adapter to behave as 𝑛 virtual network adapters for
𝑛 VLAN network segments, carrying multiple VLANs through a single network link using a trunking
protocol, where 𝑛 has a theoretical upper limit of 4096 but is typically limited to 1000. Trunking
requires that the network switch, network adapter, and the drivers for the operating system all support
VLAN tagging in order for them to trunk. A VLAN tag is added to packets going through a trunk in
order to identify the VLAN that the packet belongs to.
19
There are several different trunking technologies:

IEEE 802.1Q: the IEEE industry open standard for VLAN frame tagging and the only
technology that can be used to interoperate between devices from different vendors. In
802.1Q The VLAN frame tag is placed inside an Ethernet frame when it reaches a switch that
is a member of a VLAN. If the switch has a trunk port, the Ethernet frame can be forwarded
out the trunk link port.

Inter-Switch Link (ISL): Cisco proprietary VLAN frame tagging method that encapsulates
an Ethernet frame with an ISL header.

LAN Emulation (LANE): LANE is used to communicate with multiple VLANs over
Asynchronous Transfer Mode (ATM).

802.10 (FDDI): Protocol for sending VLAN information over FDDI.

VLAN Trunking Protocol (VTP) is a proprietary CISCO protocol for propagating VLAN
information to all the switches in a VTP domain. VTP advertisements can be sent over
802.1Q or ISL trunks. The information propagated is management domains, configuration
revision number and currently known VLANs and their specific parameters.

MAC-based approaches where the network manager specifies the set of MAC addresses that
belong to each VLAN. In this case whenever a device attaches to a port, the port is connected
into the appropriate VLAN based on the MAC address of the device.

Network-layer approaches (e.g., IPv4, IPv6, or Appletalk)
d. Multiprotocol Label Switching
Multiprotocol Label Switching (MPLS), standardised by the IETF in RFC 3031, is a data-carrying
technique generally considered to reside between traditional OSI data link and network layers. It
incorporates Virtual Circuit techniques into a routed datagram network by applying fixed-length
labels. It was designed to provide a unified data-carrying service for both circuit-based clients and
packet-switching clients which provide a datagram service model. MPLS is used on top of IP
addressing by selectively labelling datagrams and allowing routers to forward datagrams based on
fixed-length labels instead of IP addresses when possible. It can be used to carry many different kinds
of traffic, including IP packets, E1/T1, ATM, Frame Relay, SONET, and Ethernet frames but is also
beginning to replace some of these access technologies, most notably ATM. MPLS is mainly used for
traffic engineering purposes and to forward IP protocol data units and Virtual Private LAN Service
Ethernet traffic. Generalised Multi-Protocol Label Switching (GMPLS) is a protocol that extends
MPLS to manage further classes of interfaces and switching technologies other than packet interfaces
20
and switching. Examples are TDM, link layer switching, wavelength switching and fibre (port)
switching.
9
Conclusion
In this Chapter we discussed the data link layer, starting with some services it provides. We saw that
the link layer can be seen under the structure of two sublayers - the Link Layer Control sublayer and
the MAC sublayer. In terms of control, we discussed link layer error detection and correction
techniques. We saw that there are three categories of MAC protocols: fixed assigned, demand
assigned and random access MAC. Fixed MAC includes the fundamental ways of dividing network
resources in wireless networks i.e. FDMA, TDMA and CDMA. We mentioned the polling MAC
technique and some examples of its use. The ALOHA and slotted ALOHA random access protocols
were introduced, and the CSMA technique for gaining channel access. ARP was introduced and its
functioning explained in some detail for use within a subnet. It was also mentioned what alterations
are required to provide the same mapping function over different subnets. The Ethernet protocol was
introduced. The concept of a VLAN was introduced and different ways of creating VLANs. Finally
we briefly mentioned the MPLS protocol.
10
Review Questions
1. If all the links in the Internet were to provide reliable delivery service, would the TCP reliable
delivery service be redundant? Why or why not?
2. What are some of the possible services that a link-layer protocol can offer to the network
layer and mention which of these services have an implementation in TCP and IP?
3. Why would the token-ring protocol not work well if a LAN had a very large perimeter?
4. How big are the MAC address space, IPv4 address space and The IPv6 address spaces
respectively?
5. What are some desirable features of a MAC protocol in a wireless network that is spread out
over large distances?
6. If a sending device does not know the MAC address of the destination device, what protocol
is used to find the MAC address of the receiving device?
7. Will a host always update its ARP cache upon receiving an ARP request?
8. Host A wants to send data to host B. Host B is on a different segment from host A. The two
segments are connected to each other through a router. What will host B see as the source
MAC address for all frames sent from host A?
21
9. In what situations do you think contention based MAC protocols are suitable and why?
10. How does CSMA/CD improve performance over CSMA only?
11. How does replacing a hub with a switch affect CSMA/CD behaviour in an Ethernet network?
12. What is an advantage of token passing protocol over CSMA/CD protocol?
13. Suppose that N switches supporting K VLAN groups are to be connected via a trunking
protocol. How many ports are needed to connect the switches? Justify your answer.
14. When a new trunk link is configured on a switch, which VLANs are allowed over the link?
15. Using the information in this chapter, what is a way to improve network performance by
increasing the bandwidth available to hosts and limiting the size of the broadcast domains?
Answers
1.
No, it would not be redundant because although the reliable delivery service would ensure that
datagrams sent over the link are received without errors, arrival in the correct order is not
guaranteed. The IP datagrams may take different routes and arrive out of order and TCP is still
required to ensure that the byte stream is delivered in the correct order.
2.
Framing, done by both IP and TCP; reliable delivery and flow control, both provided by TCP;
error detection, provided in TCP/IP and error correction are some examples.
3.
When a node transmits a frame it must wait for the frame to propagate around the entire ring
before it can release its token so if the perimeter is large the propagation time will be long.
4.
248 MAC address, 232 IPv4 address and 2128 IPv6 addresses
5.
The protocol should be simple to implement, decentralised and require minimal overhead
6.
ARP
7.
No, the ARP cache is only updated if the ARP request is for its IP address.
8.
Because host B is on a different segment that is separated by a router, the MAC address of all
frames sent from host A will be the MAC address of the router. Anytime a frame passed
through a router, a router rewrites the MAC address to the MAC address of the router’s exit
interface for the segment and then sends the frame to the local host. In this case, the router will
change the source MAC address of the frame sent from host A with the MAC address of its
interface connecting to the segment host B is on. Host B will see that the frame came from the
MAC address of the router with the IP address of host A.
9.
Contention-based protocols are suitable for bursty traffic under light to moderate load. These
techniques are decentralised, simple and easy to implement.
10.
In CSMA, a station monitors the channel before sending a packet. Whenever a collision is
detected, it does not stop transmission leading to some time being wasted on backoffs and
retransmissions. In CSMA/CD scheme, whenever a station detects a collision, it sends a signal
to inform other stations that a collision has occurred, thus reducing time wastage and improving
in performance.
22
11.
Collisions are eliminated and CSMA/CD can be disabled.
12.
The CSMA/CD is not a deterministic protocol. A packet may be delivered after many collisions
leading to long variable delay. Some packets may not be delivered at all. On the other hand,
token passing protocol is a deterministic approach, which allows a packet to be delivered within
a known time frame. It also allows priority to be assigned to packets.
13.
The N switches can be connected together so the first and last switches each use one port for
trunking while the middle N-2 switches use two ports each. This makes the total number of
ports 2+2(N-2)=2N-2.
14.
By default, all VLANs are allowed on the trunk link and the network administrator must
remove by hand each VLAN that should not be allowed to traverse the trunked link.
15.
By creating and implementing VLANs in the switched network, it can be segmented into
smaller broadcast domains at link layer. For hosts on different VLANs to communicate, a
router or network layer switch will then be required.
11
Further reading:
1) J. Kurose and K. Ross, “Computer Networking: A Top-Down Approach (International)
Chapter 3”, Pearson Education Ltd., Essex, sixth edition, 2013.
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