WIRELESS ATM SWITCHES
358
When a signaling message for the completion of the connection setup between the switch and the new base station which a mobile terminal is
currently visiting arrives, the handoff controller updates the output port address in the output port address memory and the output VCI in the output
VCI memory for the handoff connection. Since the signaling message has information about a new connection subpath for a mobile’s handoff, a COS
can resume transmitting ATM cells of a handoff connection with a new allocated output port address and VCI. To resume transmission, the mobility
controller accesses the HP memory through the old VCI to find the HOL cell
Ž .
of the handoff connection. If the HP is not null, its AT, A pair is inserted in a sequencer that is associated with the new output port address. This can
resume transmitting cells of the handoff connection through the new output port.
12.5.2 Performance
In this subsection we present the performance of the mobility-support switch architecture described in the previous section.
For performance study, it is assumed that cells from different VCs are multiplexed in bursts, and that the bursts are interleaved as they arrive at
each input port of the switch. To quantify the traffic characteristics, the following traffic model is considered in which an arrival process to an input
Ž .
Ž .
port alternates between on active and off idle periods. Figure 12.10 shows the traffic model used for simulations. During the on
period, cells that originate from the same VC arrive at an input port continuously in consecutive cell time slots, and during the off period no cells
are generated. Both the on and off periods are assumed to be geometrically w x
w x distributed with average lengths E B and E I , respectively.
Fig. 12.10 The simulated traffic model.
MOBILITY-SUPPORT ATM SWITCH
359
Let us define p as the probability that an arriving cell is the last cell in a burst, and q as the probability that a new burst starts per time slot. Then the
probability that the burst has i cells is
iy1
w x
P B s i s 1 y p p,
i G 1, 12.1
Ž .
Ž .
and the probability that an idle period will last for j time slots is
j
w x
P I s j s 1 y q q, j G 0,
12.2
Ž .
Ž .
where we assume that there is at least one cell in each burst and the duration w x
w x of an idle period can be zero. The average lengths E B and E I
are given by
1 1 y q
w x w x
E B s and
E I s .
12.3
Ž .
p q
Given p and q, the offered load is equal to
w x
E B .
12.4
Ž .
w x w x
E B q E I In this simulation study, it is assumed that the switch size N is 16 and the
Ž .
total number of VCs is 512 32 VCs per input port . The initial route of each Ž
. VC is shown in Table 12.1. Then the offered load
of each input port i
i
TABLE 12.1 Initial Routing Table
Input Output
VC Port Address
Port Address 1
1 1
2 1
2 .
. .
. .
. .
. .
16 1
16 17
1 1
18 1
2 .
. .
. .
. .
. .
32 1
16 33
2 1
34 2
2 .
. .
. .
. .
. .
512 16
16
WIRELESS ATM SWITCHES
360
Ž .
and the offered load of each output port j are given by
j
32
s
,
i s 1, 2, . . . , 16, 12.5
Ž .
Ý
i VC
Ž iy1. =32qk
ks1
and
16
s
,
j s 1, 2, . . . , 16, 12.6
Ž .
Ý
j VC
jq16=k
ks0
where and
are the average load of VC and
VC VC
Ž iy1. =32qk
Ž iy1. =32qk
jq16=k
the average load of VC , respectively.
jq16=k
For simplicity, a uniform source distribution is considered, in which any burst to each input port has an equal probability of being originated from any
VC, and successive choices of VC are independent. Then the average load of each VC is the same and can be expressed as
s
, k s 1, 2, . . . , 512
12.7
Ž .
VC VC
k
Ž .
Ž .
and in 12.5 and in 12.6 become
i j
s s s
32 .
12.8
Ž .
i j
VC
Although it was assumed that the traffic loads to each output port are uniformly distributed, the uniform distribution is no longer valid when
handoff is considered. Since handoff changes the route of the corresponding Ž
. VC e.g., the output port address of the VC , the traffic load distribution to
each output port is dynamically changed depending on the handoff rate. It is noted that this will affect the delay and cell loss performance of a mobility-
support switch.
Figure 12.11 shows the average cell delay as a function of input offered Ž
. load
in different situations. Here N and M are the switch size and the
i
total number of VCs, respectively. BL means the average burst length, which Ž
. is given in 12.3 , and HR is the handoff rate, which is defined as the average
number of handoffs that occur in a switch during a second. It is assumed that all VCs are mobile connections, and each VC has an equal probability of
handoff. It is also assumed that each handoff is independent. As mentioned in Section 12.4, handoff requires buffering until a new connection subpath is
established between a COS and the new base station that a mobile terminal is roaming to. It is assumed that this buffering time is 10 ms, allowing for the
w x
connection setup for several hops 36 . Thus, when handoff for a VC occurs, the cells of the VC are queued in the switch memory and transmitted
through a different output port after 10 ms. In Figure 12.11, the case of 100rs handoff rate is compared with the case
of no handoff. The 100rs handoff rate means that the handoff of each VC
MOBILITY-SUPPORT ATM SWITCH
361
Fig. 12.11 Average delay as a function of offered load.
occurs in every 5.12 s on the average, and the rate is sufficiently high to see the handoff effect of the worst case. As shown in Figure 12.11, the average
cell delay of the handoff case is larger than that of the no-handoff case. This Ž
. Ž
. is so with both bursty BL s 15 and nonbursty BL s 1 traffic and is
Ž .
prominent with large input offered load
. For example, for an input
i
offered load of 0.9 and burst length BL of 15, the handoff case’s average
i
Ž .
delay 222.7 cell times is almost twice as large as that in the no-handoff case Ž
. 116.2 cell times .
There are two factors that increase the average delay in the handoff case. One is that due to handoffs of each VC, the offered loads to each output
port are unbalanced, while the load distribution of each output port is assumed to be uniform for the no-handoff case. The other is that when
handoff occurs, no incoming cells of the corresponding VC are served during the connection setup time of a new subpath.
Figure 12.12 shows simulation results on cell loss probability vs. buffer size. In this experiment, the switch size, the number total of VCs, and the
offered load to each input port are 16, 512, and 0.9, respectively. For uniform Ž
. random traffic BL s 1 , the required buffer size is much smaller than that
Ž .
for bursty traffic BL s 15 , and the required buffer size in the handoff case is larger than that in the no-handoff case. This shows that a COS needs a
large memory for buffering cells of each handoff VC, although it uses a shared memory. For example, for bursty traffic the required buffer size in
the handoff case is about 7000 cells to maintain the cell loss probability at less than 10
y 6
. This is at least twice as large as in the no-handoff case Ž
. about 3300 .
WIRELESS ATM SWITCHES
362
Fig. 12.12
Cell loss probability as a function of buffer size.
Simulation results show the impact of handoffs on the performance of the mobility-support switch architecture. It can be noticed that the average cell
delay in the handoff case is almost two times larger than that in the no-handoff case when the traffic load is heavy and bursty and the handoff
rate is high. It can be also noticed that the required memory size in the handoff case is double that in the no-handoff case for heavily loaded bursty
traffic and high handoff rate.
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H. Jonathan Chao, Cheuk H. Lam, Eiji Oki Copyright 䊚 2001 John Wiley Sons, Inc.
Ž .
Ž .
ISBNs: 0-471-00454-5 Hardback ; 0-471-22440-5 Electronic
CHAPTER 13
IP ROUTE LOOKUPS
The emergence of new multimedia networking applications and the growth of the Internet due to the exponentially increasing number of users, hosts, and
domains have led to a situation where network capacity is becoming a scarce resource. In order to maintain the good service, three key factors have been
considered in designing IP networks: large-bandwidth links, high router data
w x
throughput, and high packet forwarding rates 16 . With the advent of fiber optics, which provides fast links, and the current switching technology, which
is applied for moving packets from the input interface of a router to the corresponding output interface at gigabit speeds, the first two factors can be
readily handled. Therefore, the key to the success of the next generation IP networks is the deployment of high-performance routers to forward the
packets at the high speeds.
There are many tasks to be performed in packet forwarding; packet Ž
. header encapsulation and decapsulation, updating a time-to-live TTL field
in each packet header, classifying the packets into queues for specific service classes, etc. The major task, which seems to dominate the processing time of
the incoming packet, is searching for the next-hop information of the appro- priate prefix matching the destination address from a routing table. This is
also called an IP route lookup.
As the Internet has evolved and grown over in recent years, it has been proved that the IP address space, which is divided into classes A, B, and C, is
inflexible and wasteful. Class C, with a maximum of 254 host addresses, is too small, while class B, which allows up to 65,534 addresses, is too large for most
organizations. The lack of a network class of a size that is appropriate for mid-sized organization results in exhaustion of the class B network address
365
IP ROUTE LOOKUPS
366
space. In order to use the address space efficiently, bundles of class C addresses were given out instead of class B addresses. This also causes
massive growth of routing table entries. To reduce the number of routing table entries, classless interdomain routing
Ž . w x
CIDR 4 was introduced to allow for arbitrary aggregation of networks.
A network that has identical routing information for all subnets except a single one requires only two entries in the routing table: one for the specific
Ž .
subnet which has preference if it is matched and the other for the remain- ing subnets. This decreases the size of the routing table and results in better
usage of the available address space. On the other hand, an efficient mechanism to do IP route lookups is required.
In the CIDR approach, a routing table consists of a set of IP routes. Each ²
: IP route is represented by a route prefixrprefix length pair. The prefix
length indicates the number of significant bits in the route prefix. Search- ing is done in a longest-matching manner. For example, a routing table
² : ²
: ²
: may have the prefix routes 12.0.54.8r32 , 12.0.54.0r24 , and 12.0.0.0r16 .
² :
If a packet has the destination address 12.0.54.2 , the second prefix
route is matched, and its next hop is retrieved and used for forwarding the packet.
This chapter is organized as follows: Starting from Section 13.1, the architectures of generic routers are described and the design criteria
that should be considered for IP lookups is discussed. Recently, there have been a number of techniques proposed to provide fast IP lookups
w x
2, 3, 5, 6, 9, 12, 14, 15, 16 . From Section 13.2 to Section 13.9 we will look at several IP route-lookup schemes proposed over the past few years.
13.1 IP ROUTER DESIGN
